On the role of actomyosin ATPases in regulation of ATP turnover rates during intense exercise.

Actomyosin ATPase is the dominant ATP sink during muscle work. Its catalytic capacities in fast-twitch oxidative glycolytic fibers have long been known to exceed by about 3-fold those of slow-twitch oxidative fibers, but the relative contributions to control of metabolic rates during exercise have never been closely examined. We compared fast-twitch oxidative glycolytic and slow-twitch oxidative fibers that displayed similar mitochondrial abundance (similar activities of mitochondrial marker enzymes). During short-term, but near maximum, aerobic exercise, fast-twitch oxidative glycolytic fibers displayed ATP turnover rates that were 2-4 times higher than for slow-twitch oxidative fibers (despite similar mitochondrial metabolic capacities), implying a large ATPase contribution to control of maximum metabolic rate. Fluxes through the ATP in equilibrium ADP + Pi cycle were extremely well regulated; at the lower limit, the forward flux exceeded the backward flux by only 0.06%, whereas at the upper limit, ATPase rates exceeded ATP synthesis rates by 0.12%. This very high precision of energy coupling could not be easily explained by standard metabolic regulation models.