1) The effects of various alpha-adrenoceptor antagonists on the pressor responses to norepinephrine and phenylephrine were examined in anesthetized rabbits in an attempt to determine whether alpha1 and alpha2-adrenoceptors are located on vascular smooth muscle. 2) yohimbine and piperoxan caused a much greater reduction in the pressor responses(rise of 30~50mmHg) to norepinephrine than to phenylephrine, where as labetalol, thymoxamine, phentolamine and prazosin showed a much greater selectively in reducing the pressor responses to phnylephrine than to norepinephrine. 3) The prssor action of small dose of norepinephrine(rise of 10~20mmHg) was not significantly inhibited by the doses of labetalol, thymoxamine and prazosin which caused marked reduction of the above phenylephrine pressor responses, but yohimbine, piperoxan and phentolamine weakend the action significantly. 4) The results suggest that there are two types, alpha 1 and alpha 2, of postsynaptic alpha-adrenoceptors in the vasculature of rabbits. It seems that phenylephrine produces pressor responses by acting alpha 1-type adrenoreceptors: small doses of norepinephrine by acting on alpha 2-type: large doses of norepinephrine by acting on both types.
Labetalol ; Moxisylyte ; Muscle, Smooth, Vascular ; Norepinephrine* ; Phentolamine ; Phenylephrine* ; Piperoxan ; Prazosin ; Rabbits ; Yohimbine