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

In this study, we used 31P NMR to investigate the relationship between cardiac workload 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 T1 relaxation times for creatine phosphate and ATP which are longer than most reported values. Analysis of the T1 data indicates that chemical-shift anisotropy is a plausible mechanism for a portion of the spin-lattice relaxation rate at high field strengths.