PRESSURE-INDUCED CHANGES IN THE ISOMETRIC CONTRACTIONS OF SINGLE INTACT FROG-MUSCLE FIBERS AT LOW-TEMPERATURES

Effects of increased hydrostatic pressure (range 0.1-10 MPa) on isomet ric twitch and tetanic contractions of single intact muscle fibres, is olated from frog tibialis anterior muscle, were examined at 4-12 degre es C. The tension changes produced on exposure to steady high pressure s are compared with those produced on exposure to low concentrations o f caffeine (0.5 nM, subthreshold for contracture) and when pressure is rapidly released during a contraction. The peak twitch tension was po tentiated by pressure accompanied by increased rate of tension rise an d increased duration; the pressure sensitivity of twitch tension was s imilar to 8% MPa(-1). The correlation between the rate of tension rise and peak tension in caffeine-induced twitch tension potentiation was quantitatively similar to that in pressure-induced twitch potentiation . Experiments involving the rapid release of pressure (similar to 2 ms ) during twitch contractions demonstrate that high pressure need only be maintained for a brief period during the early part of tension deve lopment to elicit full twitch potentiation. The tetanic tension was de pressed by pressure (similar to 1% MPa(-1)). Results demonstrate that the major effect of increased hydrostatic pressure on intact muscle fi bres, which results in tension potentiation, is complete very early du ring contraction and is similar to that of caffeine.