The role of calcium ions in the activation of rabbit psoas muscle

This study tested the effects of free Ca⁺⁺ on both the small-amplitude mechanical behaviour (dynamic stiffness and phase between 1 and 500 Hz) and the large-scale filament-sliding behaviour (V_max) of single fibres of chemically-activated glycerol-extracted rabbit psoas muscle. Small-amplitude vibrations (0.1% peak-to-peak of initial length L_o) were used to elicit f_w, the frequency for maximum oscillatory work-production per cycle. The unloaded contraction velocity V_max was measured during the same contractions, using the slack-test method with shortening length steps of up to 10%L_o. These produced nonlinear length-time plots demonstrating that the unloaded contraction velocity was not constant as contraction progressed, but fell with time. This behaviour was approximated by two velocities, V₁ the velocity observed for about the first 15 ms and V₂ the velocity after this time. V₁ and V₂ were found to have different sensitivities to Ca⁺⁺. The value of V₂ fell as the level of [Ca⁺⁺] was reduced, and was linearly proportional to the active tension over the range 0.2 P_max to P_max (where P_max is the isometric tension for saturating amounts of Ca⁺⁺). In contrast to this V₁ remained insensitive to changes in [Ca⁺⁺] for levels of activation corresponding to active tensions ranging from P_max to 0.6 P_max, and then fell as the level of activation was further reduced. It was found that the level of [Ca⁺⁺] did not affect the magnitude of f_w over the range of concentrations yielding active tensions from 0.2 P_max to P_max. These results are discussed in terms of the kinetic processes underlying transient readjustments to perturbations from isometric equilibrium.