Agonist action of yohimbine on the perfused rabbit central ear artery.

The agonist action of yohimbine, an alpha-adrenoceptor antagonist, was studied on the perfused ear artery of the rabbit. Yohimbine, 5-hydroxytryptamine (5-HT) and noradrenaline produced concentration-dependent vasoconstriction of the central ear artery and gave EC50 values of 3.5 X 10(-7), 7.5 X 10(-9) and 3.0 X 10(-10) M, respectively. Phentolamine significantly reduced the effects of yohimbine, 5-HT and noradrenaline, and gave pA2 values of 8.40 (slope = 1.0), 8.60 (slope = 0.94) and 8.20 (slope = 1.04), respectively. Methysergide, a serotonergic antagonist, significantly reduced yohimbine- and 5-HT-induced vasoconstriction but failed to influence the effect of noradrenaline. Reserpine treatment induced hypersensitivity of the ear artery to yohimbine and noradrenaline but not to 5-HT. It is suggested that the agonist action of yohimbine on the ear artery may be mediated via activation of alpha-noradrenoceptors.