Dual effect of alpha-adrenoceptor antagonists in rat isolated vas deferens

1 In rat isolated vas deferens, the isotonic contractile responses to low doses of noradrenaline or adrenaline were antagonized, and those to high doses were potentiated, by yohimbine, piperoxan, phentolamine and tolazoline. Effects due to intermediate doses were not affected, or were potentiated within about 30 min, following an initial inhibition. 2 The alpha-adrenoceptor blockers thus caused a shift to the right and an increase of the maximum height of log dose-response curves of alpha-adrenoceptor stimulants. For a given dose of antagonist, the onset was slower for the potentiating than for the blocking effect. 3 The shift to the right induced by piperoxan and yohimbine on dose-response curves of noradrenaline and adrenaline was analysed with the Schild plot, and the slopes obtained, around 0.3, were lower than expected from receptor theory. When cocaine was used to block neuronal uptake, the slopes were close to 1.0. 4 The increase in maximum response to noradrenaline and adrenaline induced by alpha-adrenoceptor blockers was dependent on the time of incubation, on the dose of antagonist, and on the initial height of responses to the agonist. A less pronounced potentiation was also obtained when acetylcholine was used as agonist. 5 The findings are explained in terms of receptor theory as being due to a dual effect of alpha-adrenoceptor antagonists; competitive antagonism proper, which may be disclosed after blockade of neuronal uptake, and an interaction at a different locus, which results in potentiation of the effects of noradrenaline and adrenaline.