Molecular Pharmacology, Vol 11, 10-18, Copyright © 1975 by the American Society for Pharmacology and Experimental Therapeutics

On the Mechanism of Release of Norepinephrine from Sympathetic Nerves Induced by Depolarizing Agents and Sympathomimetic Drugs

¹ Laboratory of Clinical Science, National Institute of Mental Health, National Institutes of Health, Bethesda, Maryland 20014

Depolarization of isolated guinea pig vasa deferentia in vitro by either hypertonic KCl or veratridine resulted in a dose-dependent, proportional release of norepinephrine and dopamine -hydroxylase. The ratio of norepinephrine to dopamine -hydroxylase activity released by depolarizing agents was similar to that obtained by electrical stimulation of the hypogastric nerve. Thus exocytosis from sympathetic nerve terminals may be elicited by depolarizing drugs as well as by electrical stimulation. Incubation of vasa deferentia in vitro in the presence of reserpine or the sympathomimetic amines tyramine, d-amphetamine, or metaraminol resulted in the dose-dependent release of norepinephrine but not of dopamine -hydroxylase. When calcium was omitted from the incubation medium, exocytotic release of norepinephrine and dopamine -hydroxylase induced by depolarizing agents was blocked whereas norepinephrine release induced by sympathomimetic agents was not affected. Tetrodotoxin blocked veratridine but not tyramine-induced release. Colchicine, but not hexamethonium or atropine, blocked exocytotic release. None of these drugs affected tyramine-induced release of norepinephrine. In vasa deferentia from untreated animals, the ratio of supernatant (S₂) to particulate (P₂) norepinephrine concentration after centrifugation at 100,000 x g for 1 hr was 0.7. In animals treated with reserpine and a monoamine oxidase inhibitor this ratio increased to 1.3, suggesting a release of norepinephrine from vesicular into intraneuronal cytoplasmic stores. The release of norepinephrine by tyramine was enhanced in vasa deferentia from reserpine- and monoamine oxidase inhibitor-treated animals and resulted in a fall in the norepinephrine content of the S₂ fraction. This suggests that tyramine may act by displacing norepinephrine from cytoplasmic stores into the synaptic space. After treatment of guinea pigs with reserpine alone or in combination with a monoamine oxidase inhibitor, electrical stimulation of the hypogastric nerve to the guinea pig vas deferens resulted in release of dopamine -hydroxylase but not of norepinephrine. This suggests that release by exocytosis may occur in the presence of a depleted amine store and that the norepinephrine released by exocytosis is derived predominantly from vesicular reserpine-sensitive stores.

Note:
ACKNOWLEDGMENT We thank Mrs. P. Cover for her competent technical assistance with the experiments.