Depression of force by phosphate in skinned skeletal muscle fibers of the frog

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Abstract
The relation between isometric force and phosphate concentration in skinned skeletal muscle fibers of the frog is found to depend on fiber size. Force decreased with increasing phosphate concentration, but depression of force in thick fibers was smaller than in thin segments. When the external phosphate concentration was abruptly altered during a sustained contracture, force changed. The half-time of the force change was proportional to the cross-sectional area of the preparation. From this relation, a value for the diffusion constant of phosphate in skinned fibers of 0.9 x 10(-10) m2/s was derived. The rate of phosphate production was determined photometrically via the enzymatic coupling of the resynthesis of ATP to the oxidation of nicotinamide adenine dinucleotide. The average value (+/- SE) of the rate of ATP hydrolysis (at 4 degrees C) was 2.7 +/- 0.3 mumol.s-1.g dry wt-1, which corresponds to 0.34 mmol.l-1.s-1. From a calculation based on the diffusion constant and the rate of phosphate production determined, it follows that the dependency of the force-phosphate relation on fiber diameter is due to phosphate accumulation inside the fiber.