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 µM (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 - µM 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 >4), which is consistent with the Hill coefficient estimated from the entire [Ca²⁺]_itension 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.