Prejunctional α-Adrenoceptor-Mediated Inhibition of Norepinephrine Release in Blood-Perfused Skeletal Muscle In Situ

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Abstract
The influence of .alpha.-adrenoceptor antagonists on the overflow of endogenous norepinephrine (NE) and vasoconstrictor responses elicited by sympathetic nerve stimulation (1-4 Hz, 2 min) was investigated in desipramine-pretreated canine blood-perfused skeletal muscle in situ. The nonselective .alpha.-adrenoceptor antagonist phentolamine enhanced stimulation-evoked NE overflow and reduced vasoconstrictor responses concentration-dependently. Similar effects were obtained with phenoxybenzamine (irreversible .alpha.-adrenoceptor antagonist). Desipramine pretreatment attenuated the enhancement of stimulation-evoked NE overflow produced by phenoxybenzamine, indicating that phenoxybenzamine also inhibits neuronal uptake. The enhancement by phenoxybenzamine was independent of the stimulation frequency, suggesting a similar engagement of prejunctional .alpha.-adrenoceptor-mediated inhibition of transmitter release over the frequency range studied here. The .alpha.2-selective adrenoceptor antagonist yohimbine enhanced nerve stimulation-evoked NE overflow at concentrations similar to those required to antagonize vasoconstrictor responses to exogenous NE; 10-fold higher concentrations were required, however, to antagonize nerve stimulation-induced vasoconstriction. The concept of a quantitatively important prejunctional .alpha.2-adrenoceptor-mediated feedback inhibition of NE release in vivo is supported by our findings in the skeletal muscle vasculature. Postjunctional .alpha.2-adrenoceptors appear to be preferentially activated by circulating catecholamines but also seem to be involved in the nervous control of vascular tone.