α₁-adrenoceptor subtypes and two receptor systems in vascular tissues

The subtypes of α₁-adrenoceptor are coexpressed in many tissues. We examined the relationship between coexpressed α₁-adrenoceptor subtypes and their functions in blood vessels. Rat and rabbit aortas coexpressed three subtypes (α(1A), α(1B), α(1D)) and four subtypes (α(1A), α(1B), α(1D), α(1L) respectively. In rat aorta however, noradrenaline-induced contraction was mediated predominantly through the α(1D) subtype, and oxymetazoline produced α(1B)-mediated contraction. In rabbit aorta, concentration-response curves for noradrenaline were composed of two components (α(1B) and α(1L)- mediated), while oxymetazoline produced α(1L)-mediated contraction. Therefore, the inhibitory actions of some antagonists varied markedly among tissues and agonists. These results demonstrate diversity of the two receptor systems and suggest that the heterogeneity of physiological responses reflects the differences in functional subtypes among tissues and in their sensitivities to agonists and antagonists.