Influence of Sympathetic Nerve Stimulation on Ventricular Function in the Newborn Lamb

Abstract

Cardiac responses to supramaximal electrical stimulation of postganglionic sympathetic nerve fibers were studied in 17 lambs, 10 hours to 3 days of age. In all lambs left ventricular contractility increased within 3 seconds and was unaltered by atropine, ganglionic blockade, or nerve sectioning proximal to the stimulating electrodes but was abolished by beta-receptor blockade. The responses were repeatedly demonstrated in two lambs subjected to bilateral adrenalectomy. Acidemia (pH 6.9) produced by lactic acid infusion failed to diminish the inotropic responses. Intravenous or leftatrial injections of tyramine produced chronotropic and inotropic responses comparable to sympathetic nerve stimulation. Glucagon, 50 to 200 µg/kg, failed to elicit cardiac responses in lambs from 1 to 60 days of age. It is concluded that sympathetic neural mechanisms may strongly influence myocardial contractility in the newborn lamb and that these responses are independent of adrenal medullary release of catecholamines. These findings further suggest that the lamb possesses a myocardial adenyl cyclase system that responds only to catecholamines and may be blocked with propranolol.