Relation between steady-state force and intracellular Ca2+ in intact human myocardium. Index of myofibrillar responsiveness to Ca2+.

A novel approach was developed allowing the measurement of steady-state force and intracellular calcium concentration (Ca2+i) in tetanized human ventricular trabeculae carneae without pharmacological intervention. We compared and contrasted three methods of assessing calcium sensitivity of the myofilaments: 1) force-Ca2+ relations in skinned fiber preparations, 2) peak twitch force-peak Ca2+i relations, and 3) steady-state force-Ca2+i relations in intact muscles. Steady-state contractile activation was achieved rapidly by tetanizing intact human trabeculae, loaded with aequorin, a Ca2(+)-sensitive bioluminescent protein, at a stimulation frequency of 15-20 Hz. Steady-state force and Ca2+i were measured during tetani, and the force versus Ca2+i relation was obtained by varying the extracellular calcium concentration (Ca2+o). Force-Ca2+i relations obtained from control and myopathic hearts were fitted to the Hill equation: %Force = Ca2+inh/(Ca2+inh50% + Ca2+inh), where nh is the Hill coefficient, and Ca2+50% is the Ca2+ required for 50% activation. The curves of tetani had Hill coefficients of 5.21 +/- 0.20 (n = 6) and 5.61 +/- 0.60 (n = 10) and Ca2+50% of 0.56 +/- 0.05 microM (n = 6) and 0.54 +/- 0.09 microM (n = 10) in control and myopathic muscles, respectively. We also constructed peak force-peak Ca2+i relations using isometric twitches from the same muscles. These curves were shifted toward higher Ca2+i compared with the steady-state force-Ca2+i curve derived from tetani. Ryanodine (1 microM), which increased the time course of the Ca2+ and force transients, shifted the peak force-peak Ca2+i relation to the left, without affecting the steady-state force-Ca2+i relation. Exposure to 10 mM caffeine shifted the steady-state force-Ca2+i relation to the left, whereas exposure to 3 microM isoproterenol shifted this relation to the right. Experiments using skinned fiber preparations were performed in parallel with experiments on intact muscles from the same hearts. The force-pCa (-logCa2+) relations in saponin-skinned trabeculae from control and myopathic tissue were superimposable. Ryanodine (1 microM) had no effect on the force-pCa relation in skinned fibers. Maximal tension was evoked by the posttetanic twitch, which was larger than the tetanus. This potentiation was abolished in the presence of ryanodine, a sarcoplasmic reticulum inhibitor. We propose that the changes in the steady-state force-Ca2+i relations are correlated with alterations in the sensitivity of the myofilaments to Ca2+, whereas changes in the peak force-peak Ca2+i relations represent temporal changes in the twitch transient.(ABSTRACT TRUNCATED AT 400 WORDS)