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 Ca activation of varying magnitude in mechanically disrupted EGTA[ethylene glycol bis (.beta.-aminoethyl ether) N,N,N'',N'' tetraacetic acid]-treated ventricular bundles from guinea pigs. The results were as follows: 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. CP above 10 mM was sufficient to buffer MgATP2- intracellularly. The activated bundles exhibited an exponential stress-strain relationship, and the series elastic properties did not vary appreciably with degree of activation or CP. At a muscle length 20% beyond just taut, peak tension increased with Ca2+ concentration over the range slightly below 10-6 slightly above 10-4 M. 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 10-4 M Ca2+ and 6% at 4 .times. 10-6 M Ca2+. The rate of shortening after a release was greater at lower loads. At identical loads (relative to maximum force at a given Ca2+ level), velocity at a given time after the release was less at lower Ca2+ concentrations; at 10 M-5, velocity was 72% of that at 10-4 M. At 4 .times. 10-6 M, active shortening was usually delayed and was 40% of the velocity at 10-4 M. Both velocity and peak tension depend on the level of Ca2+ activation over a similar range of Ca2+ concentration.