Further studies of the effects on myocardial energy utilization elicited by nitroglycerin.

Since sympathetic stimulation could play an important part in the short term attacks of angina peetoris, the effects of nitroglycerin on the myocardial response to levarterenol were investigated. Levarterenol infusions directly increased the isometric systolic tension (1ST) developed by the cardiac muscle. Nitroglycerin had no apparent direct effect on this increase in 1ST either in the normal heart or in the heart following partial coronary ligntion. In experiments in which 1ST was measured with a lever system adjusted to the end-diastolic length and tension occurring just prior to the contraction, nitro-glycerin was found to have an indirect depressant effect on the levarterenol response. This decrease in contractility was shown to be secondary to a decrease in heinodynamic work load elicited by nitroglycerin. It is suggested that the chief effect of nitroglycerin in the relief of angina peetoris is due to this agent's ability to decrease a work load imposed on the myocardium by sympathetic stimulation. T HE effectiveness of nitroglycerin in the relief of angina peetoris has been largely interpreted in terms of the ability of this agent to dilate the coronary vessels. 1 However , Gorlin and coworkers-have reported that nitroglycerin does not change coronary vascular resistance in patients with severe heart disease. l a these patients nitroglycerin did not affect myocardial oxygen consumption or coronary flow, but the cardiac work decreased resulting in a reduction in cardiac efficiency. These and other investigators 51 suggested the possibility of an altered energy metabolism by nitroglycerin. A number of investigators have presented evidence which indicates tin increased synipatho-adrenal function during periods of angina pectoris. 4 " 7 Raab 4 suggested that nitroglycerin interfered with the metabolic anoxia-producing effects of the sympathomiinetic amines. Eckstein and co-workers 8 concluded that nitroglycerin did not alter the adrenergic-produced increase in oxygen consumption, in rate, or in estimated sys-tolic vigor of the normal dog's heart. Olson and Piatnek 9 have suggested that myocardial metabolism might be divided into 3 general phases: 1) energy liberation, 2) energy conservation , and 3) energy utilization. It is possible sympathetic stimulation may affect any one or all. 3 such phases. Further, nitro-glycerin could alter the effects of sympathetic stimulation on any of these possible metabolic phases. In a previous series of experiments nitro-glycerin was found to be without a direct effect on isometric systolic tension (1ST) measured at a fixed initial tension and fixed initial length. The hypotension elicited by …