Transmitter release modulated by α-adrenoceptor antagonists in the rabbit mesenteric artery: a comparison between noradrenaline outflow and electrical activity

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Abstract
Effects of .alpha.-adrenoceptor blockers (prazosin, yohimbine, phentolamine and phenoxybenzamine) on the outflow of noradrenaline (NA) [norepinephrine] and 3,4-dihydroxyphenylglycol (DOPEG) during perivascular nerve stimulation were observed in relation to electrical events in the rabbit mesenteric artery. Cocaine or imipramine increased the NA outflow and reduced the DOPEG outflow induced by nerve stimulation. In the absence of stimulation, cocaine and imipramine did not significantly modify the NA and DOPEG outflows. The .alpha.-adrenoceptor blockers enhanced the NA and DOPEG outflow during nerve stimulation in a dose-dependent manner; the potency of the enhancement was higher for phentolamine and phenoxybenzamine than for prazosin and yohimbine. Higher concentrations (10-5 M) of yohimbine reduced the NA and DOPEG outflows induced by nerve stimulation. Prazosin increased the DOPEG outflow in the absence of stimulation, and this effect was not inhibited by pretreatment with cocaine. Guanethidine increased the NA and DOPEG outflow in the absence of stimulation, and the NA outflow was reduced during nerve stimulation. These effects of guanethidine were prevented by pretreatment with cocaine or imipramine. Perivascular nerve stimulation evoked excitatory junction potentials (e.j.p.) and with high frequency stimulation, slow depolarization and spike potentials. Application of phentolamine, phenoxybenzamine or yohimbine enhanced (prazosin had no effect) on the amplitude of the e.j.p. Spike potentials were not affected by these .alpha.-adrenoceptor blockers. Slow depolarization ceased in the presence of prazosin, phentolamine or phenoxybenzamine, and was slightly inhibited by yohimbine. Guanethidine blocked all of these electrical responses induced by perivascular nerve stimulation. Application of prazosin, phentolamine or phenoxybenzamine did not alter the resting membrane potential of the smooth muscle cells. Depolarizations of smooth muscle membrane produced by exogenously applied NA were inhibited by prazosin, phentolamine or phenoxybenzamine. Yohimbine itself depolarized the membrane and the inhibitory effects on the NA-induced depolarization were weaker. The smooth muscle membrane of the rabbit mesenteric artery evidently possesses .alpha.1-adrenoceptors. Increase in NA outflow by .alpha.-adrenoceptor antagonists during nerve stimulation was not always consistent with increase in e.j.p. amplitude, presumably due to involvement of actions other than .alpha.-adrenoceptor blockade with each of these antagonists.
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