Intrinsic shortening speed of temperature-jump-activated intact muscle fibers. Effects of varying osmotic pressure with sucrose and KCl

Abstract

Effects of intracellular ionic strength on the isotonic contraction properties of both intact fibers and skinned fibers give insights into the cross-bridge mechanism, but presently there is fundamental disagreement in the results on the two fiber preparations. This paper, which studies the effects on contraction of varying the osmotic pressure of the bathing medium with impermeant and permeant solutes, explains the above controversy and establishes the physiological significance of the previous results on skinned fibers. Fast-twitch fibers, isolated singly from tibialis and semitendinosus muscles of frogs, were activated by a temperature-jump technique in hyperosmotic solutions with either 100 or 150 mM sucrose (impermeant), or 50 or 75 mM KCl (permeant). Intracellular ionic strength was expected to rise in these solutions from the standard value of approximately 190 to 265 mM. Cell volume and the speed of unloaded shortening both decreased with sucrose and were constant with KCl. On the other hand, isometric force decreased equally with equiosmolar addition of either solute; this is additional evidence that contractile force decreases with ionic strength and is independent of fiber volume. Therefore, for the main cross-bridges, force per bridge is constant with changes in the lateral separation between the myofilaments. The next finding, that at a fixed cell volume the contraction speed is constant with KCl, provides clear evidence in intact fibers that the intrinsic speed of shortening is insensitive to increased ionic strength. The data with KCl are in agreement with the results on skinned fibers. The results suggest that in the cross-bridge kinetics in vivo the rate-limiting step is different for force than that for shortening. On the other hand, the decrease in speed with sucrose is associated with the shrinkage in cell volume, and is explained by the possibility of an increased internal load. A major fraction of the internal load may arise from unusual interactions between the sliding filaments; these interactions are enhanced in the fibers compressed with sucrose, but this does not affect the intrinsic kinetics of the main cross-bridges.

The effect of hypertonicity on force generation in tetanized single fibres from frog skeletal muscle.

“…It has been shown (April, Brandt, Reuben & Grundfest, 1968;Gulati & Babu, 1984, 1985 that the factor responsible for the tonicity-induced depression of force generation is the increase in ionic strength that accompanies the reduction in lattice spacing caused by hypertonicity. Recent advances in the study of the structure of the actin-myosin complex (Rayment et al 1993) indicate that the first step in the interaction between actin and myosin is an electrostatic interaction, sensitive to ionic strength.…”

Contraction kinetics of intact and skinned frog muscle fibers and degree of activation. Effects of intracellular Ca2+ on unloaded shortening.

“…1 A. The step was applied by using a four-chamber device as described earlier (Gulati and Babu, 1984x). The fiber was initially equilibrated in the caffeine-containing solution for 30-60 min at 25'C, removed from the solution (arrow a in Fig.…”

Shortening velocity extrapolated to zero load and unloaded shortening velocity of whole rat skeletal muscle.

“…The slack test has been used subsequently to measure the VI of both intact (Julian & Morgan, 1981; Lannergren, Lindblom & Johansson, 1982;Gulati & Babu, 1984) and skinned (Julian & Moss, 1981; Moss, 1982) single skeletal muscle fibres. In D. R. CLAFLIN AND J.…”