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Calcium-dependent activation of cardiac myofibrils. The mechanisms that modulate myofibrillar ATPase and tension and their significance for heart function.

Myocardial function can be modulated at the level of the sarcolemma, the intracellular Ca2+ stores, and the myofilaments. A way of following myofibrillar modulation of mechanochemical activity is by studying the Ca2+-dependent activation of myofibrils. In this study, the role of Mg2+ in the Ca2+ activation of myofibrillar ATPase was evaluated. The concentrations of both free Ca2+ and free Mg2+ were varied at a given concentration of MgATP (3.16 mM), ionic strength (0.12), and pH 7.0. The experimental ATPase-vs.-Ca2+ data were fit to the model-independent Hill equation and to Tawada's model of intertropomyosin cooperation. In the micromolar range, Mg2+ did not affect the Ca2+ sensitivity, whereas it was reduced at high Mg2+ (5 mM), corresponding to a shift of the activation curve to higher Ca2+ concentrations. Positive cooperativity (n = 2.4) and ATPase activity at saturating Ca2+ concentrations were affected only at low Mg2+. At 0.032 mM Mg2+, the positive cooperation was partially lost (n = 1.3), and maximum ATPase was reduced by 23%. The consequences of a change in the activation curves for the mechanogram are shown. The influence of other modulatory mechanisms on the activation properties was also analyzed on the basis of the Hill equation and the intertropomyosin cooperation model. Acute changes in contractility can arise from changes in the intracellular pH, in sarcomere length, and from phosphorylation of troponin I. Long-term modifications of cardiac performance involve genetic expression of different myosin isoenzymes. The relevance of these different modulatory mechanisms for myocardial function is shown.