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

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Abstract
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 lengthL o) were used to elicitf w, the frequency for maximum oscillatory work-production per cycle. The unloaded contraction velocityV 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,V 1 the velocity observed for about the first 15 ms andV 2 the velocity after this time.V 1 andV 2 were found to have different sensitivities to Ca++. The value ofV 2 fell as the level of [Ca++] was reduced, and was linearly proportional to the active tension over the range 0.2P max toP max (whereP max is the isometric tension for saturating amounts of Ca++). In contrast to thisV 1 remained insensitive to changes in [Ca++] for levels of activation corresponding to active tensions ranging fromP max to 0.6P 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 off w over the range of concentrations yielding active tensions from 0.2P max toP max. These results are discussed in terms of the kinetic processes underlying transient readjustments to perturbations from isometric equilibrium.