Crossbridge kinetics in chemically skinned rabbit psoas fibres when th actin-myosin lattice spacing is altered by dextran T-500

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Abstract
The action-myosin lattice spacing of chemically skinned rabbit psoas fibres was osmotically altered by dextran T500, and the transient kinetic response of tension arising from maximally cycling crossbridges was measured by sinuosidal length perturbations. The lattice spacing was estimated from the width of the fibres measured under a light microscope. As the dextran concentration was increased, the widths during both relaxation and Ca-activation decreased monotonically. The tension increased to a maximum at 7% dextran, and decreased again at further increases in dextran. Dynamic modulus (stiffness) increased monotonically with compression by dextran; this increase is primarily due to the elastic modulus. The rate constants slightly decreased between 0% and 4% dextran, then decreased rapidly at higher concentrations. The rate of oscillatory work output stayed approximately constant between 0% and 4% dextran, and sharply decreased at higher concentrations. apparently, two indpendent effect occur as the lattice is compressed by dextran: (1) a compensation for the spacing change through an increase in tension and a decrease in the rate constants (this takes place at low dextran concentrations); and (2) an alteration of the crossbridge kinetics by grossly decreasing both the tension and the rate constants (at high dextran concentrations). The first effect is interpreted as a decrease in the detachment rate, while the second effect is interpreted as a decrase in the rate of the ‘power stroke’ reaction.