DIFFERENTIAL BLOCKING EFFECTS OF PRAZOSIN AND YOHIMBINE ON PRESSOR RESPONSES TO NEURONALLY RELEASED AND EXOGENOUSLY ADMINISTERED NORADRENALINE IN THE PITHED RAT

The effects of prazosin and yohimbine on pressor responses to sympathetic nerve stimulation and intravenous injections of noradrenaline, phenylephrine and clonidine were examined in pithed rats to determine the postjunctional location of alpha 2-adrenoceptors in the vascular smooth muscle. Prazosin antagonized the pressor responses to phenylephrine and to sympathetic nerve stimulation more effectively than the responses to noradrenaline and to clonidine. Yohimbine antagonized the pressor responses to noradrenaline and to clonidine more effectively than the responses to sympathetic nerve stimulation and to phenylephrine. These results suggest that alpha 2-adrenoceptors as well as alpha 1-adrenoceptors produce vasoconstriction in the rat vasculature and support the hypothesis that alpha 1-adrenoceptors are predominantly located within the neuroeffector junction in contrast to an extrajunctional location of alpha 2-adrenoceptors.