Tissue accumulation and release of newly synthesized 3H-dopamine and 3H-noradrenaline in canine saphenous veins incubated with 3H-(--)-tyrosine

Abstract

Experiments were designed to study the release of newly synthesized catecholamines in the isolated canine saphenous vein. Unlabelled (--)-tyrosine did not affect the contractions caused by electrical stimulation or exogenous noradrenaline and did not influence the basal efflux and the stimulation-induced overflow of 3H-noradrenaline in veins, previously incubated with the radiolabelled transmitter. The precursor increased the stimulation-induced overflow of the principal intraneuronal metabolite of 3H-noradrenaline, 3H-3,4-dihydroxyphenylglycol, and augmented the concentration of endogenous dopamine. Available column chromatographic procedures were modified to measure small amounts of 3H-catecholamines in the presence of large concentrations of tritiated (--)-tyrosine. Incubation of isolated veins with 3H-(--)-tyrosine caused concentration- and time-dependent tissue accumulation of newly synthesized catecholamines; the amounts of 3H-noradrenaline and 3H-dopamine were roughly comparable and were augmented by raising the external K+ concentration. In isolated veins, first incubated with 3H-(--)-tyrosine, and then superfused, a small basal efflux of 3H-noradrenaline was detected. Electrical stimulation caused a frequency-dependent overflow of 3H-catecholamines consisting mainly of 3H-noradrenaline; a stimulation-evoked efflux of 3H-dopamine (10% of total 3H-catecholamines) could be detected only when long stimulation periods were applied. After a 70 min period of superfusion, the 3H-dopamine content of the tissues decreased while that of 3H-noradrenaline remained unchanged, irrespective of whether the tissues had been stimulated electrically or not. The present results show that the isolated canine saphenous vein can synthesize and release 3H-noradrenaline and 3H-dopamine when incubated with 3H-(--)-tyrosine.