Interaction between Neuronal Amine Uptake and Prejunctional Alpha-Adrenergic Receptor Activation in Smooth Muscle from Canine Blood Vessels and Spleen

Abstract

Experiments were performed to determine the conditions in which norepinephrine release from adrenergic nerve terminals in smooth muscle from canine blood vessels and spleen might be inhibited by prejunctional α-adrenergic receptor activation. Strips of aorta, mesenteric and splenic arteries, splenic capsule and portal and saphenous veins were labeled with 7-³H-norepinephrine and mounted for superfusion. In the portal vein, an inhibitory effect of prejunctional receptor activation with exogenous norepinephrine (1.2 × 10^–6 M) on transmitter efflux could be demonstrated during electrical stimulation (9 V, 2 Hz) of the nerve terminals. By contrast, in the other tissues, inhibition of transmitter release during electrical stimulation or depolarization of the nerve terminals with K⁺ (40 mEq/l) could only be demonstrated after blockade of the neuronal uptake mechanism. That activation of prejunctional α-adrenergic receptors in blood vessels inhibits the exocytotic process is suggested by the failure of exogenous norepinephrine to affect either the basal efflux of ³H-norepinephrine or the displacement of ³H-norepinephrine by tyramine.