Free
Clinical Science  |   July 1998
Dobutamine Antagonizes Epinephrine's Biochemical and Cardiotonic Effects  : Results of an In Vitro Model Using Human Lymphocytes and a Clinical Study in Patients Recovering from Cardiac Surgery
Author Notes
  • (Prielipp) Associate Professor of Anesthesiology.
  • (MacGregor) Assistant Professor of Anesthesiology and Medicine.
  • (Royster) Professor of Anesthesiology and Vice Chairman, Department of Anesthesiology.
  • (Kon) Associate Professor of Surgery.
  • (Hines) Assistant Professor of Surgery.
  • (Butterworth) Professor of Anesthesiology and Vice Chairman for Research, Department of Anesthesiology.
Article Information
Clinical Science
Clinical Science   |   July 1998
Dobutamine Antagonizes Epinephrine's Biochemical and Cardiotonic Effects  : Results of an In Vitro Model Using Human Lymphocytes and a Clinical Study in Patients Recovering from Cardiac Surgery
Anesthesiology 7 1998, Vol.89, 49-57. doi:
Anesthesiology 7 1998, Vol.89, 49-57. doi:
DRUG combinations can provide therapeutic advantages and improved outcome compared with monotherapy for the treatment of many disorders. Most drug combinations use agents with differing mechanisms of action to achieve additive or synergistic effects. Combinations of a catecholamine and a phosphodiesterase inhibitor have been used to treat chronic congestive heart failure and low cardiac output syndrome after cardiac surgery. [1,2] Two (or more) catecholamines are often infused simultaneously in critically ill patients. The theoretical rationale for confusing drugs in this manner is to combine the positive attributes of both drugs, limit the detrimental side effects of high doses of a single agent, and, perhaps, to achieve a greater therapeutic response than possible with either drug alone (pharmacologic synergism, also known as super-additivity). However, two agonist drugs may interact in an additive, synergistic, or pseudoantagonistic manner. This study was designed to determine whether the [Greek small letter beta]-receptor agonist dobutamine is additive to the effect of epinephrine.
The investigation was performed in two phases. In phase 1, using human lymphocytes studied in vitro, we measured cyclic adenosine monophoshate (cAMP) generation by the [Greek small letter beta]-adrenergic receptor-adenylyl cyclase-cAMP pathway in response to dobutamine and epinephrine stimulation. [3] Phase 2 was a clinical investigation of patients who had aortocoronary bypass surgery to determine the cardiotonic effects of epinephrine and dobutamine, alone and in combination.
Materials and Methods
These studies were approved by the Clinical Research Practices Committee of The Bowman Gray School of Medicine, Wake Forest University, with informed consent obtained from all participants.
Phase 1
The lymphocyte cAMP method we used was similar to that previously published. [4–7] Samples of 50 ml venous blood were obtained from 10 healthy volunteers before 10: A.M. on the experimental day. Volunteers were excluded if any medication was taken 30 days before blood sampling and if [Greek small letter beta]-adrenergic antagonist medications had ever been taken. Blood from only one volunteer was used in any one experimental preparation. Blood was anticoagulated with heparin sodium and immediately diluted with an equal volume of phosphate-saline buffer (NaCl = 140 mM, KCl = 2.7 mM, KH2PO4= 1.5 mM, K2HPO4= 6 mM). Lymphocytes were isolated as described by Boyum [4] by applying diluted blood to Ficoll-Paque (Pharmacia Laboratory Separation, Pharmacia, Piscataway, NJ) and subjecting it to density-gradient centrifugation at 1,300 g for 30 min to generate a lymphocyte-rich (70% T lymphocytes and 10% B lymphocytes) band of cells. This lymphocyte band was aspirated, washed twice in ice-cold phosphate-buffered saline solution, and divided into separate test tubes, each of which contained approximately 1 x 106lymphocytes/ml, quantified by Coulter Counter (Coulter Electronics, Hialeah, FL) set to recognize only mononuclear cells. 3-isobutyl-1-methylxanthine (IBMX, 0.01 M) was added to each test tube to block cAMP hydrolysis.
Adenylyl cyclase activity was determined in an incubation mixture containing 140 mM NaCl 1.5 mM CaCl2, and IBMX. The reaction was initiated by adding cells to the incubation mixture, for a final reaction volume of 500 [micro sign]l. Two tubes from each group were not exposed to any agonist drug to determine baseline cAMP production, whereas the remaining tubes of lymphocytes were mixed with graded concentrations of the [Greek small letter beta]-agonists epinephrine, dobutamine, or a combination of epinephrine plus dobutamine. The concentrations of each drug used in this study were determined from previous experience with the lymphocyte-cAMP experimental model [6,7] and ranged from 10-8M to 10-4M dobutamine, and from 10-9M to 10-5M epinephrine. The tubes were incubated for 10 min at 27 [degree sign]C.
Incubation was terminated with the addition of a solution of cold 3% perchloric acid and with subsequent addition of 2% KHCO3. All samples were chilled to 1–2 [degree sign]C, and cAMP was measured in duplicate using a radioimmunoassay method modified from Gilman [5] (RIA, Diagnostic Products Corporation, Los Angeles, CA). Measured cAMP production was standardized to 1 x 106lymphocytes. The values for cAMP production for the two samples exposed to the same concentration of agonists were averaged, resulting in a single cAMP production value used for statistical analysis. Based on our experience with these procedures, we used an a priori quality control wherein cAMP concentrations from the two lymphocyte samples exposed to the same concentration of agonist that varied by <25% were averaged and considered valid. Our experience with this model finds that only 1–2% of samples deviate outside this range. The entire process was repeated on separate study days for each of the 10 volunteer blood samples.
Phase 2
After patients gave written informed consent, those undergoing elective aortocoronary bypass graft surgery with normal preoperative left-ventricular function (ejection fraction >or= to 0.50) were studied in the cardiac surgical intensive care unit (ICU) on their first postoperative day. Inclusion criteria required that the patient be extubated in the ICU, demonstrate respiratory and hemodynamic stability, and have manifest no need for any inotropic drug support during the entire postoperative period. Previous infusion of vasoactive drugs (such as phenylephrine, nitroglycerin, sodium nitroprusside, and so on) was permitted if such infusions were terminated before the study was begun. In addition, diuretic therapy was temporarily withheld during the clinical protocol. Only mild sedatives or analgesics were administered concomitant with study medications, if necessary.
