CONTRACTILE ACTIVATION AND FORCE GENERATION IN SKINNED RABBIT MUSCLE-FIBERS - EFFECTS OF HYDROSTATIC-PRESSURE

1. Effects of hydrostatic pressure (range 0.1-10 MPa) on the isometric tension of skinned (rabbit psoas) muscle fibres were examined at 12 degrees C and at different levels of Ca2+ activation (pCa range 4-7); the effects on both the steady tension and the tension transients induced by rapid pressure release (< 1 ms) are described. 2. The steady tension was depressed by increased pressure (similar to 1% MPa(-1)) at a high level of Ca2+ activation (pCa similar to 4) whereas it was potentiated at lower Ca2+ levels (pCa > 6); the effects were reversible. 3. At maximal Ca2+ activation, the tension recovery following pressure release (10 MPa to atmospheric) consisted of a fast (similar to 30 s(-1)) and a slow (2-3 s(-1)) phase; the rate and the normalized amplitude (normalized to the steady tension at atmospheric pressure for a particular pCa) of the fast phase were invariant with changes in Ca2+ level. 4. The effects of changing: Ca2+ level on the slow phase were complex; its positive amplitude at high Ca2+ levels changed to negative and the rate decreased to similar to 1 s(-1) at low Ca2+ levels (pCa > 6.0). 5. Results are discussed in relation to previous studies on the effect of pressure on intact muscle fibres and the actin-myosin interaction. This work supports calcium regulation of cross-bridge recruitment rather than calcium regulation of the rate of a specific step in the cross-bridge cycle.