ALPHA(2)-ADRENOCEPTORS IN THE GUINEA-PIG UTERUS - HETEROGENEITY IN THE CIRCULAR AND LONGITUDINAL SMOOTH-MUSCLE LAYERS

Homogenate binding and functional studies have been undertaken to investigate the role of alpha2-adrenoceptors on the circular and longitudinal myometrial layers of the dioestrous guinea-pig uterus. Each myometrial layer contained a single population of [H-3]rauwolscine binding sites (K(D) values almost-equal-to 3 nM) for which yohimbine exhibited a higher affinity than prazosin, and xylazine a higher affinity than phenylephrine, indicating the presence of alpha2-adrenoceptor binding sites. In circular myometrium, xylazine enhanced contractile responses, was more potent than either noradrenaline or phenylephrine in inhibiting forskolin-stimulated accumulation of cyclic AMP, and reduced the inhibitory effect of forskolin on phenylephrine-induced phosphatidyl inositol hydrolysis. In longitudinal myometrium xylazine enhanced contractile responses to phenylephrine but did not inhibit forskolin-stimulated accumulation of cyclic AMP. We conclude that alpha2-adrenoceptor binding sites are present in both uterine layers and mediate uterine contractility possibly through different mechanisms.