Eight patients qualified for the study in the ICU. Baseline hemodynamic measurements obtained included heart rate (HR), mean arterial pressure, pulmonary artery pressure, pulmonary artery occlusion pressure, and cardiac output in duplicate (a third determination was done if the first two varied by more than 10%). Cardiac index (CI) and the percentage changes after therapy were calculated by standard formulas. All study inotrope medications were administered into a central catheter, with a secondary carrier solution of D5W infusing at a constant 100 ml/h to ensure rapid drug bioavailability. An epinephrine infusion at 10 ng [middle dot] kg-1[middle dot] min-1was infused for 8 min, and hemodynamic measurements were repeated. We previously determined that the clinical (cardiotonic) effects of these drugs reach steady state within 8 min, and that 15 min is adequate to allow for return to baseline. [2,8–10] The infusion was increased to 20 and then to 40 ng [middle dot] kg-1[middle dot] min-1with data recorded after each incremental dose was infused for 8 min. After a 15-min recovery period allowed for hemodynamic parameters to return to baseline, dobutamine was administered (1.25 [micro sign]g [middle dot] kg (-1)[middle dot] min-1) and data were recorded after 8 min. Then, while dobutamine was being infused, the epinephrine infusion was administered at 10, 20, and 40 ng [middle dot] kg-1[middle dot] min-1with hemodynamic data obtained 8 min after each incremental dose (see the flow diagram in Table 1). The epinephrine infusion was then discontinued and the dobutamine infusion was increased to 2.5 [micro sign]g [middle dot] kg-1[middle dot] min-1and the epinephrine infusion was added at the same three doses as previously described. The epinephrine infusion was again discontinued and the dobutamine infusion increased to 5 [micro sign]g [middle dot] kg-1[middle dot] min-1. After 15 min, hemodynamic measurements were performed and the epinephrine infusion was again added at the three doses, as previously described. No other intravenous fluids were administered during the study. Data were recorded with dobutamine alone and after each combination of dobutamine and epinephrine.
Table 1. Clinical Study Design Performed in Each Cardiac Surgery Patient 
Image not available
Table 1. Clinical Study Design Performed in Each Cardiac Surgery Patient 
×
Statistical Analyses
Basal cAMP production was compared using analysis of variance. To evaluate drug efficacy, the maximal cAMP production was used, divided by baseline cAMP production before the statistical analysis to adjust for baseline differences among lymphocyte groups. To evaluate drug potency, the minimum, maximum, and the median effective concentrations were determined by fitting the data to a sigmoidal distribution curve with a Hill coefficient of 1, defined by the Equation 1where E = the drug effect (cAMP production), Emin = the minimum cAMP concentration, Emax = the maximum cAMP concentration, EC50= the concentration producing 50% of the maximal effect, and conc = the concentration of the agonist. This Equation assumesthat the agonists bind freely with the receptor sites without negative or positive cooperativity. [11] The slopes for change in absolute cAMP production between consecutive dobutamine concentrations along the epinephrine concentration curves were estimated and then tested for significant difference from 0. Statistical contrasts were performed to compare similar slopes at different dobutamine concentrations.
In phase 2, the actual recorded parameters (i.e., HR and CI) were analyzed to evaluate the interaction between epinephrine and dobutamine with a doubly repeated-measures mixed-effects analysis of variance. [12] Within-subject factors included epinephrine, dobutamine, and their interaction. Corrections were made for multiple comparisons using Fisher's protected least significant difference test as appropriate. Statistical analyses were performed using software (Proc Mixed) in the SAS statistical package (version 6.08; SAS Institute, Cary, NC). [12] In addition, a CI isobologram was constructed. This graphical technique uses equieffective concentrations of individual drugs and the combination of two drugs. [13–16] We plotted dose-response curves with dobutamine, epinephrine, and the dobutamine-epinephrine combinations and chose the dose from these curves that resulted in a 10% increase in CI. On the horizontal and vertical axes of the isobologram, the dose of dobutamine and epinephrine that would result in a 10% increase in CI is plotted. A line of additivity is drawn between these two points. It would be expected that any combination of drugs along this line would produce a 10% increase in CI. If a 10% increase in CI results from a drug dose combination below this line (i.e., smaller doses than predicted), drug synergism is present. If the dose combination producing a 10% increase in CI lies above this line (i.e., larger doses than predicted), then pseudoantagonism is present. Data are presented as mean +/- SD, and alpha levels of 0.05 were used throughout.
Results
Phase 1
Dobutamine alone resulted in a small but significant increase in cAMP production of 41%(3.2 +/- 0.9 to 4.6 +/- 0.9 pmole [middle dot] 10-6lymphocytes [middle dot] 10 min-1; P = 0.005) in concentrations varying from 10-8M to 10-4M dobutamine (Figure 1). Previously we showed that concentrations of dobutamine <or= to 10-8M are below the stimulatory threshold of cAMP production in this model and not significantly different from a zero drug concentration. [7] Epinephrine concentrations from 10-9M to 10-5M increased cAMP above baseline by [approximately] 200%(3.6 +/- 0.9 to 10.6 +/- 2.5 pmole [middle dot] 10-6lymphocytes [middle dot] 10 min-1; P < 0.0001;Figure 1).
Figure 1. Dose-response curves for epinephrine and dobutamine. Cyclic adenosine monophosphate (cAMP) production was measured in duplicate for each sample from 10 healthy human volunteers. Epinephrine is about 10 times more potent (median effective concentration, 10-7.335 M) compared with dobutamine (median effective concentration, 10-6.124 M) at the [Greek small letter beta]-adrenergic receptor (primarily [Greek small letter beta](2) receptors) in this human lymphocyte model.
Figure 1. Dose-response curves for epinephrine and dobutamine. Cyclic adenosine monophosphate (cAMP) production was measured in duplicate for each sample from 10 healthy human volunteers. Epinephrine is about 10 times more potent (median effective concentration, 10-7.335 M) compared with dobutamine (median effective concentration, 10-6.124 M) at the [Greek small letter beta]-adrenergic receptor (primarily [Greek small letter beta](2) receptors) in this human lymphocyte model.
Figure 1. Dose-response curves for epinephrine and dobutamine. Cyclic adenosine monophosphate (cAMP) production was measured in duplicate for each sample from 10 healthy human volunteers. Epinephrine is about 10 times more potent (median effective concentration, 10-7.335 M) compared with dobutamine (median effective concentration, 10-6.124 M) at the [Greek small letter beta]-adrenergic receptor (primarily [Greek small letter beta](2) receptors) in this human lymphocyte model.
×
(Figure 2) shows cyclic AMP production stimulated by varying concentrations of epinephrine plus dobutamine. For epinephrine at 10-6M, cAMP production decreased significantly when dobutamine was increased from 10-6M to 10-5M (P = 0.0011), and also from 10-5M to 10-4M (P = 0.0009). Similarly, an epinephrine concentration of 10-7M was significantly inhibited by dobutamine concentrations from 10-6M to 10 (-5) M (P = 0.024). Cyclic AMP production did not decrease when epinephrine was mixed with more dilute concentrations of dobutamine (i.e., dobutamine = 10-8M to 10-6M). Furthermore, concentrations of epinephrine > 10 (-8) M showed no evidence for inhibition by dobutamine (Figure 2).
