Distributions of alpha 2 adrenoceptor subtypes in porcine coronary vasculature

Abstract

Alpha 2-adrenoreceptor agonists affect vascular tone of coronary vasculature by multiple mechanisms. One of the mechanisms is the direct action on endothelial {alpha}2 adrenoreceptors (mainly {alpha}2A), which induces an endothelial-dependent vasorelaxation via release of nitric oxide. We sought to investigate the subtypes of {alpha}2 adrenoceptors and their relative abundance in coronary blood vessels of different sizes (arteries vs arterioles). Result of RT-PCR clearly demonstrated that mRNA of {alpha}2A and {alpha}2B adrenoceptors was present in coronary arteries but mRNA of {alpha}2C-adrenoceptors was absent. In contrast, mRNA of {alpha}2B and {alpha}2C adrenoceptors was present in subendocardial myocardium (which contains coronary arterioles) but mRNA of {alpha}2A adrenoceptors was not. While mRNA of {alpha}2B adrenoceptors were detected in both coronary arteries and subendocardial myocardium, quantitative analysis revealed that its expression was higher in coronary arteries. Immunohistochemical studies showed that {alpha}2A and {alpha}2B adrenoceptor proteins were located mainly on the endothelial cells of coronary arteries but none was visualized in coronary arterioles. Distributions and cellular localizations of {alpha}2C adrenoceptor proteins had not yet been studied since specific antibody was not available. This preliminary data suggests that coronary blood vessels of different sizes may respond differently to pharmacological agents.