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 µm, 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 µM with a Hill coefficient of 5.2 µ 1.2 (mean ± SD, n = 9), and the maximal stress produced at saturating [Ca²⁺]_i equalled 121 ± 35 mN/mm 2 (n = 9). The dependence of steady state force on [Ca²⁺]_i was identical in muscles tetanized in the presence of the Ca²⁺₋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 µM 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.