Phosphocreatine kinetics at the onset of contractions in skeletal muscle of MM creatine kinase knockout mice
Phosphocreatine (PCr) depletion during isometric twitch stimulation at 5 Hz was measured by (31)P-NMR spectroscopy in gastrocnemius muscles of pentobarbital-anesthetized MM creatine kinase knockout (MMKO) vs. wild-type C57B (WT) mice. PCr depletion after 2 s of stimulation, estimated from the difference between spectra gated to times 200 ms and 140 s after 2-s bursts of contractions, was 2.2 +/- 0.6% of initial PCr in MMKO muscle vs. 9.7 +/- 1.6% in WT muscles (mean +/- SE, n = 7, P < 0.001). Initial PCr/ATP ratio and intracellular pH were not significantly different between groups, and there was no detectable change in intracellular pH or ATP in either group after 2 s. The initial difference in net PCr depletion was maintained during the first minute of continuous 5-Hz stimulation. However, there was no significant difference in the quasi-steady-state PCr level approached after 80 s (MMKO 36.1 +/- 3.5 vs. WT 35.5 +/- 4.4% of initial PCr; n = 5-6). A kinetic model of ATPase, creatine kinase, and adenylate kinase fluxes during stimulation was consistent with the observed PCr depletion in MMKO muscle after 2 s only if ADP-stimulated oxidative phosphorylation was included in the model. Taken together, the results suggest that cytoplasmic ADP more rapidly increases and oxidative phosphorylation is more rapidly activated at the onset of contractions in MMKO compared with WT muscles.