Isometric force development, isotonic shortening, and elasticity measurements from Ca2+-activated ventricular muscle of the guinea pig

Abstract

Isometric tension and isotonic shortening were measured at constant levels of calcium activation of varying magnitude in mechanically disrupted EGTA-treated ventricular bundles from guinea pigs. The results were as follows: (a) The effect of creatine phosphate (CP) on peak tension and rate of shortening saturated at a CP concentration > 10 mM; below that level tension was increased and shortening velocity decreased. We interpreted this to mean that CP above 10 mM was sufficient to buffer MgATP intracellularly. (b) The activated bundles exhibited an exponential stress-strain relationship, and the series elastic properties did not vary appreciably with degree of activation or creatine phosphate level, (c) At a muscle length 20% beyond just taut, peak tension increased with Ca-- concentration over the range slightly below 10-5 to slightly above 10-4 M. (d) By releasing the muscle to different loads and measuring the length at which shortening stopped, muscle length-active tension curves were constructed. Force declined to 20% peak tension with a decrease in muscle length (after the recoil) of only 11% at 10- M Ca- and 6% at 4 × 10-4 M Ca-. (e) The rate of shortening after a release was greater at lower loads. At identical loads (relative to maximum force at a given Ca-- level), velocity at a given time after the release was less at lower Ca-- concentrations; at 10 M-4, velocity was 72% of that at 10-4 M, and at 4 × 10-4 M, active shortening was usually delayed and was 40% of the velocity at 10-4 M. Thus, under the conditions of these experiments, both velocity and peak tension depend on the level of Ca-- activation over a similar range of Ca-- concentration. © 1978, Rockefeller University Press., All rights reserved.