CONSTITUTIVE NONEXOCYTOTIC NOREPINEPHRINE RELEASE IN SYMPATHETIC-NERVES OF IN-SITU CANINE HEART

Nonexocytotic norepinephrine bulk overflow from sympathetic nerves has been previously demonstrated in the perfused isolated heart only after inhibition of sympathetic nerve metabolism by hypoxia, ischemia, or metabolic inhibitors. The measurement, however, ignores simultaneous uptake of norepinephrine by sympathetic nerves. We quantitated simultaneous norepinephrine uptake and release in pentobarbital-anesthetized dogs by the use of a transient tracer approach, the multiple indicator dilution technique, combined with measurement of endogenous arterial and venous plasma norepinephrine levels. Sympathetic vesicles were previously depleted by reserpine, and desipramine was used to inhibit the neuronal membrane pump. Labeled albumin, sucrose, and norepinephrine were injected into the coronary artery, and sequential samples were collected from the coronary sinus. Reserpine pretreatment significantly depleted tissue and decreased plasma norepinephrine levels; tracer norepinephrine uptake increased slightly, and there was a substantial decrease in the rate of release of unlabeled norepinephrine. Desipramine then decreased tracer uptake and virtually eliminated norepinephrine release. We conclude that desipramine-suppressible, constitutive nonexocytotic norepinephrine release is present in cardiac sympathetic nerves.