AJP - Cell Physiology, Vol 259, Issue 2 C349-C357, Copyright © 1990 by American Physiological Society
Depression of force by phosphate in skinned skeletal muscle fibers of the frog
G. J. Stienen, M. C. Roosemalen, M. G. Wilson and G. Elzinga
Laboratory for Physiology, Free University, Amsterdam, The Netherlands.
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.
