Activity of phosphorylcreatine shuttle enzymes in rat cardiac, fast-, and slow-twitch skeletal muscles

Energy producing and energy utilizing reactions in muscle are coherently linked by a phosphorylcreatine shuttle mechanism. Operational components of the shuttle were evaluated in muscles encompassing a wide metabolic and contractile spectrum. Consistent CPK/ATPase ratios suggest homogeneous function of the myofibrillar component of the shuttle. Mitochondrial CPK/ATPase ratios vary due to 4-fold differences in CPK activity with respect to oxidative phosphorylation. The phosphorylcreatine shuttle mechanism, and the mitochondrial component in particular, seems a relevant, physiological regulator of skeletal and cardiac muscle function.