Inhibition of norepinephrine 3H release from sympathetic nerve endings in veins by acetylcholine

Abstract

Experiments were designed to determine if the relaxation obtained with acetylcholine during electric stimulation of cutaneous veins results from inhibition of norepinephrine release. Helical strips of dog saphenous veins were incubated (4 hr) in Krebs Ringer solution which contained norepinephrine-7-3H (5 x 10-8 g/ml). The preparations were then rinsed and mounted for superfusion (3 ml/min) and isometric tension recording. The superfusate was collected at timed intervals for estimation of total radioactivity or for column chromatographic separation of norepinephrine and its metabolites. Electric stimulation increased the tension and the total radioactivity of the superfusate; it increased the amount of intact norepinephrine-3H present more than the metabolite fractions. Acetylcholine (5 x 10-8 to 5 x 10-7 g/ml) depressed the contractions and diminished the norepinephrine-3H efflux. Isoproterenol depressed the contractions but did not affect the efflux of radioactivity. Tyramine (2 x 10-6 g/ml) augmented both tension and efflux of radioactivity, but these actions were not depressed by acetylcholine. These experiments show that acetylcholine causes relaxation of venous smooth muscle constricted by sympathetic stimulation and does so by inhibiting the release of norepinephrine from the nerve endings. By contrast, acetylcholine does not inhibit the release of norepinephrine by tyramine.