CA-DEPENDENCE OF ISOMETRIC FORCE KINETICS IN SINGLE SKINNED VENTRICULAR CARDIOMYOCYTES FROM RATS

Objectives: The effects of Ca2+ on the rate of tension redevelopment f ollowing a brief release/restretch were investigated in single chemica lly-skinned ventricular myocytes from the rat. Methods: The myocytes w ere enzymatically isolated and skinned using Triton-X100. They were th en attached with an optical adhesive glue to glass micropipettes fixed to a piezoelectric translator and a force transducer. Tension redevel opment was measured at various levels of Ca activation after disruptin g force-generating crossbridges by a brief (20 ms) step release/restre tch equivalent to 20% of the original 2.1 mu m sarcomere length. Most of tension redevelopment was well fitted by a monoexponential function . Results: At maximal Ca concentration, pCa 4.5 maximal force was obta ined at 2.1 mu m sarcomere length and averaged 11.8 +/- 0.7 mu N. The rate of tension redevelopment (k(tr)) increased with increasing Ca con centrations up to 5.19 +/- 0.37 . s(-1) at maximal Ca activation. The relation between the rate of tension redevelopment and Ca concentratio n was sigmoidal and could be fitted by the Hill equation with coeffici ents similar to those describing the tension-pCa relation. The relatio n between relative rate of tension redevelopment and relative steady a ctivated tension was curvilinear increasing with increasing Ca concent ration. Conclusions: In cardiac muscle, Ca2+ modulates both the number and the kinetics of force-generating crossbridges in a manner similar to that previously reported in skeletal muscle.