Figure 2. Dose-response curves of cyclic adenosine monophosphate (cAMP) production stimulated by various concentrations of epinephrine combined with dobutamine. For 10-6M epinephrine, cAMP production decreased significantly when dobutamine was increased from 10-6M to 10-5M and from 10-5M to 10-4M (*P = 0.001). Similarly, an epinephrine concentration of 10-7M was significantly inhibited by dobutamine concentrations from 10-6M to 10-5M (P = 0.02). Production of cAMP did not decrease significantly when epinephrine was mixed with more dilute concentrations of dobutamine (i.e., dobutamine = 10-8M to 10-6M). Data were plotted using the method of Ariens et al. [18] 
Figure 2. Dose-response curves of cyclic adenosine monophosphate (cAMP) production stimulated by various concentrations of epinephrine combined with dobutamine. For 10-6M epinephrine, cAMP production decreased significantly when dobutamine was increased from 10-6M to 10-5M and from 10-5M to 10-4M (*P = 0.001). Similarly, an epinephrine concentration of 10-7M was significantly inhibited by dobutamine concentrations from 10-6M to 10-5M (P = 0.02). Production of cAMP did not decrease significantly when epinephrine was mixed with more dilute concentrations of dobutamine (i.e., dobutamine = 10-8M to 10-6M). Data were plotted using the method of Ariens et al. [18]
Figure 2. Dose-response curves of cyclic adenosine monophosphate (cAMP) production stimulated by various concentrations of epinephrine combined with dobutamine. For 10-6M epinephrine, cAMP production decreased significantly when dobutamine was increased from 10-6M to 10-5M and from 10-5M to 10-4M (*P = 0.001). Similarly, an epinephrine concentration of 10-7M was significantly inhibited by dobutamine concentrations from 10-6M to 10-5M (P = 0.02). Production of cAMP did not decrease significantly when epinephrine was mixed with more dilute concentrations of dobutamine (i.e., dobutamine = 10-8M to 10-6M). Data were plotted using the method of Ariens et al. [18] 
×
In a subset of lymphocytes, blood samples (n = 5) were examined to determine whether the [Greek small letter beta]-agonist inhibition demonstrated above is competitive. If so, then very high doses of the full agonist epinephrine should overcome the pseudoantagonism produced by the partial agonist dobutamine. In this subset analysis, cAMP production produced by 10-5M epinephrine was not significantly inhibited by dobutamine concentrations in the range of 10-6M to 10-5M.
Phase 2
Eight patients (of 37 who gave initial, preoperative consent) qualified for the study in the ICU. Twenty-three patients were excluded because of use of concomitant medications (epinephrine, amrinone, dopamine, nitroglycerin, or nitroprusside), one patient could not be studied because no investigator was available, one patient was still intubated, one patient was having gastrointestinal distress, and three patients had their surgery canceled. Figure 3and Figure 4show the HR and CI responses for both drugs alone and for the various combinations. Dobutamine significantly increased HR with each incremental increase in dose (overall dobutamine dose effect on HR; P <or= to 0.0001). Epinephrine significantly increased HR from baseline only at the highest dose (40 ng [middle dot] kg-1[middle dot] min-1; P = 0.0081). Combining epinephrine and dobutamine resulted in the same incremental increase in HR as that produced by doses of dobutamine alone.
Figure 3. Heart rate responses for dobutamine and epinephrine used alone, and their various combinations, in eight patients having aortocoronary bypass surgery who received dobutamine at doses of 0, 1.25, 2.5, and 5 [micro sign]g [middle dot] kg-1[middle dot] min-1, and epinephrine at doses of 0, 10, 20, and 40 ng [middle dot] kg-1[middle dot] min-1. See “Results” for a discussion of significant changes and Table 1for a summary of the drug administration protocol. Data points are slightly offset along the abscissa to eliminate overlap of the symbols in Figure 3and Figure 4.
Figure 3. Heart rate responses for dobutamine and epinephrine used alone, and their various combinations, in eight patients having aortocoronary bypass surgery who received dobutamine at doses of 0, 1.25, 2.5, and 5 [micro sign]g [middle dot] kg-1[middle dot] min-1, and epinephrine at doses of 0, 10, 20, and 40 ng [middle dot] kg-1[middle dot] min-1. See “Results” for a discussion of significant changes and Table 1for a summary of the drug administration protocol. Data points are slightly offset along the abscissa to eliminate overlap of the symbols in Figure 3and Figure 4.
Figure 3. Heart rate responses for dobutamine and epinephrine used alone, and their various combinations, in eight patients having aortocoronary bypass surgery who received dobutamine at doses of 0, 1.25, 2.5, and 5 [micro sign]g [middle dot] kg-1[middle dot] min-1, and epinephrine at doses of 0, 10, 20, and 40 ng [middle dot] kg-1[middle dot] min-1. See “Results” for a discussion of significant changes and Table 1for a summary of the drug administration protocol. Data points are slightly offset along the abscissa to eliminate overlap of the symbols in Figure 3and Figure 4.
×
Figure 4. Cardiac index responses for both drugs alone and for their various combinations. The cardiac index response of eight patients having aortocoronary bypass surgery who received dobutamine at doses of 0, 1.25, 2.5, and 5 [micro sign]g [middle dot] kg-1[middle dot] min-1, and epinephrine at doses of 0, 10, 20, and 40 ng [middle dot] kg-1[middle dot] min-1. See “Results” for a discussion of significant changes.
Figure 4. Cardiac index responses for both drugs alone and for their various combinations. The cardiac index response of eight patients having aortocoronary bypass surgery who received dobutamine at doses of 0, 1.25, 2.5, and 5 [micro sign]g [middle dot] kg-1[middle dot] min-1, and epinephrine at doses of 0, 10, 20, and 40 ng [middle dot] kg-1[middle dot] min-1. See “Results” for a discussion of significant changes.
Figure 4. Cardiac index responses for both drugs alone and for their various combinations. The cardiac index response of eight patients having aortocoronary bypass surgery who received dobutamine at doses of 0, 1.25, 2.5, and 5 [micro sign]g [middle dot] kg-1[middle dot] min-1, and epinephrine at doses of 0, 10, 20, and 40 ng [middle dot] kg-1[middle dot] min-1. See “Results” for a discussion of significant changes.
×
Dobutamine doses of 1.25 and 2.5 [micro sign]g [middle dot] kg-1[middle dot] min-1produced equivalent increases in CI (Figure 4), whereas 5 [micro sign]g [middle dot] kg-1[middle dot] min-1of dobutamine produced the greatest increase in CI (28%). Epinephrine alone increased CI (compared with baseline) with each incremental increase in dose (10, 20, and 40 ng [middle dot] kg-1[middle dot] min-1). However, the combination of epinephrine with the two larger doses of dobutamine (2.5 and 5 [micro sign]g [middle dot] kg-1[middle dot] min-1) was no better at increasing CI than that achieved with epinephrine plus the lowest dose of dobutamine (1.25 [micro sign]g [middle dot] kg-1[middle dot] min-1; P > 0.05).
