Cardiac transfer function relating energy metabolism to workload in different species as studied with 31P NMR

Cardiac metabolism was studied with 31P NMR in 7 dogs and 4 cats to determine whether animals adapted for different life-styles (stalk and sprint vs endurance running) respond to increased work loads (heart rate X blood pressure product) with different high-energy phosphate kinetics. Hearts were exposed via a left lateral thoracotomy under Nembutal anesthesia (40 mg/kg). Two-turned solenoid surface coils were placed on the left ventricles; pacing wires were sutured into the left ventricular apices. The femoral artery and vein were cannulated for blood pressure and arterial blood gas monitoring and fluid and drug infusion, respectively. Animals were placed in a plexiglass holder into a 2.1-T, 31-cm-bore, superconducting magnet. 31P spectra were obtained from the heart using respiratory and electrocardiogram gating. Cardiac work loads were changed by pacing the heart at 4, 4.5, and 5 Hz. Heart rate X blood pressure product "work" was correlated with Pi/PCr ratios. Dog hearts were more resistant than those of cats to changes in Pi/PCr with increasing work load. It is possible that animals adapted to different life-styles may have cardiovascular systems which are metabolically and mechanically adapted for different forms of stress. These differences may be elicited and effectively delineated using in vivo NMR techniques during various physiological interventions, such as pacing. The basis for these differences may be related to cardiac microvasculature or to intrinsic differences in enzyme kinetics. Delineation of these mechanisms may be helpful in the understanding of the physiological basis of cardiac function in health and disease.