Isotonic contraction of skinned muscle fibers on a slow time base: effects of ionic strength and calcium.

Abstract

The force development by Ca-activated skinned frog skeletal muscle fibers and the motion on a slow time base after a quick decrease in load were studied at 0-1.degree. C as a function of the ionic strength and the degree of activation. The ionic strength was varied between 50-190 mM by adding appropriate concentrations of KCl to the bathing solution. Under these conditions, the fibers could be maximally activated for several cycles at low ionic strength without developing residual tension. The steady isometric force in fully activated fibers linearly decreased when the KCl concentration was increased from 0 to 140 mM. The steady isotonic motion at a given relative load in fully activated fibers was almost the same at KCl concentrations .gtoreq. 50 mM. In 0 and 20 mM KCl, the isotonic velocity decreased continuously for > 300 ms. At a given relative load, the initial velocity of the motion in 0 and 20 mM KCl was .apprx. 0.6 and 0.9 times, respectively, that in 140 mM KCl. The initial velocity decreased further when residual tension developed; this observation provides additional evidence that residual tension may reflect the presence of an internal load. The effect of Ca on the motion was examined at 70 mM KCl. In this solution, the motion during the velocity transient at a given relative load appeared to be the same at different levels of activation. The speed of the subsequent motion was almost steady at high Ca levels but decreased continuously in low Ca levels. These results support the idea that at low ionic strength the response of the fiber to Ca is switch-like, but that other factors affect the contraction mechanism under these conditions.