Cyclical changes in high-energy phosphates during the cardiac cycle by pacing-Gated 31P nuclear magnetic resonance.

Whether cyclical changes in energy-related phosphate metabolites arise during a cardiac cycle in isolated rat hearts and are affected by differences in myosin isozyme composition was determined. Myocardial adenosine triphosphate (ATP), phosphocreatine (PCr), inorganic phosphate (Pi), and intracellular pH in normal, hypothyroid and hyperthyroid rat hearts were measured using the pacing-gated 31P nuclear magnetic resonance technique. Maximal decrease in ATP and PCr, and maximal increase in Pi at the peak-systole in normal rat hearts were observed. In hypothyroid and hyperthyroid rats, similar cyclical changes in phosphate metabolites were observed during the cycle. However, the magnitude of fluctuations was smaller in hypothyroid rats and larger in hyperthyroid rats compared with that observed in normal rats. Cardiac myosin isozyme patterns were also different amongst the experimental groups. The results suggest that cyclical changes and the magnitude of fluctuations in energy-related phosphate metabolites during a cardiac cycle may depend on the cardiac workload and the intrinsic properties in the enzyme kinetics of myosin.