Determination of creatine kinase kinetic parameters in rat brain by NMR magnetization transfer. Correlation with brain function.

The pseudo first-order rate constant kf of the creatine kinase (CK) forward reaction as well as the CK forward flux FCK,f have been shown to correlate better with cardiac performance than the steady-state levels of ATP and PCr (Bittl, J. A., and Ingwall, J. S. (1985) J. Biol. Chem. 260, 3512-3517). In order to elucidate the relationship between the CK kinetic parameters and brain activity, we have determined, using the non-invasive NMR technique of magnetization transfer, kf and FCK,f in rats, in which brain activity was experimentally varied by administration of either thiopental sodium or bicuculline to decrease or increase electro-encephalogram (EEG) intensity, respectively. The steady-state levels of ATP and PCr, as well as the accumulation of deoxyglucose 6-phosphate (DG-6P) in brain following intraperitoneal administration of deoxyglucose, were determined simultaneously by the NMR technique, whereas the cortical EEG was recorded in a separate experiment. The EEG intensity (range, 1-20 Hz), taken as a measure for brain performance, as well as the amount of DG-6P formed in brain, reflecting the synthesis rate of high energy phosphates (ATP and PCr), linearly correlated with kf. Despite large changes in both EEG intensity (50-250%) and kf (0.12-0.69 s-1) between thiopental sodium- and bicuculline-treated rats, the ATP levels remained constant, whereas the PCr levels decreased with high EEG activity. In contrast to the expectation based on model calculations of CK kinetics, the PCr levels did not increase above control values at reduced EEG intensity (50% of controls). At EEG intensities exceeding control values (bicuculline-treated rats) FCK,f increased as predicted by CK equilibrium. In conclusion, we have shown that in the rat brain, like in the heart, the CK forward rate constant kf, in contrast to ATP and PCr levels, is a sensitive reliable indicator of both increased and reduced function.