Spontaneous and ouabain-induced efflux of catecholamines and dihydroxyphenylglycol in two canine blood vessels

Abstract

The spontaneous efflux of endogenous noradrenaline, dopamine, dihydroxyphenylglycol (DOPEG) from adrenergic nerve endings of 2 canine blood vessels (the mesenteric artery and the saphenous vein) were studied during 8 successive incubation periods of 15 min each. Extraneuronal uptake and O-methylation were minimized by the presence of adequate concentrations of tropolone and hydrocortisone. Both vessels had an efflux characterized by a decline in the 3 catechols, which was most marked for noradrenaline; the mesenteric artery lost larger amounts than the saphenous vein. Ouabain caused a large increase in the efflux of noradrenaline and dopamine and a reduction of DOPEG efflux. Cocaine had only a modest effect, more evident in the case of the mesenteric artery, increasing noradrenaline and reducing DOPEG effluxes. The combination of ouabain and cocaine had no additive effects, and the effects of ouabain were even reduced (on some parameters) by cocaine. Accordingly, the noradrenaline:DOPEG ratio was markedly increased by ouabain, but not by cocaine; cocaine significantly reduced the effects of ouabain. The ratio dopamine:noradrenaline was decreased by cocaine and by ouabain. Comparison of tissue content and efflux allowed us to conclude that apparently no significant de novo synthesis of noradrenaline occurred during the incubation period. We conclude that a fast and early component of spontaneous efflux is due to loss from the neurons and that its greater magnitude in the mesenteric artery may be due to differences in neuronal [Na+] and/or to differences in neuronal membrane adenosine triphosphatase activity. The results also suggest that neuronal reuptake plays only a minor role in the handling of spontaneously released noradrenaline.