KINETICS OF TENSION DEVELOPMENT IN SKINNED CARDIAC MYOCYTES MEASURED BY PHOTORELEASE OF CA2+

The rate of activation of tension development by free Ca2+ was examined in skinned rat ventricular myocytes. Pulse photolysis of the photosensitive Ca2+ chelator, Nitr-7, was used to rapidly elevate Ca2+ in the vicinity of the myofilaments. Tension increased exponentially with a first-order rate constant (k(Ca)) that depended on the level of Ca2+ activation. k(Ca) increased approximately linearly from 0.9 +/- 0.2 s(-1) at 20% maximal Ca2+-activated tension (P-o) to 4.0 +/- 0.9 s(-1) at 85% P-o, representing a fourfold increase in k(Ca) with activation. Reducing free Mg2+ from 1 to 0.1 mM accelerated k(Ca) by about twofold at all levels of Ca2+. Tension development kinetics were significantly different in skinned rabbit psoas fibers: k(Ca) increased nonlinearly from 1.2 +/- 0.2 s(-1) at 15% P-o to a maximum of 17.5 +/- 1.3 s(-1) at 85% P-o, representing a 15-fold increase in k(Ca) with activation. Moreover, lowering free Mg2+ increased k(Ca) only at submaximal Ca2+ in psoas fibers. Extraction of troponin C (TNC) from psoas fibers and recombination with bovine cardiac TNC did not significantly alter any of these characteristics of tension development kinetics. We conclude that significant differences exist between cardiac and fast-twitch skeletal muscles in terms of the effects of Ca2+ and Mg2+ on contraction kinetics and that these differences cannot be attributed solely to differences in TNC isoforms.