AGE-RELATED-CHANGES IN SWINE BRAIN CREATINE KINASE-CATALYZED P-31 EXCHANGE MEASURED IN-VIVO USING P-31 NMR MAGNETIZATION-TRANSFER

P-31 exchange rates through the creatine kinase-catalyzed interconversion of phosphocreatine and gamma-ATP were measured in a total of 27 miniature swine ranging in age from 5 days preterm to 5 weeks old. A steep increase in the forward rate constant for P-31 exchange from phosphocreatine (PCr) to gamma-ATP was observed between 2 days preterm and 3 days postterm, with a more gradual increase for older ages. In contrast, the [PCr]/[NTP] ratio measured by in vivo P-31 nuclear magnetic resonance (NMR) remained constant throughout this age interval and close to unity. Forward and reverse rate constants and the rate of flux for P-31 exchange were equal to each other for both preterm and S-week-old animals, suggesting that the creatine kinase reaction is near-equilibrium for this span of age. Multifrequency steady-state saturation of P-i and PCr compared to single-frequency saturation of PCr produced the same extent of saturation transfer to gamma-ATP, and the saturation of P-i alone had no effect on the gamma-ATP P-31 NMR signal. These results suggest that even for immature swine brain, creatine kinase activity should be adequate to buffer against changes in [ATP] when there is a mismatch between energy supply and energy demand, during conditions such as ischemia or hypoxia. The results from the present study indicate the unlikelihood that previously reported discrepancies between forward and reverse P-32 flux rates in rat brain (Shoubridge et al., FEBS Lett 140:288-292, 1982) were due to neglect of gamma-ATP to P-i exchange. If the contribution of nonadenosine triphosphate to the in vivo rat brain P-31 NMR Signal is accounted for in the calculation of reverse flux and a literature value for rat brain [PCr] is used in the calculation of forward flux, then forward and reverse flux rates are equal.