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 mM) on active t ension was examined at a range of temperatures (5-30 degreesC) in chemicall y skinned (0.5% Brij) rabbit psoas muscle fibres. Three types of experiment s were carried out. 2. In one type of experiment, a muscle fibre was maximally activated at low temperature (5 degreesC) and its tension change was recorded during stepwi se heating to high temperature in similar to 60 s. As found in previous stu dies, the tension increased with temperature and the normalised tension-(re ciprocal) temperature relation was sigmoidal, with a half-maximal tension a t 8 degreesC. In the presence of 25 mM added P-i, the temperature for half- maximal tension of the normalised curve was similar to5 degreesC higher tha n in the control. The difference in the slope was small. 3. In a second type of experiment, the tension increment during a large tem perature jump (from 5 to 30 degreesC) was examined during an active contrac tion. The relative increase of active tension on heating was significantly higher in the presence of 25 mM added Pi (30/5 degreesC tension ratio of 6- 7) than in the control with no added P-i (tension ratio of similar to3). 4. In a third type of experiment, the effect on the maximal Ca2+-activated tension of different levels of added P-i (3-25 mM) (and P-i mop adequate to reduce contaminating P-i to micromolar levels) was examined at 5, 10, 20 a nd 30 degreesC. The tension was depressed with increased [P-i] in a concent ration-dependent manner at all temperatures, and the data could be fitted w ith a hyperbolic relation. The calculated maximal tension depression in exc ess [P-i] was similar to 65 % of the control at 5-10 degreesC, in contrast to a maximal depression of 40 % at 20 degreesC and 30 % at 30 degreesC. 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 q ualitatively predicted by a simplified actomyosin ATPase cycle where a pre- phosphate release, force-generation step is enhanced by temperature.