Differential influences of several .ALPHA.-adrenoceptor antagonists on .ALPHA.-adrenoceptor-mediated inotropic responses in the left atria of guinea pigs.

Abstract

Abstract-Influencesof several a-adrenoceptor antagonists on the triphasic time response curves of phenylephrine, which are characterized by an initial increasing phase followed by a declining phase and then a second increasing phase, were investigated in the guinea-pig left atria pretreated with sotalol (5X10-5 M) and driven at 0.2 Hz.

Effect of E-643 on .ALPHA.-adrenoceptor-mediated inotropic responses to phenylephrine in the left atria of guinea pigs.

“…The PIE of Phe was abolished by 10-6 M E-643, but a reduction in the developed tension still occurred gradually 5 min after the adminis tration of Phe. These observations are in accordance with our previous results with prazosin (2).…”

“…We previously reported that stimulation of a-adrenoceptors produced a triphasic time response-curve composed of an initial positive phase followed by a negative phase and then a second positive inotropic phase in the left atria of guinea pigs (1); and the positive phase was preferentially inhibited by prazosin, whereas the negative one was inhibited by yohimbine and phentolamine (2). In view of the reported selectivity of these antagonists on a, and a2-adrenoceptors (3)(4)(5), these results may indicate that the positive inotropic effect (PIE) and the negative one induced by stimulation of a-adrenoceptors are mediated by a, and a2-adrenoceptors, respectively.…”