The influence of varied tonicity of the extracellular medium on the depressant effect of active shortening in vertebrate striated muscle

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

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 influence of dantrolene, diazepam, pentobarbital and 4-aminopyridine on shortening induced depression in isolated muscle fibres of the frog

The influence of dantrolene, diazepam, pentobarbital and 4-aminopyridine on the depressant effect of active shortening during twitch and tetanus was investigated in isolated muscle fibres of the frog. These drugs are all presumed to alter the kinetics of the release and re-uptake of activator calcium during the excitation-contraction process. The shortening induced depression was calculated as the difference in redeveloped force after a small (control) and a larger (test) release step and was expressed in per cent of the control. The depressant effect in response to a given amount of shortening during twitch contraction as well as during the plateau of a tetanus was approximately doubled after addition of 10 microM dantrolene. In contrast, diazepam (150 microM) reduced the shortening effect during the twitch to about one tenth of the value obtained in ordinary Ringer solution, while the force depression by shortening during tetanic stimulation remained essentially unaltered. Pentobarbital (1.0 mM) reduced the shortening induced depression during the twitch by 95-100%. 4-aminopyridine (3.0 mM) completely abolished the depressant effect of shortening obtained during the twitch and approximately halved the shortening effect during tetanic contraction.