(Figure 5) illustrates the isobologram for dobutamine and epinephrine on CI. Using dose-response curves with dobutamine, epinephrine, and the dobutamine-epinephrine combinations, the dose that resulted in a 10% increase in CI was determined. On the horizontal and vertical axes of the isobologram, the dose of dobutamine and epinephrine that would result in a 10% increase in CI is plotted. A line of additivity is drawn between these two points. If the effects were additive, it would be expected that any combination of drugs that achieved a 10% increase in CI would result in doses of drugs taken from this line. If the 10% increase in CI results from drug dose combinations below this line (smaller doses than predicted), drug synergism is present. If the dose combination results in drug doses above this line (larger doses than predicted), then pseudoantagonism is present. Visual inspection reveals that the observed data points fall above and to the right of the line of additivity, suggesting clinical pseudoantagonism between these two [Greek small letter beta]-agonists.
Figure 5. An isobolographic analysis of the cardiotonic effects of dobutamine and epinephrine. On the horizontal and vertical axes of this isobologram, the dose of dobutamine and epinephrine that would result in a 10% increase in the cardiac index (CI) is plotted. A line of additivity is drawn between these two points. It would be expected that any combination of drugs that achieved a 10% increase in CI would result in doses of drugs from along this line. If the 10% increase in CI results from drug dose combinations below this line (smaller doses than predicted), then drug synergism is present. If the drug doses are above this line (larger doses than predicted), then drug antagonism is present. Visual inspection reveals that the observed data points fall above and to the right of the line of additivity, suggesting clinical antagonism between dobutamine and epinephrine.
Figure 5. An isobolographic analysis of the cardiotonic effects of dobutamine and epinephrine. On the horizontal and vertical axes of this isobologram, the dose of dobutamine and epinephrine that would result in a 10% increase in the cardiac index (CI) is plotted. A line of additivity is drawn between these two points. It would be expected that any combination of drugs that achieved a 10% increase in CI would result in doses of drugs from along this line. If the 10% increase in CI results from drug dose combinations below this line (smaller doses than predicted), then drug synergism is present. If the drug doses are above this line (larger doses than predicted), then drug antagonism is present. Visual inspection reveals that the observed data points fall above and to the right of the line of additivity, suggesting clinical antagonism between dobutamine and epinephrine.
Figure 5. An isobolographic analysis of the cardiotonic effects of dobutamine and epinephrine. On the horizontal and vertical axes of this isobologram, the dose of dobutamine and epinephrine that would result in a 10% increase in the cardiac index (CI) is plotted. A line of additivity is drawn between these two points. It would be expected that any combination of drugs that achieved a 10% increase in CI would result in doses of drugs from along this line. If the 10% increase in CI results from drug dose combinations below this line (smaller doses than predicted), then drug synergism is present. If the drug doses are above this line (larger doses than predicted), then drug antagonism is present. Visual inspection reveals that the observed data points fall above and to the right of the line of additivity, suggesting clinical antagonism between dobutamine and epinephrine.
×
Discussion
This study shows pseudoantagonism of biochemical and clinical endpoints when the two [Greek small letter beta]-agonists dobutamine and epinephrine are combined. Phase 1 tested the intracellular molecular signaling via the [Greek small letter beta]-adrenergic receptor-adenylyl cyclase-cAMP pathway in human lymphocytes. We found that dobutamine (>or= to 10-6M), combined with epinephrine (concentrations of 10-6M and 10 (-7) M), significantly reduced cAMP production in the in vitro model (compared with epinephrine alone). In addition, the cardiotonic effects of epinephrine and dobutamine were prospectively analyzed in postoperative cardiac surgery patients via isobolographic analysis of CI. The combination of dobutamine with epinephrine requires more drug (than predicted) to produce a 10% increase in CI, based on the epinephrine-dobutamine isobologram. We conclude that the combination of these two [Greek small letter beta]-adrenergic agonists (at these concentrations) results in inotropic drug pseudoantagonism.
The classic drug-receptor interaction model postulates an agonist binding to a cellular receptor, activating it, and producing a maximal physiologic response (usually defined as drug efficacy), whereas a partial agonist binds to the same receptor but generates only a fraction of this maximal response. [17] By these definitions, epinephrine is a full agonist in our model (maximal cAMP response = 10.6 pmole [middle dot] 10-6lymphocytes in 10 min) and dobutamine is a partial agonist (maximal cAMP response = 4.6 pmole [middle dot] 10-6lymphocytes in 10 min). We previously identified similar drug efficacy and potency while describing six [Greek small letter beta]-adrenergic receptor agonists in the human lymphocyte model. [7] This classic model of drug-receptor interaction implies that partial agonists should have “pseudoantagonistic properties”; that is, they occupy receptors while producing weaker effects. [18] This pharmacologic phenomenon has been well characterized previously with other drugs. [18] 
However, a new model for receptor-mediated response suggests that the key step in transmembrane signal transduction is the transition of the [Greek small letter beta]-receptor from the inactive (R) to the active (R*) state (Figure 6). [17,19,20] In the traditional model, the occupation of these receptors by an agonist results in activation (R*). However, evidence now suggests that hormones may not actually be involved in the conversion from the inactive to the fully active receptor. Rather, the receptor is in a natural state of flux between R and R* conformations, and agonists preferentially select the active form, thereby shifting the equilibrium toward the active (R*) conformation. [17,19,20] Thus, in this model, agonists do not activate receptors but rather bind selectively to the already active conformation, favoring movement of the equilibrium in that direction. An antagonist binds with equal affinity to both active and inactive forms but does not alter the equilibrium while occupying receptors. Finally, a partial agonist has only slightly greater affinity for R* compared with R, which means that even at saturating concentrations, the magnitude of the observed effect will be less (i.e., less cAMP production by dobutamine compared with epinephrine). Many drugs used clinically are partial agonists and are predicted to produce subadditive effects when combined with a full agonist. [18] 
Figure 6. A hypothetical [Greek small letter beta]-receptor exists in equilibrium between the R*(active, productive) and the R (inactive, nonproductive) forms, representing two conformations of the [Greek small letter beta]-receptor-G-protein-coupled-adenylyl cyclase enzyme system. The extent to which the equilibrium is perturbed depends on the relative affinity of a drug for the R* or the R conformations. A full agonist is sufficiently selective that it will drive the receptor completely to the active state (R*). A partial agonist is structurally analogous but exhibits only slightly greater affinity for R* than for R, meaning that even at saturating concentrations, the magnitude of the observed effect will be less. An antagonist binds with equal affinity to both conformations and does not alter the activation equilibrium while occupying receptor sites.
