Pressure-induced changes in the isometric contractions of single intact frog muscle fibres at low temperatures.

Effects of increased hydrostatic pressure (range 0.1-10 MPa) on isometric twitch and tetanic contractions of single intact muscle fibres, isolated from frog tibialis anterior muscle, were examined at 4-12 degrees C. The tension changes produced on exposure to steady high pressures are compared with those produced on exposure to low concentrations of caffeine (0.5 mM, subthreshold for contracture) and when pressure is rapidly released during a contraction. The peak twitch tension was potentiated by pressure accompanied by increased rate of tension rise and increased duration; the pressure sensitivity of twitch tension was approximately 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 (approximately 2 ms) during twitch contractions demonstrate that high pressure need only be maintained for a brief period during the early part of tension development to elicit full twitch potentiation. The tetanic tension was depressed by pressure (approximately 1% MPa-1). Results demonstrate that the major effect of increased hydrostatic pressure on intact muscle fibres, which results in tension potentiation, is complete very early during contraction and is similar to that of caffeine.