Dynamic relations among length, tension, and intracellular Ca2+ in activated ferret papillary muscles

To study the effects of mechanical constraints on the Ca2+ affinity of card iac troponin C, we analyzed the tension and aequorin light (AL) responses t o sinusoidal length changes (5-10% of the initial muscle length) in aequori n-injected, tetanized cardiac muscles. The amplitude of the quasi-sinusoida l tension and AL responses decreased with increasing length-perturbation fr equency from 0.5 to 1 Hz at 24 degrees C and from 1 to 3 Hz at 30 degrees C . The increase in AL corresponded well to the decrease in tension; likewise , the decrease in AL to the increase in tension and the tension response la gged behind the length change. A further increase in frequency (>1 Hz at 24 degrees C and >3 Hz at 30 degrees C) markedly increased the amplitude of t he tension responses but decreased the amplitude of the AL responses. The i ncrease in AL lagged behind the decrease in tension; likewise, the decrease in AL lagged behind the increase in tension, and the tension response led the length change. From previous mechanistic interpretations of the frequen cy dependence of the amplitude of tension response, we argue that the Ca2affinity of cardiac troponin C changes in parallel with the active tension (i.e., the number of active cross bridges) but not with the passive tension produced by the length perturbation-induced cross-bridge strain.