Figure 6. A hypothetical [Greek small letter beta]-receptor exists in equilibrium between the R*(active, productive) and the R (inactive, nonproductive) forms, representing two conformations of the [Greek small letter beta]-receptor-G-protein-coupled-adenylyl cyclase enzyme system. The extent to which the equilibrium is perturbed depends on the relative affinity of a drug for the R* or the R conformations. A full agonist is sufficiently selective that it will drive the receptor completely to the active state (R*). A partial agonist is structurally analogous but exhibits only slightly greater affinity for R* than for R, meaning that even at saturating concentrations, the magnitude of the observed effect will be less. An antagonist binds with equal affinity to both conformations and does not alter the activation equilibrium while occupying receptor sites.
Figure 6. A hypothetical [Greek small letter beta]-receptor exists in equilibrium between the R*(active, productive) and the R (inactive, nonproductive) forms, representing two conformations of the [Greek small letter beta]-receptor-G-protein-coupled-adenylyl cyclase enzyme system. The extent to which the equilibrium is perturbed depends on the relative affinity of a drug for the R* or the R conformations. A full agonist is sufficiently selective that it will drive the receptor completely to the active state (R*). A partial agonist is structurally analogous but exhibits only slightly greater affinity for R* than for R, meaning that even at saturating concentrations, the magnitude of the observed effect will be less. An antagonist binds with equal affinity to both conformations and does not alter the activation equilibrium while occupying receptor sites.
×
From our previous experience and characterization of this human lymphocyte model, epinephrine is about 10 times more potent (EC50= 10 (-7).335 M) than dobutamine (EC50= 10-6.124 M) at the [Greek small letter beta]2-adrenergicreceptor. [7] This would be consistent with inspection of the data shown in Figure 1and Figure 2. That is, pseudoantagonism of 10-7M epinephrine requires 10 to 100 times greater concentrations of dobutamine, concentrations that presumably are sufficient to account for the differences in molecular affinity at the [Greek small letter beta]-receptor. Similarly, for 10-6M epinephrine, it requires 10 to 100 times greater concentrations of dobutamine for subadditive effects to be observed. Conversely, however, higher concentrations of epinephrine (10 (-5) M) overcome the inhibition of dobutamine in the range of 10-6M to 10-4M.
Are the concentrations and doses of epinephrine and dobutamine used in this study relevant to clinical practice? Most patients receiving infusions of catecholamines exhibit highly variable serum concentrations of these drugs. [21–23] Standard dobutamine infusions in patients in the ICU produce a 50-fold range of serum concentrations (0.05 x 10-6M to 2.5 x 10-6M). [23] We found that postoperative epinephrine infusions of 0.03 [micro sign]g [middle dot] kg-1[middle dot] min-1patients having coronary artery bypass grafting produced blood concentrations of about 6 x 10-6M. [24] Thus the drug concentrations and doses we used in the in vitro phase of this study appear to be within, or near, the concentrations achieved clinically with typical infusion rates for these two catecholamines in critically ill patients.
The effects of dobutamine and epinephrine on CI are displayed in Figure 4and in the isobologram of Figure 5. The low dose of dobutamine (1.25 [micro sign]g [middle dot] kg-1[middle dot] min-1) increases CI as it is added to low doses of epinephrine, and this is clinically what we would expect, and indeed demonstrated. However, the combination of epinephrine with the two larger doses of dobutamine (2.5 and 5 [micro sign]g [middle dot] kg-1[middle dot] min-1) was no better at increasing CI than that achieved with epinephrine plus the lowest dose of dobutamine (1.25 [micro sign]g [middle dot] kg-1[middle dot] min-1). This is a less-than-additive effect between a partial agonist (dobutamine) and a full agonist (epinephrine). [18] We would not expect to see an actual decrease in the CI by the addition of dobutamine unless higher doses of epinephrine were being used. Obviously, we had to restrict the clinical doses of these inotropes to reasonable, safe, and prudent ones. However, the clinical effects on CI are less than additive (less than the predicted amount), as seen by isobolographic analysis. Although we wanted to enroll additional patients into the clinical investigation to increase the statistical power, the clinical pathway for cardiac surgery commonly called “fast tracking” significantly altered the recovery paradigm for these patients (early removal of pulmonary artery catheter, aggressive diuresis, early mobilization, and ICU discharge) which restricted our opportunity to study these patients under the same clinical conditions we describe here.
Additional limitations are evident. First, our findings may be applicable only to epinephrine and dobutamine used in postoperative cardiac patients. However, we specifically selected these patients, and these drugs, because of the frequency that these drugs are used after this common surgical procedure. Second, acute [Greek small letter beta]-receptor desensitization could occur during repeated catecholamine infusions during the study protocol (Table 1). However, we have not seen acute, demonstrable alterations in the clinical response to repetitive catecholamine infusions in this patient population in our past investigations. [2,8–10,24] Nonetheless, others have identified acute receptor desensitization of both [Greek small letter beta]1and [Greek small letter beta]2receptors after cardiopulmonary bypass. [25,26] This pattern may be reversed by 30 min after extracorporeal circulation (although the time course of [Greek small letter beta] receptor downregulation is more variable). [25] Because our cardiac surgery patients were tested 24 h after operation, we assumed that many of these alterations abated at the time of this study. A third potential limitation is the in vitro model of human lymphocytes, which contains a homogenous population of [Greek small letter beta]2receptors. [27] Despite some continued debate, most investigators acknowledge that the [Greek small letter beta]2-adrenoreceptoron circulating lymphocytes accurately predicts adrenergic function in general, [27,28] and [Greek small letter beta]2receptor function specifically in the healthy [29] and septic heart. [30] We have extensive experienced with this model that shows the interaction of local anesthetics [31] and sepsis [32] with harvested lymphocytes. Although only 40%(range, 14–59%) of [Greek small letter beta]-adrenergic receptor subtype, [33] the [Greek small letter beta]2receptor subtype is particularly critical for intracellular cAMP generation, because it may account for as much as 10 times more cAMP production per activated receptor than the [Greek small letter beta]1receptor. [34] Thus we believe this combined evidence supports the validity of the circulating lymphocyte-cAMP model as an acceptable in vitro model to evaluate human [Greek small letter beta]2adrenoreceptor function, and that the advantages of studying actual human tissue offset potential limitations. Finally, we recognize that it would be ideal to test the isobole interaction at several CI endpoints, such as drug doses required to produce increases in CI of 10%, 30%, and 50%. However, clinical limitations, patient safety, and patient tolerance did not allow for such an ideal drug titration.
