Determinants of force decline during relaxation in isolated cardiac muscle

Determinants of the time course of isometric force decline are less well known than those of peak rate of force decline. We studied the influence of loading history, nonuniformity, extracellular calcium, and muscle length on force decline in isolated right ventricular cat papillary muscle. Changes in contraction load affected the time of onset but not time course of force decline at any given length and contractile state. Despite delayed time of onset of force decline (up to 80% of time to peak twitch), rate of force decline was identical at equal force levels. No direct influence of nonuniform behavior of longitudinal segments on the pattern of muscle force decline was observed. The rate of terminal force decline (toward the end of relaxation) was not influenced by extracellular calcium but significantly increased at short muscle lengths: its time constant (means +/- SE) averaged 85 +/- 12 ms at optimal length and decreased by 20 +/- 6 ms per 10% reduction in length. Force control of rate of force decline and its modulation by length can be understood by considering the influence of force and length on sensitivity of contractile proteins to calcium.