A metabolic control analysis of kinetic controls in ATP free energy metabolism in contracting skeletal muscle

A system analysis of ATP free energy metabolism in skeletal muscle was made using the principles of metabolic control theory. We developed a network m odel of ATP free energy metabolism in muscle consisting of actomyosin ATPas e, sarcoplasmic reticulum (SR) Ca2+-ATPase, and mitochondria. These compone nts were sufficient to capture the major aspects of the regulation of the c ytosolic ATP-to-ADP concentration ratio (ATP/ADP) in muscle contraction and had inherent homeostatic properties regulating this free energy potential. As input for the analysis, we used ATP metabolic flux and the cytosolic AT P/ADP at steady state at six contraction frequencies between 0 and 2 Hz mea sured in human forearm flexor muscle by P-31-NMR spectroscopy. We used the mathematical formalism of metabolic control theory to analyze the distribut ion of fractional kinetic control of ATPase flux and the ATP/ADP in the net work at steady state among the components over this experimental range and an extrapolated range of stimulation frequencies (up to 10 Hz). The control analysis showed that the contractile actomyosin ATPase has dominant kineti c control of ATP flux in forearm flexor muscle over the 0- to 1.6-Hz range of contraction frequencies that resulted in steady states, as determined by P-31-NMR. However, flux control begins to shift toward mitochondria at >1 Hz. This inversion of flux control from ATP demand to ATP supply control hi erarchy progressed as the contraction frequency increased past 2 Hz and was nearly complete at 10 Hz. The functional significance of this result is th at, at steady state, ATP free energy consumption cannot outstrip the ATP fr ee energy supply. Therefore, this reduced, three-component muscle ATPase sy stem is inherently homeostatic.