Contractile action of yohimbine in the dog basilar artery

Yohimbine (10(-8) to 10(-4) M) produced a concentration-dependent contraction of the isolated dog basilar artery. The maximal amplitude of the contraction induced by yohimbine was approximately 50% of that induced by 5-hydroxytryptamine (5-HT) and twice as large as that of noradrenaline. This response to yohimbine (3 X 10(-7) M) was attenuated by 5-HT receptor antagonists such as 5-methoxygramine, methysergide and ketanserin. Phentolamine also antagonized the yohimbine response but prazosin and DG-5128 did not. The 5-HT antagonists and phentolamine produced similar inhibition of the response to 5-HT. Propranolol, atropine and diphenhydramine did not modify the contractile responses to yohimbine and 5-HT. These results suggest that the contractile response to yohimbine is mediated through 5-HT receptors. In addition to the contractile action, yohimbine at concentrations over 10(-6) M inhibited the contractile response to 5-HT. Thus, the possibility that yohimbine is a partial agonist on 5-HT receptors in the dog basilar artery has to be considered.