Different characteristics of postjunctional alpha-adrenoceptors on arterial and venous smooth muscle

The postsynaptic alpha-adrenoceptors of rabbit ear artery, dog splenic artery and dog splenic vein were characterized in vitro by the use of the selective antagonists prazosin (alpha 1) and rauwolscine (alpha 2). In isolated segments of rabbit ear artery, prazosin was a potent antagonist of norepinephrine-induced contraction, with a receptor dissociation constant (KB) of 6.4 nM. In contrast, rauwolscine was effective only at very high concentrations (KB = 4,000 nM), suggesting that the alpha-receptor of the ear artery has conventional alpha 1-characteristics. The opposite order of potency was observed in a presynaptic alpha-receptor model, the isolated guinea pig atrium, where rauwolscine was potent (KB = 4.5 nM) and prazosin essentially inactive. Rauwolscine had an intermediate potency in the dog splenic vasculature, with a KB of 125 nM in splenic arterial segments and 5-15 nM in segments or helical strips of splenic vein. In contrast to the results with rauwolscine, prazosin was essentially equipotent in dog and rabbit tissues as an antagonist of norepinephrine-induced contraction (KB = 3-9 nM). Schild analysis showed competitive antagonism for both rauwolscine and prazosin in all tissues. These results are consistent with the presence of a homogeneous population of alpha-receptors in dog vasculature having characteristics of both the alpha 1 and alpha 2 receptor subtypes.