Although many physician believe that all inotropic drug combinations are additive or even synergistic, we found that specific concentrations of dobutamine and epinephrine are, in fact, less than additive. It is unclear how these findings might influence the management of critically ill patients, but we believe clinicians should be aware of the complexity of drug interactions when drugs have an affinity for the same receptor. Decreased responsiveness to catecholamine infusions is recognized in a wide variety of clinical scenarios, including congestive heart failure, [35] after thoracic and abdominal surgical operations, [36,37] after reversible myocardial ischemia, [38] and during sepsis. [39–42] Although receptor desensitization and downregulation certainly play a role in the decreased responsiveness to catecholamine infusions, [43] we speculate that drug pseudoantagonism also may occur during concurrent administration of two or more [Greek small letter beta]-adrenergic inotropic drugs. We would advise close monitoring of the hemodynamic and cardiovascular system to observe for possible drug interactions.
The authors thank Jian P. Leith and Yonggu A. Lin for their help in performing the cAMP assays.
REFERENCES
Butterworth JF IV, Royster RL, Prielipp RC, Lawless ST, Wallenhaupt SL: Amrinone in cardiac surgical patients with left-ventricular dysfunction. A prospective, randomized placebo-controlled trial. Chest 1993; 104:1660-7
Royster RL, Butterworth JF IV, Prielipp RC, Zaloga GP, Lawless SG, Spray BJ, Kon ND, Wallenhaupt SL, Cordell AR: Combined inotropic effects of amrinone and epinephrine after cardiopulmonary bypass in humans. Anesth Analg 1993; 77:662-72
Levitzki A: From epinephrine to cyclic AMP. Science 1988; 241:800-6
Boyum A: Isolation of mononuclear cells and granulocytes from human blood. Scand J Clin Invest 1968; 21(suppl 97):77-89
Gilman AG: A protein binding assay for adenosine 3':5'-cyclic monophosphate. Proc Natl Acad Sci U S A 1970; 67:305-12
Abernethy WB, Butterworth IV JF, Prielipp RC, Leith JP, Zaloga GP: Calcium entry attenuates adenylyl cyclase activity: A possible mechanism for calcium-induced catecholamine resistance. Chest 1995; 107:1420-5
MacGregor DA, Prielipp RC, Butterworth JF IV, James RL, Royster RL: Relative efficacy and potency of [Greek small letter beta]-adrenoreceptor agonists for generating cAMP in human lymphocytes. Chest 1996; 109:194-200
Prielipp RC, Zaloga GP, Butterworth FJ IV, Robertie PG, Dudas LM, Black KW, Royster RL: Magnesium inhibits the hypertensive but non the cardiotonic actions of low-dose epinephrine. Anesthesiology 1991; 74:973-9
Butterworth JF IV, Zaloga GP, Prielipp RC, Tucker WY Jr, Royster RL: Calcium inhibits the cardiac stimulating properties of dobutamine but not of amrinone. Chest 1992; 101:174-80
Butterworth JF IV, Prielipp RC, Royster RL, Spray BJ, Kon ND, Wallenhaupt SL, Zaloga GP: Dobutamine increases heart rate more than epinephrine in patients recovering from aortocoronary bypass surgery. J Cardiothorac Vasc Anesth 1992; 6:535-41
Hull CJ: Pharmacokinetics for Anesthesia. Oxford. Butterworth-Heinemann, 1991, pp 87-90
SAS Institute: The mixed procedure, SAS Technical Report. Cary, NC, SAS Institute, 1992, pp 289-366
Smith NT: Use of isobolograms in anesthesia. Anesth Analg 1966; 45:467-73
McKay AC: Synergism among i.v. anaesthetics. Br J Anaesth 1991; 67:1-3
Tallarida RJ, Porreca F, Cowan A: Statistical analysis of drug-drug and site-site interactions with isobolograms. Life Sci 1989; 45:947-61
Plummer JL, Short TG: Statistical modeling of the effects of drug combinations. J Pharmacol Methods 1990; 23:297-309
Hardman JG, Limbird LE: Goodman & Gilman's The Pharmacological Basis of Therapeutics, 9th ed. New York, McGraw-Hill, 1996, pp 30-41
Ariens EJ, Simonis AM, Van Rossum JM: Drug-receptor interaction: Interaction of one or more drugs with one receptor system, Molecular Pharmacology, vol. 1. Edited by EJ Ariens. New York, Academic Press, 1964, p 171
Noda K, Feng Y-H, Liu X-p, Saad Y, Husain A, Karnik SS: The active state of the AT1angiotensin receptor is generated by angiotensin II induction. Biochemistry 1996; 35:16435-42
Lefkowitz RJ: Turned on to ill effect. Nature 1993; 365:603-4
Fisher DG, Schwartz PH, Davis AL: Pharmacokinetics of exogenous epinephrine in critically ill children. Crit Care Med 1993; 21:111-7
Berg RA, Donnerstein RL, Padbury JF: Dobutamine infusions in stable, critically ill children: Pharmacokinetics and hemodynamic actions. Crit Care Med 1993; 21:678-86
Klem C, Dasta JF, Reilley TE, Flancbaum LJ: Variability in dobutamine pharmacokinetics in unstable critically ill surgical patients. Crit Care Med 1994; 22:1926-32
Zaloga GP, Strickland RA, Butterworth JF IV, Marks IJ, Mills SA, Lake CR: Calcium attenuates epinephrine's [Greek small letter beta]-adrenergic effects in postoperative heart surgery patients. Circulation 1990; 81:196-200
Schwinn DA, Leone BJ, Spahn DR, Chestnut LC, Page SO, McRae RL, Liggett SB: Desensitization of myocardial [Greek small letter beta]-adrenergic receptors during cardiopulmonary bypass. Evidence for early uncoupling and late downregulation. Circulation 1991; 84:2559-67
Smiley RM, Pantuck CB, Chadburn A, Knowles DM: Down-regulation and desensitization of the [Greek small lettr beta]-adrenergic receptor system of human lymphocytes after cardiac surgery. Anesth Analg 1993; 77:653-61
Brodde OE, Wang XL: [Greek small letter beta]-Adrenoceptor changes in blood lymphocytes and altered drug responsiveness. Ann Clin Res 1988; 20:311-23
Johnson JA: Racial differences in lymphocyte beta-receptor sensitivity to propranolol. Life Sci 1993; 53:297-304
Brodde OE, Beckeringh JJ, Michel MC: Human heart [Greek small letter beta]-adrenoreceptors: A fair comparison with lymphocyte [Greek small letter beta]-adrenoreceptors? Trends Pharmacol Sci 1987; 8:403-7
Singh M, Notterman DA, Metakis L: Tumor necrosis factor produces homologous desensitization of lymphocyte beta 2-adrenergic responses. Circ Shock 1993; 39:275-8
Butterworth JF IV, Brownlow RC, Leith JP, Prielipp RC, Cole LR: Bupivacaine inhibits cyclic-3',5'-adenosine monophosphate production. A possible contributing factor to cardiovascular toxicity. Anesthesiology 1993; 79:88-95
