The direct myocardial energetic response to nitroglycerin in the rabbit heart.

The coronary resistance, total heat production, oxygen consumption and isovolumic mechanical performance were measured simultaneously in the isolated beating rabbit heart. The direct effects of nitroglycerin (0.12 mg/l) were determined. The expected coronary dilatation occurred; coronary resistance fell by 21.5% independent of mechanical performance. There was a 14% fall in mechanical performance at the apex of the Frank-Starling curve and lesser reductions at less-than-apical diastolic volumes. Oxidative metabolism was not affected; there was no change in the calorific equivalent of oxygen (20.97 mJ/mul:O2) nor was there any significant anerobic metabolism. The ergonic cost of the force-independent metabolism which includes calcium activation energy was reduced by 17.7%. The unit metabolic cost of mechanical performance, or the performance-dependent metabolism, was unchanged resulting in a net increase in overall mechanical economy. It is concluded that it is against a background of a more metabolically efficient but less mechanically active myocardium that nitroglycerin exhibits its well known peripheral and reflex in the intact organism.