Correspondence  |   May 2000
Vasoconstrictive versus  Vasodilatory Effects of Alpha-2 Adrenergic Agonists on the Spinal Microcirculation
Author Notes
  • Perioperative, and Pain Medicine
  • Department of Anesthesiology
  • Brigham and Women’s Hospital
  • Harvard Medical School
  • Boston, Massachusetts
Article Information
Correspondence   |   May 2000
Vasoconstrictive versus  Vasodilatory Effects of Alpha-2 Adrenergic Agonists on the Spinal Microcirculation
Anesthesiology 5 2000, Vol.92, 1488. doi:
Anesthesiology 5 2000, Vol.92, 1488. doi:
To the Editor:—
I read with interest the study about the effect of α-2 adrenergic agonists on the spinal pial microcirculation in dogs. 1 Using an in vivo  technique, the authors observed a significant constriction of both pial arterioles and venules after topical application of clonidine or dexmedetomidine. They suggested that α-2 agonists might be superior to epinephrine or phenylephrine as local anesthetics additives since the latter induced a greater constriction of pial microvessels and therefore may jeopardize spinal blood flow. It has been known that α-2 adrenoceptors are present on both vascular smooth muscles and endothelial cells. The stimulation of α-2 adrenoceptors on vascular smooth muscles is associated with constriction in most vascular beds, whereas activation of endothelial α-2 adrenoceptors relaxes the vascular smooth muscle through a release of endothelium-derived nitric oxide. Distribution of α-2 adrenoceptors may be heterogeneous in various vascular beds. While α-2 adrenergic stimulation induces constriction of canine pial 1 and rat intestinal arterioles, 2 vasodilation predominates in response to clonidine in the porcine coronary 3 and rat coronary 4 and uterine 5 microcirculation in which nitric oxide plays an important role in the regulation of regional blood flow. Thus, net effect of vascular α-2 adrenergic stimulation is determined by basal α-2 adrenergic activity in the vascular smooth muscle and endothelium. In addition, alterations in endothelial function and structure may affect endothelium-dependent α-2-adrenoceptor-mediated relaxation because of changes in the signal transduction of α-2 adrenergic pathway and production of nitric oxide. Previous study has shown that endothelial injury increases constriction of coronary arterioles to clonidine in septic rats. 4 An increase in relaxation response to clonidine is observed in uterine arterioles of pregnant rats, 5 likely caused by increased release of nitric oxide. However, no prior study has determined the effect of activation of endothelial α-2 adrenoceptors on vascular reactivity of the spinal microcirculation under different physiologic and pathophysiologic states (e.g.  , pregnancy and arteriosclerosis). It is plausible that changes in the endothelium-dependent α-2 adrenoceptor-mediated relaxation of pial microvessels may alter vasoconstrictive effect of α-2 agonists and even duration of spinal anesthesia. Therefore, the conclusion about effectiveness of α-2 adrenergic agonists as local anesthetics additives needs to be reached from the experiment in which the contribution of endothelial α-2 adrenoceptors is determined.
Iida H, Ohata H, Iida M, Watanabe Y, Dohi S: Direct effects of alpha-1 and alpha-2 adrenergic agonist on spinal and cerebral pial vessels in dogs. A nesthesiology 1999; 91:479–85Iida, H Ohata, H Iida, M Watanabe, Y Dohi, S
Cameron EM, Wang SY, Fink MP, Sellke FW: Mesenteric and skeletal muscle microvascular responsiveness in subacute sepsis. Shock 1998; 9:184–92Cameron, EM Wang, SY Fink, MP Sellke, FW
Wang SY, Friedman M, Johnson RG, Weintraub RM, Sellke FW: Adrenergic regulation of coronary microcirculation after extracorporeal circulation and crystalloid cardioplegia. Am J Physiol 1994; 67:H2462–H2470Wang, SY Friedman, M Johnson, RG Weintraub, RM Sellke, FW
Wang SY, Cameron EM, Fink MP, Sellke FW: Chronic septicemia alters alpha adrenergic mechanisms in the coronary circulation. J Surg Res 1997; 69:61–6Wang, SY Cameron, EM Fink, MP Sellke, FW
Wang SY, Segal S, Thomas JA, Datta S: Pregnancy alters adrenergic mechanisms in the uterine microcirculation in rats. A nesthesiology 1998(suppl): A4