Force and ATPase rate in skinned skeletal muscle fibers

Measurements of ATPase, force, and their ratio in chemically and mechanically skinned segments of muscle fibers provide a measure of steady-state kinetics of actomyosin interactions under force-generating conditions and thereby provide unique chemomechanical information. Preparations in which nonactomyosin ATPases are inhibited are available. The ratio of steady-state ATPase to steady-state isometric force (tension cost) was not altered significantly at different degrees of activation of contraction by Ca2+ or by shrinking the fiber lattice progressively to reduce force to zero. A fivefold decrease in tension cost was measured at low MgATP concentrations; data indicate either a decreased cross-bridge cycling time or an increased force per cross-bridge.