Experiments were done on (1) four-week-old rats, containing biochemically verified V₁ only, and (2) thyroidectomized adult rats, treated with propylthiouracil, verified to contain V₃ only. Contracture tension was induced in isolated papillary muscles either by high potassium solution or 0.5 mmol 1⁻¹ Ba²⁺. Small amplitude length perturbations with peak-to-peak value not exceeding 0.15% L₀ were applied to the activated muscle. Both the applied length perturbations and the corresponding resulting force changes were analysed by computer for dynamic stiffness and phase values. In order to reduce data acquisition time, pseudo-random binary noise length changes, rather than the conventional sinusoidal length changes, were used. The plot of the dynamic stiffness against frequency displays a minimum, akin to a resonance phenomenon. The frequency,f _min, at which this resonance occurs, reflects crossbridge kinetics. It was found that thef _min1 values for the two types of papillary muscles differed by a factor of two. Experiments were also done on chemically skinned muscles containing V₁ or V₃ isomyosin activated by different concentrations of either barium or calcium ions. It was found thatf _min values of skinned fibres were higher than those obtained from intact fibres. However, for each type of muscle thef _min was independent of the activator used as well as the level of activation. The ratio off_min for V₃ to that for V₁ remained the same as for intact preparations. We conclude that the difference in mechanical parameters did not arise from a possible difference in excitation-contraction coupling mechanism, but rather is a difference in the dynamic properties of the two types of crossbridges.

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