Cytosolic pH during a rest-to-work transition in red muscle: application of enzyme equilibria.

Muscles sampled from a vascularly isolated autoperfused dog gracilis by fast freezing techniques at 5, 10, 15, 30, 60, and 180 s after the initiation of twitch contractions at 4 Hz were analyzed for phosphocreatine, creatine, ATP, lactate, pyruvate, 3-phosphoglycerate, and dihydroxyacetonephosphate contents. Metabolite concentrations were used with equilibrium constants of triosephosphate isomerase, glyceraldehyde-3-phosphate dehydrogenase, phosphoglycerate kinase, lactate dehydrogenase, and creatine kinase to estimate cytosolic pH changes during the rest-to-work transition. Magnesium and hydrogen binding were taken into account. Limits to this approach include errors in the intermediate measurements and uncertainties in values of the equilibrium constants. The former leads to maximum errors of +/- 0.15 pH units, whereas the latter affects the absolute pH value but not estimates of the changes in pH. The estimated pH increases from a resting value of 7.05 to approximately 7.8 by 5 s of stimulation and then falls to a pH value of approximately 6.5 after 3 min of stimulation. The results are consistent with previous studies but permit identification of a larger early alkaline shift. Potential causes for the pH changes are discussed.