Relationship between force and intracellular Ca2+ in tetanized mammalian heart muscle.

To determine features of the steady state Ca2+-tension relationship in intact heart, we measured steady force and intracellular Ca2+ (Ca2+i) in tetanized ferret papillary muscles. Ca2+i was estimated from the luminescence emitted by muscles that had been microinjected with aequorin, a Ca2+-sensitive, bioluminescent protein. We found that by raising extracellular Ca2+ and/or by exposing muscles to the Ca2+ channel agonist Bay K 8644, tension development could be varied from rest to an apparently saturating level, at which increases in Ca2+i produced no further rise in force. 95% of maximal Ca2+-activated force was reached at a Ca2+i of 0.85 +/- 0.06 microM (mean +/- SEM; n = 7), which suggests that the sensitivity of the myofilaments to Ca2+i is far greater than anticipated from studies of skinned heart preparations (or from previous studies using Ca2+-sensitive microelectrodes in intact heart). Our finding that maximal force was reached by approximately 1 microM also allowed us to calculate that the steady state Ca2+i-tension relationship, as it might be observed in intact muscle, should be steep (Hill coefficient of greater than 4), which is consistent with the Hill coefficient estimated from the entire Ca2+i-tension relationship derived from families of variably activated tetani (6.08 +/- 0.68; n = 7). Finally, with regard to whether steady state measurements can be applied directly toward understanding physiological contractions, we found that the relation between steady force and Ca2+i obtained during tetani was steeper than that between peak force and peak Ca2+i observed during physiological twitches.