The relationship between contractile force and intracellular Ca2+ in intact rat cardiac trabeculae.

The control of force by Ca2+ was investigated in rat cardiac trabeculae loaded with fura-2 salt. At sarcomere lengths of 2.1-2.3 microns, the steady state force-Ca2+i relationship during tetanization in the presence of ryanodine was half maximally activated at a Ca2+i of 0.65 +/- 0.19 microM with a Hill coefficient of 5.2 +/- 1.2 (mean +/- SD, n = 9), and the maximal stress produced at saturating Ca2+i equalled 121 +/- 35 mN/mm2 (n = 9). The dependence of steady state force on Ca2+i was identical in muscles tetanized in the presence of the Ca(2+)-ATPase inhibitor cyclopiazonic acid (CPA). The force-Ca2+i relationship during the relaxation of twitches in the presence of CPA coincided exactly to that measured at steady state during tetani, suggesting that CPA slows the decay rate of Ca2+i sufficiently to allow the force to come into a steady state with the Ca2+i. In contrast, the relationship of force to Ca2+i during the relaxation phase of control twitches was shifted leftward relative to the steady state relationship, establishing that relaxation is limited by the contractile system itself, not by Ca2+ removal from the cytosol. Under control conditions the force-Ca2+i relationship, quantified at the time of peak twitch force (i.e., dF/dt = 0), coincided fairly well with steady state measurements in some trabeculae (i.e., three of seven). However, the force-Ca2+i relationship at peak force did not correspond to the steady state measurements after the application of 5 mM 2,3-butanedione monoxime (BDM) (to accelerate cross-bridge kinetics) or 100 microM CPA (to slow the relaxation of the Ca2+i transient). Therefore, we conclude that the relationship of force to Ca2+i during physiological twitch contractions cannot be used to predict the steady state relationship.