PREJUNCTIONAL INHIBITORY ALPHA-ADRENOCEPTORS AND DOPAMINOCEPTORS OF THE RAT VAS-DEFERENS AND THE GUINEA-PIG ILEUM INVITRO

The effects of α-adrenoceptor and dopaminoceptor agonists and antagonists were investigated on prejunctional receptors of the rat vas deferens and the guinea-pig ileum. The order of potency of the agonists for twitch inhibition of the rat vas deferens was clonidine > oxymetazoline > dopamine > apomorphine > noradrenaline whilst the order of potency for inhibiting the stimulated guinea-pig ileum was clonidine > oxymetazoline > noradrenaline > dopamine > apomorphine. Yohimbine readily blocked the inhibitory effects of clonidine, oxymetazoline and noradrenaline in both tissues but was less effective against dopamine and apomorphine. Pimozide selectively blocked the effects of dopamine and apomorphine on the rat vas deferens and was almost completely ineffective against clonidine, oxymetazoline and noradrenaline. However, pimozide significantly antagonized the noradrenaline-induced twitch inhibition of the stimulated guinea-pig ileum in addition to antagonising dopamine and apomorphine action. The pA2 values for pimozide against dopamine, apomorphine and noradrenaline in both tissues were significantly different. It is concluded that the prejunctional α-adrenoceptors of the rat vas deferens are the same as those located on the terminal cholinergic neurones of the guinea-pig ileum whilst the prejunctional dopaminoceptors in these tissues appear to differ from one another. © 1979.