Induced potential model for muscular contraction mechanism, including two attached states of myosin head

The model for myosin head motion along an actin filament as proposed by Mitsui & Chiba [(1996). J. theor. Biol. 182, 147-159] is here modified so that it can explain the isometric tension and isotonic velocity transients having the same parameter values as the stationary filament sliding. The modified model differs in that a myosin head forms a complex with two actin molecules in an actin filament and has two attached states in the complex instead of three. Thus an incremental step in the myosin head motion is equal to the F-actin monomer repeat (5.46 nm). Muscle properties concerning the stationary filament sliding are calculated with new parameters in a manner similar to that of Mitsui-Chiba, with the results being qualitatively similar to theirs. In studying the transient phenomena, a quantitative expression is given for the potential energy of the myosin head in the complex, and two rate constants are applied to the kinetics of the head. The time course of tension recovery after a quick length change is determined by calculating the statistical distribution of the head in the two attached states, which conforms to experimental observations by Ford et al. [(1977). J. Physiol. 269, 441-515]. The tension variations T1/T0 and T2/T0 calculated with parameters determined from the analysis of stationary filament slidings are in fairly good agreement with the experimental data by Ford et al. The model suggests that a large fluctuation exists in the relative position between the actin and myosin filaments even when the load on a muscle is kept constant. Taking this fluctuation into account explains the characteristics of the isotonic velocity transient observed by Civan & Podolsky.