Estimated time course of Ca2+ bound to troponin C during relaxation in isolated cardiac muscle

We present a mechanical assay for estimating the time course of Ca2+ bound to low-affinity sites on troponin C (TnC) in twitching rabbit papillary muscle. The assay is based on a theoretical correlation between the rate of force redevelopment after detachment of all cross-bridges and the amount of Ca2+ bound to TnC. Experimentally, we applied length impulses at different times to detach all cross-bridges; the initial rate of force redevelopment after each impulse was taken as an index of bound Ca2+ at that time. Under control conditions, the magnitude of this index decreased to 10% of its maximum during early relaxation, when force had declined only slightly 78 +/- 12% of its peak isometric value. The time course of this index was examined after addition of either isoproterenol or ryanodine, which are known to shorten and prolong, respectively, the intracellular free Ca2+ transient. As expected, changes previously reported in the free Ca2+ time course were qualitatively reflected in the time course of the bound Ca2+ index. We conclude that this index constitutes a reasonable method for estimating the time course of bound Ca2+ and that bound Ca2+ declines well ahead of force in isometrically contracting rabbit myocardium at 24 degrees C.