Changes of myocardial high‐energy phosphates with the cardiac cycle during acute or chronic myocardial stress

Whether changes of cardiac high-energy phosphate concentrations occur over the cardiac cycle remains controversial. The hypothesis was that such cyclical changes are accentuated during acute or chronic myocardial stress. Isolated rat hearts were perfused under four conditions: (1) control, (2) inotropic stimulation by doubling of perfusate [Ca2+], (3) acute hypoxia (buffer PO2 approximately 150 torr), and (4) failing, chronically infarcted hearts. 31P-MR spectra were obtained at seven time points of the cardiac cycle. Under control conditions, cyclical changes ("cycling") of ATP (11+/-3%*, *P < 0.05) and phosphocreatine (9+/-2%*) were detected, inorganic phosphate cycling did not reach statistical significance. At high [Ca2+] perfusion, cycling of phosphocreatine (9+/-5%*) was not accentuated, cycling of ATP and inorganic phosphate did not reach significance. During acute hypoxia, cycling of ATP (10+/-4%*) and inorganic phosphate (11+/-4%*) occurred, but cyclical changes of phosphocreatine were not significant. In chronically infarcted hearts, the extent of cyclical changes of ATP, phosphocreatine, and inorganic phosphate was not accentuated. Thus, in perfused rat heart, small oscillations of high-energy phosphates during the cardiac cycle are detectable, but such changes are not accentuated during acute or chronic stress. The concentrations of high-energy phosphates over the cardiac cycle are tightly regulated.