Zaloga GP, Washburn D, Black KW, Prielipp RC: Human sepsis increases lymphocyte intracellular calcium. Crit Care Med 1993; 21:196-202
Summers RJ, Molnaar P, Russell F, Elnatan J, Jones CR, Buxton BF, Chang V, Hambley J: Coexistence and localization of [Greek small letter beta]1and [Greek small letter beta]2adrenoceptors in the human heart. Eur Heart J 1989; 10(suppl B):11-21
Kaumann AJ, Hall JA, Murray KJ, Wells FC, Brown MJ: A comparison of the effects of adrenaline and noradrenaline on human heart: the role of [Greek small letter beta]1-and [Greek small letter beta]2-adrenoreceptorsin the stimulation of adenylate cyclase and contractile force. Eur Heart J 1989; 10(suppl B):29-37
Fowler MB, Laser JA, Hopkins GL, Minobe W, Bristow MR: Assessment of the beta-adrenergic receptor pathway in the intact failing human heart: Progressive receptor down-regulation and subsensitivity to agonist response. Circulation 1986; 74:1290-302
Marty J, Nimier M, Rocchiccioli C, Mantz J, Luscombe F, Henzel D, Loiseau A, Desmonts JM: [Greek small letter beta]-adrenergic receptor function is acutely altered in surgical patients. Anesth Analg 1990; 71:1-8
Smiley RM, Pantuck CB, Morelli JJ, Chadburn A, Knowles DM: Alterations of the [Greek small letter beta]-adrenergic receptor system after thoracic and abdominal surgery. Anesth Analg 1994; 79:821-8
Strasser RH, Marquetant R, Kubler W: Adrenergic receptors and sensitization of adenylyl cyclase in acute myocardial ischemia. Circulation 1990; 82(suppl II):II-23-9
Goldfarb RD: Cardiac mechanical performance in circulatory shock: A critical review of methods and results. Circ Shock 1982; 9:633-53
Sibbald WJ: Myocardial function in the critical ill: Factors influencing left and right ventricular performance in patients with sepsis and trauma. Surg Clin North Am 1985; 65:867-93
Shepherd RE, McDonough KH, Burns AH: Mechanism of cardiac dysfunction in hearts from endotoxin-treated rats. Circ Shock 1986; 19:371-84
Romanosky AJ, Giaimo ME, Shepherd RE, Burns AH: The effect of in vivo endotoxin on myocardial function in vitro. Circ Shock 1986; 19:1-12
Liggett SB: Desensitization of the [Greek small letter beta]-adrenergic receptor: Distinct molecular determinants of phosphorylation by specific kinases. Pharmacol Res 1991; 24:29-41
Figure 1. Dose-response curves for epinephrine and dobutamine. Cyclic adenosine monophosphate (cAMP) production was measured in duplicate for each sample from 10 healthy human volunteers. Epinephrine is about 10 times more potent (median effective concentration, 10-7.335 M) compared with dobutamine (median effective concentration, 10-6.124 M) at the [Greek small letter beta]-adrenergic receptor (primarily [Greek small letter beta](2) receptors) in this human lymphocyte model.
Figure 1. Dose-response curves for epinephrine and dobutamine. Cyclic adenosine monophosphate (cAMP) production was measured in duplicate for each sample from 10 healthy human volunteers. Epinephrine is about 10 times more potent (median effective concentration, 10-7.335 M) compared with dobutamine (median effective concentration, 10-6.124 M) at the [Greek small letter beta]-adrenergic receptor (primarily [Greek small letter beta](2) receptors) in this human lymphocyte model.
Figure 1. Dose-response curves for epinephrine and dobutamine. Cyclic adenosine monophosphate (cAMP) production was measured in duplicate for each sample from 10 healthy human volunteers. Epinephrine is about 10 times more potent (median effective concentration, 10-7.335 M) compared with dobutamine (median effective concentration, 10-6.124 M) at the [Greek small letter beta]-adrenergic receptor (primarily [Greek small letter beta](2) receptors) in this human lymphocyte model.
×
Figure 2. Dose-response curves of cyclic adenosine monophosphate (cAMP) production stimulated by various concentrations of epinephrine combined with dobutamine. For 10-6M epinephrine, cAMP production decreased significantly when dobutamine was increased from 10-6M to 10-5M and from 10-5M to 10-4M (*P = 0.001). Similarly, an epinephrine concentration of 10-7M was significantly inhibited by dobutamine concentrations from 10-6M to 10-5M (P = 0.02). Production of cAMP did not decrease significantly when epinephrine was mixed with more dilute concentrations of dobutamine (i.e., dobutamine = 10-8M to 10-6M). Data were plotted using the method of Ariens et al. [18] 
Figure 2. Dose-response curves of cyclic adenosine monophosphate (cAMP) production stimulated by various concentrations of epinephrine combined with dobutamine. For 10-6M epinephrine, cAMP production decreased significantly when dobutamine was increased from 10-6M to 10-5M and from 10-5M to 10-4M (*P = 0.001). Similarly, an epinephrine concentration of 10-7M was significantly inhibited by dobutamine concentrations from 10-6M to 10-5M (P = 0.02). Production of cAMP did not decrease significantly when epinephrine was mixed with more dilute concentrations of dobutamine (i.e., dobutamine = 10-8M to 10-6M). Data were plotted using the method of Ariens et al. [18]
Figure 2. Dose-response curves of cyclic adenosine monophosphate (cAMP) production stimulated by various concentrations of epinephrine combined with dobutamine. For 10-6M epinephrine, cAMP production decreased significantly when dobutamine was increased from 10-6M to 10-5M and from 10-5M to 10-4M (*P = 0.001). Similarly, an epinephrine concentration of 10-7M was significantly inhibited by dobutamine concentrations from 10-6M to 10-5M (P = 0.02). Production of cAMP did not decrease significantly when epinephrine was mixed with more dilute concentrations of dobutamine (i.e., dobutamine = 10-8M to 10-6M). Data were plotted using the method of Ariens et al. [18] 
×
Figure 3. Heart rate responses for dobutamine and epinephrine used alone, and their various combinations, in eight patients having aortocoronary bypass surgery who received dobutamine at doses of 0, 1.25, 2.5, and 5 [micro sign]g [middle dot] kg-1[middle dot] min-1, and epinephrine at doses of 0, 10, 20, and 40 ng [middle dot] kg-1[middle dot] min-1. See “Results” for a discussion of significant changes and Table 1for a summary of the drug administration protocol. Data points are slightly offset along the abscissa to eliminate overlap of the symbols in Figure 3and Figure 4.
