HETEROGENEITY OF SMOOTH-MUSCLE ALPHA-ADRENOCEPTORS IN RAT TAIL ARTERY INVITRO

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  • 1 January 1984
    • journal article
    • research article
    • Vol. 229  (3) , 823-830
Abstract
Vascular smooth muscle .alpha.-adrenoceptors in the proximal end of the rat isolated tail artery were classified by determining pA2 [competitive antagonistic activity] values and -log KB values for the antagonists prazosin, corynanthine and idazoxan (RX 781094, a new synthetic .alpha.2-adrenoceptor antagonist) against norepinephrine. The effects of the antagonists on responses to intramural sympathetic nerve stimulation were also assessed. Artery segments were perfused and superfused with Krebs'' solution containing cocaine (4 .mu.M) and propranolol (1 .mu.M). Maximum responses to KCl, norepinephrine and nerve stimulation were not significantly different from one another. Corynanthine (0.1-100 .mu.M), prazosin (10 nM-1 .mu.M) and idazoxan (1-100 .mu.M) caused competitive antagonism of norepinephrine responses with pA2 values consistent with the presence of .alpha.1 adrenoceptors. Idazoxan (10-100 nM) also caused parallel shifts in the concentration-response curves to norepinephrine with -log KB values higher than those consistent with the presence of .alpha.1-adrenoceptors. A predominance of smooth muscle .alpha.1-adrenoceptors in addition to a subpopulation of smooth muscle .alpha.2-adrenoceptors which also contribute to vasoconstrictor responses to norepinephrine, is suggested. In contrast to the results obtained with exogenous norepinephrine, responses to electrical stimulation were exquisitely sensitive to blockade by prazosin but resistant to idazoxan, suggesting an involvement of an .alpha.1-adrenoceptor in these responses. Idazoxan may be used to distinguish .alpha.1- and .alpha.2-adrenoceptors on vascular smooth muscle in vitro and the results favor the existence of .alpha.1- and .alpha.2-adrenoceptors in terms of the existing subclassification scheme for .alpha.-adrenoceptor subtypes. Exogenous norepinephrine activates both .alpha.1- and .alpha.2-adrenoceptors in the vascular smooth muscle of the rat tail artery whereas the transmitter released by sympathetic nerves stimulates only postjunctional .alpha.1-adrenoceptors.

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