Fluorometric and myothermic studies of cardiac energetics.
Energy production during isometric and isotonic contractions of cardiac muscle has been studied using the isolated papillary muscle of the rabbit at room temperature. Muscles were mounted on thermopiles to record heat production associated with small numbers of contractions and then were mounted in an organ bath for fluorometric recording of the level of reduction of mitochondrial NAD. After each twitch there was a decrease in the level of fluorescence followed by an exponential return to the resting level, or further decreases after subsequent twitches. The fluorescence-time integral (integral delta Fl-dt), or area enclosed by a fluorometric transient during and after a series of twitches, was found to vary in magnitude in a manner similar to the magnitude of the total heat production when the mechanical conditions of contraction were altered by varying the load. The influence of the metabolic state of the muscle was also studied by using either glucose or sodium pyruvate as the exogenous substrates in the bathing medium. When pyruvate was substituted for glucose, maximum external work, maximum isometric force, and maximum mechanical efficiency were all increased. The kinetics of oxidative recovery metabolism, either measured fluorometrically or derived from the heat production, was accelerated when pyruvate replaced glucose.