Age-Related Changes in Swine Brain Creatine Kinase-Catalyzed31P Exchange MeasuredIn vivoUsing31P NMR Magnetization Transfer

Abstract

31P exchange rates through the creatine kinase-catalyzed interconversion of phosphocreatine and γ-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 31P exchange from phosphocreatine (PCr) to γ-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 31P 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 31P exchange were equal to each other for both preterm and 5-week-old animals, suggesting that the creatine kinase reaction is near-equilibrium for this span of age. Multifrequency steady-state saturation of Pi and PCr compared to single-frequency saturation of PCr produced the same extent of saturation transfer to γ-ATP, and the saturation of Pi alone had no effect on the γ-ATP 31P NMR signal. Th...