STRAIN-DEPENDENT CROSS-BRIDGE CYCLE FOR MUSCLE .2. STEADY-STATE BEHAVIOR

Quantitative predictions of steady-state muscle properties from the st rain-dependent cross-bridge for muscle are presented. With a stiffness of 5.4 x 10(-4) N/m per head, a throw distance of II nm, and three al lowed actin sites/head, isometric properties and their dependence on p hosphate and nucleotide levels are well described if the tension-gener ating step occurs before phosphate release. At very low ATP levels, ri gorlike states with negative strain are predicted. The rate-limiting s tep for cycling and ATP consumption is strain-blocked ADP release for isometric and slowly shortening muscle. Under rapid shortening, ATP hy drolysis on detached heads is the rate-limiting step, and the ratio of bound ATP to bound ADP.Pi increases by a factor of 7. At large positi ve strains, bound heads must be forcibly detached from actin to accoun t for tension in rapid extension, but forced detachment in shortening has no effect without destroying isometric attached states. Strain-blo cked phosphate release as proposed produces modest inhibition of the A TPase rate under rapid shortening, sufficient to give a maximum for on e actin site per helix turn. Alternative cross-bridge models are discu ssed in the light of these predictions.