The effects of alpha-adrenoceptor agonists, noradrenaline, phenylephrine and clonidine, and antagonists, coryn;anthine yohimbine, on the postsynaptic alpha receptors were examined in the dog basilar, mesenteric and renal arteries and the following results were obtained:
In the basilar artery, noradrenaline, phenylephrine and clonidine produced almost the same maximal contraction. Corynanthine (10-6 M) had no effect on contractile responses induced by these agonists. Yohimbine (1-3x10-´ M) inhibited the contractile responses to noradrenaline, phenylephrine and clonidine. In the mesenteric and renal arteries, the maximal responses to noradrenaline and phenylephrine were markedly greater than those to clonidine. Yohimbine (10-´-i0-5 M) and corynanthine (10-´-10-5 M) both antagonized noradrenaline in these vessels. When the contraction induced by noradrenaline (10-4 M) were compared with those induced by KC1(6x10-2 M) and serotonin (10-5 M) in the three kinds of arteries, the maximum response to noradrenaline in the basilar artery was much smaller than were produced by KCl or serotonin. In the mesenteric and renal arteries, however, noradrenaline produced contractions larger than those of KCl or serotonin.
These results indicate that alpha receptors of dog basilar artery are largely alpha2 type or receptors which differ from classical postsynaptic alpha receptors.
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