Myocardial creatine kinase exchange rates and 31 P NMR relaxation rates in intact pigs

In this study, we used 31 P NMR to investigate the relationship between cardiac work load and creatine kinase flux in intact pigs. NMR measurements were performed on anesthetized miniature swine in which a surface coil was surgically implanted on the surface of the left ventricle. Cardiac workload was varied by infusion of norepinephrine. Phosphate exchange between creatine phosphate and ATP was measured by a combined saturation transfer, saturation recovery pulse sequence. Exchange measurements showed that creatine kinase flux and concentrations of PCr and ATP were independent of workload for a 2.5‐fold range of cardiac rate‐pressure products. It appears that, if creatine kinase flux is coupled to work load, the pig heart operates in a regime where small changes in metabolite concentrations or creatine kinase flux are sufficient to maintain elevated workloads. Exchange and relaxation measurements, at 2.0 and 4.7 T, yielded T 1 relaxation times for creatine phosphate and ATP which are longer than most reported values. Analysis of the T 1 data indicates that chemical‐shift anisotropy is a plausible mechanism for a portion of the spin‐lattice relaxation rate at high field strengths. © 1989 Academic Press. Inc.