Contractile repriming of single muscle fibers: inactivation and tubular diffusion

Following maximal potassium contractures of isolated single fibers from Rana pipiens, a brief recovery period is required to restore the ability to produce maximal tension by subsequent depolarization. At normal potassium concentration (2.5 mM), there is a minimal lag time of 5 s before any tension is produced by a second exposure to elevated K+; half-maximal tension can be produced in 13 s. The rate of repriming is directly related to the square of the fiber radius and is slowed by lowering external calcium concentration (1.5-0.4 mM). There is no significant difference in any of these respects between normal fibers and fibers dissected from semitendinosus muscles which had been chronically denervated for periods of up to 8 wk. Increased potassium concentration (5-20 mM) in the recovery solution decreased the steady-state repriming level, affecting denervated fibers more than controls. No repriming occurred at concentrations above 20 mM (-48 mV membrane potential.) At these intermediate concentrations, repriming was less complete in denervated muscle, reflecting the increased speed of delayed contractile inactivation. From the times to 50% repriming, which were proportional to the square of fiber radius, we estimated an upper limit for the diffusion coefficient for K+ in the transverse tubular system of 0.3 X 10(-6) cm2/s for both control and chronically denervated fibers.