An examination of pre- and postsynaptic alpha-adrenoceptors in the autoperfused rabbit hindlimb

Pre- and postsynaptic alpha-adrenoceptors in autoperfused rabbit hindlimbs were examined employing the yohimbine diastereomers rauwolscine and corynanthine, as well as other drugs with varying selectivities for alpha 1-and alpha 2-adrenoceptors. Vasoconstrictor responses were elicited by sympathetic nerve stimulation and by intraarterial injection of agonists. The alpha 2-selective antagonist rauwolscine (10(-7) and 10(-6) M) reduced responses to injected noradrenaline (mixed alpha 1- and alpha 2-agonist) but not to injected phenylephrine (alpha 1-selective agonist); rauwolscine (10(-6) M) actually enhanced nerve-mediated responses. Corynanthine (alpha 1-selective antagonist) was about equipotent against agonist- and nerve-mediated responses. In another series of experiments, rauwolscine (10(-6) M) strongly antagonized the response to xylazine (alpha 2-selective agonist), antagonized the responses to alpha-methylnoradrenaline and noradrenaline (mixed agonists) less strongly, and did not affect the response to phenylephrine, Conversely, prazosin (alpha 1-selective antagonist, 10(-8) M) strongly antagonized phenylephrine, antagonized noradrenaline and alpha-methylnoradrenaline less strongly, and did not antagonize xylazine. Potentiation of neuroeffector responses by rauwolscine demonstrates the operation of an endogenous autoinhibition of noradrenaline release mediated by presynaptic alpha 2-adrenoceptors. However, in comparison with previous in vitro results on rabbit pulmonary artery, the potentiation was small due to the presence of postsynaptic alpha 2-adrenoceptors.