Release of 3H-norepinephrine from adrenergic nerves in chick heart

The metabolic disposition of 3H-norepinephrine (3H-NE) was studied in adrenergic nerves of the chick heart. Spontaneous overflow of 3H was associated with considerable metabolism of NE; about 90% of the 3H-NE appeared as acid and neutral metabolites. Overflow of 3H increased markedly during depolarization by K+ and over 70% of the 3H was unmetabolized 3H-NE. This 3H-NE seems to have been protected from intraneuronal metabolism (probably by vesicles) and to have been released by exocytosis. By contrast, tyramine-induced overflow included a large proportion of metabolites (approximately 70%) perhaps because of a nonexocytotic release mechanism similar to that in adult tissues.