Chronic norepinephrine infusion and adrenergic function of hypertrophied hearts

The ability of chronic subpressor norepinephrine infusion to prevent depletion of myocardial norepinephrine stores and deterioration of myocardial adrenergic nerve function in congestive heart failure was examined. Four groups of adult male Wistar rats were subjected to aortic constriction or sham aortic constriction, along with either norepinephrine infusion or infusion of vehicle alone. After 7 days of treatment, these animals were evaluated for myocardial norepinephrine content, ventricular mass, and resting hemodynamics. Chronotropic and inotropic responses to electrical stimulation of the left and right cervical sympathetic trunks and to intravenous injections of tyramine and norepinephrine were measured. Chronic norepinephrine infusion normalized myocardial norepinephrine content in aortic-constricted rats but did not significantly affect heart size or mortality. Norepinephrine infusion diminished responses to sympathetic nerve stimulation, despite apparently normal myocardial norepinephrine sensitivity. The results suggest that chronic norepinephrine infusion suppresses neurotransmission in the heart by some means unrelated to its norepinephrine store. This suppression may be compensatory and reversible.