Changes produced by increased hydrostatic pressure in isometric contractions of rat fast muscle.

1. Muscle fibre bundles isolated from the extensor digitorum longus (a fast muscle) of the rat were exposed to different hydrostatic pressures (range 0.1-10 MPa), in order to determine the pressure dependence of their isometric contractions. 2. The pressure dependent changes in the contractions were reversible and linearly related to pressure. 3. The peak tension, the time to peak and the time to half-relaxation of a twitch contraction increased with pressure; the mean (+/- S.E.M.) percentage increases were 5.9 +/- 0.5% MPa-1, 2.7 +/- 0.2% MPa-1 and 2.7 +/- 0.4% MPa-1, respectively. 4. In a fused tetanus, the tension was typically depressed at high pressure (0.9 +/- 0.16% MPa-1); the half-time of tension rise was decreased (2.1 +/- 0.2% MPa-1) and the half-time of exponential relaxation was increased (2.4 +/- 0.3% MPa-1).