Temperature dependence of active tension in mammalian (rabbit psoas) muscle fibres: effect of inorganic phosphate

1 The effect of added inorganic phosphate (P i , range 3–25 m m ) on active tension was examined at a range of temperatures (5–30 °C) in chemically skinned (0.5 % Brij) rabbit psoas muscle fibres. Three types of experiments were carried out. 2 In one type of experiment, a muscle fibre was maximally activated at low temperature (5 °C) and its tension change was recorded during stepwise heating to high temperature in ≈60 s. As found in previous studies, the tension increased with temperature and the normalised tension‐(reciprocal) temperature relation was sigmoidal, with a half‐maximal tension at 8 °C. In the presence of 25 m m added P i , the temperature for half‐maximal tension of the normalised curve was ≈5 °C higher than in the control. The difference in the slope was small. 3 In a second type of experiment, the tension increment during a large temperature jump (from 5 to 30 °C) was examined during an active contraction. The relative increase of active tension on heating was significantly higher in the presence of 25 m m added P i (30/5 °C tension ratio of 6–7) than in the control with no added P i (tension ratio of ≈3). 4 In a third type of experiment, the effect on the maximal Ca 2+ ‐activated tension of different levels of added P i (3–25 m m ) (and P i mop adequate to reduce contaminating P i to micromolar levels) was examined at 5, 10, 20 and 30 °C. The tension was depressed with increased [P i ] in a concentration‐dependent manner at all temperatures, and the data could be fitted with a hyperbolic relation. The calculated maximal tension depression in excess [P i ] was ≈65 % of the control at 5–10 °C, in contrast to a maximal depression of 40 % at 20 °C and 30 % at 30 °C. 5 These experiments indicate that the active tension depression induced by P i in psoas fibres is temperature sensitive, the depression becoming less marked at high temperatures. A reduced P i ‐induced tension depression is qualitatively predicted by a simplified actomyosin ATPase cycle where a pre‐phosphate release, force‐generation step is enhanced by temperature.