Heterogeneity of smooth muscle alpha adrenoceptors

Vascular smooth muscle alpha adrenoceptors in the proximal end of the rat isolated tail artery have been classified by deter-mining pA2 values and-log KB values for the antagonists pra-zosin, corynanthine and idazoxan (RX 781 094, a new synthetic alpha-2 adrenoceptor antagonist) against norepinephnne. The effects of the antagonists on responses to intramural sympa-thetic nerve stimulation were also assessed. Artery segments were perfused and superfused with Krebs ’ solution containing cocaine (4 M) and propranolol (1,M). Maximum responses to KCI, norepinephnne and nerve stimulation were not significantly different from one another. Corynanthine (0.1-1 00 zM), prazosin (1 0 nM-i M) and idazoxan (1-1 00 M) caused competitive antagonism of norepinephnne responses with pA2 values con-sistent with the presence of alpha-i adrenoceptors. However, idazoxan (1 0-1 00 nM) also caused parallel shifts in the concen