Figure 3. Heart rate responses for dobutamine and epinephrine used alone, and their various combinations, in eight patients having aortocoronary bypass surgery who received dobutamine at doses of 0, 1.25, 2.5, and 5 [micro sign]g [middle dot] kg-1[middle dot] min-1, and epinephrine at doses of 0, 10, 20, and 40 ng [middle dot] kg-1[middle dot] min-1. See “Results” for a discussion of significant changes and Table 1for a summary of the drug administration protocol. Data points are slightly offset along the abscissa to eliminate overlap of the symbols in Figure 3and Figure 4.
Figure 3. Heart rate responses for dobutamine and epinephrine used alone, and their various combinations, in eight patients having aortocoronary bypass surgery who received dobutamine at doses of 0, 1.25, 2.5, and 5 [micro sign]g [middle dot] kg-1[middle dot] min-1, and epinephrine at doses of 0, 10, 20, and 40 ng [middle dot] kg-1[middle dot] min-1. See “Results” for a discussion of significant changes and Table 1for a summary of the drug administration protocol. Data points are slightly offset along the abscissa to eliminate overlap of the symbols in Figure 3and Figure 4.
×
Figure 4. Cardiac index responses for both drugs alone and for their various combinations. The cardiac index response of eight patients having aortocoronary bypass surgery who received dobutamine at doses of 0, 1.25, 2.5, and 5 [micro sign]g [middle dot] kg-1[middle dot] min-1, and epinephrine at doses of 0, 10, 20, and 40 ng [middle dot] kg-1[middle dot] min-1. See “Results” for a discussion of significant changes.
Figure 4. Cardiac index responses for both drugs alone and for their various combinations. The cardiac index response of eight patients having aortocoronary bypass surgery who received dobutamine at doses of 0, 1.25, 2.5, and 5 [micro sign]g [middle dot] kg-1[middle dot] min-1, and epinephrine at doses of 0, 10, 20, and 40 ng [middle dot] kg-1[middle dot] min-1. See “Results” for a discussion of significant changes.
Figure 4. Cardiac index responses for both drugs alone and for their various combinations. The cardiac index response of eight patients having aortocoronary bypass surgery who received dobutamine at doses of 0, 1.25, 2.5, and 5 [micro sign]g [middle dot] kg-1[middle dot] min-1, and epinephrine at doses of 0, 10, 20, and 40 ng [middle dot] kg-1[middle dot] min-1. See “Results” for a discussion of significant changes.
×
Figure 5. An isobolographic analysis of the cardiotonic effects of dobutamine and epinephrine. On the horizontal and vertical axes of this isobologram, the dose of dobutamine and epinephrine that would result in a 10% increase in the cardiac index (CI) is plotted. A line of additivity is drawn between these two points. It would be expected that any combination of drugs that achieved a 10% increase in CI would result in doses of drugs from along this line. If the 10% increase in CI results from drug dose combinations below this line (smaller doses than predicted), then drug synergism is present. If the drug doses are above this line (larger doses than predicted), then drug antagonism is present. Visual inspection reveals that the observed data points fall above and to the right of the line of additivity, suggesting clinical antagonism between dobutamine and epinephrine.
Figure 5. An isobolographic analysis of the cardiotonic effects of dobutamine and epinephrine. On the horizontal and vertical axes of this isobologram, the dose of dobutamine and epinephrine that would result in a 10% increase in the cardiac index (CI) is plotted. A line of additivity is drawn between these two points. It would be expected that any combination of drugs that achieved a 10% increase in CI would result in doses of drugs from along this line. If the 10% increase in CI results from drug dose combinations below this line (smaller doses than predicted), then drug synergism is present. If the drug doses are above this line (larger doses than predicted), then drug antagonism is present. Visual inspection reveals that the observed data points fall above and to the right of the line of additivity, suggesting clinical antagonism between dobutamine and epinephrine.
Figure 5. An isobolographic analysis of the cardiotonic effects of dobutamine and epinephrine. On the horizontal and vertical axes of this isobologram, the dose of dobutamine and epinephrine that would result in a 10% increase in the cardiac index (CI) is plotted. A line of additivity is drawn between these two points. It would be expected that any combination of drugs that achieved a 10% increase in CI would result in doses of drugs from along this line. If the 10% increase in CI results from drug dose combinations below this line (smaller doses than predicted), then drug synergism is present. If the drug doses are above this line (larger doses than predicted), then drug antagonism is present. Visual inspection reveals that the observed data points fall above and to the right of the line of additivity, suggesting clinical antagonism between dobutamine and epinephrine.
×
Figure 6. A hypothetical [Greek small letter beta]-receptor exists in equilibrium between the R*(active, productive) and the R (inactive, nonproductive) forms, representing two conformations of the [Greek small letter beta]-receptor-G-protein-coupled-adenylyl cyclase enzyme system. The extent to which the equilibrium is perturbed depends on the relative affinity of a drug for the R* or the R conformations. A full agonist is sufficiently selective that it will drive the receptor completely to the active state (R*). A partial agonist is structurally analogous but exhibits only slightly greater affinity for R* than for R, meaning that even at saturating concentrations, the magnitude of the observed effect will be less. An antagonist binds with equal affinity to both conformations and does not alter the activation equilibrium while occupying receptor sites.
Figure 6. A hypothetical [Greek small letter beta]-receptor exists in equilibrium between the R*(active, productive) and the R (inactive, nonproductive) forms, representing two conformations of the [Greek small letter beta]-receptor-G-protein-coupled-adenylyl cyclase enzyme system. The extent to which the equilibrium is perturbed depends on the relative affinity of a drug for the R* or the R conformations. A full agonist is sufficiently selective that it will drive the receptor completely to the active state (R*). A partial agonist is structurally analogous but exhibits only slightly greater affinity for R* than for R, meaning that even at saturating concentrations, the magnitude of the observed effect will be less. An antagonist binds with equal affinity to both conformations and does not alter the activation equilibrium while occupying receptor sites.
Figure 6. A hypothetical [Greek small letter beta]-receptor exists in equilibrium between the R*(active, productive) and the R (inactive, nonproductive) forms, representing two conformations of the [Greek small letter beta]-receptor-G-protein-coupled-adenylyl cyclase enzyme system. The extent to which the equilibrium is perturbed depends on the relative affinity of a drug for the R* or the R conformations. A full agonist is sufficiently selective that it will drive the receptor completely to the active state (R*). A partial agonist is structurally analogous but exhibits only slightly greater affinity for R* than for R, meaning that even at saturating concentrations, the magnitude of the observed effect will be less. An antagonist binds with equal affinity to both conformations and does not alter the activation equilibrium while occupying receptor sites.
×
Table 1. Clinical Study Design Performed in Each Cardiac Surgery Patient 
Image not available
Table 1. Clinical Study Design Performed in Each Cardiac Surgery Patient 
×