Stimulation of Norepinephrine Release By Peripheral Presynaptic β-Adrenoceptors

Increases in plasma norepinephrine (NE) concentration induced by β-agonist infusion have been taken as evidence for the existence of peripheral presynaptic β-adrenoceptors, facilitating NE release. Concomitant hemodynamic changes, however, could invoke reflex mechanisms and thus hamper the interpretation of the results. We studied the presence of peripheral presynaptic β-adrenoceptors by giving intra-arterial infusions of EPI in the forearm before and during uptake inhibition, and the effects of systemic infusions of very low-dose epinephrine (EPI) and isoproterenol (ISO) on hemodynamics and arterial and venous plasma NE concentrations. An i.a. infusion of EPI, 0.1 ng.kg−1.min−1. increased the net overflow of NE from 0.3 ± 1.0 to 5.4 ± 1.8 pmol.min−1 (M ± SEM, p < 0.05) and raised the calculated arterial plasma EPI concentration to 0.92 ± 0.22 nmol.l−1. The uptake inhibition increased the net overflow of NE to 15.4 ± 2.4 pmol.min−1 and during addition of EPI, 0.1 ng.kg−1.min−1. it further increased to 23....