Modulation of Rigor and Myosin ATPase Activity in Rat Cardiomyocytes

Ischaemic myocardium undergoes calcium-independent contracture at millimolar tissue ATP, though in actomyosin solutions ATP must be reduced to micromolar before rigor complexes form. This contracture is associated with myosin ATPase activity that may contribute to tissue de-energization. Here we used isolated rat cardiomyocytes permeabilized with digitonin to analyse in parallel how rigor and myosin ATPase activity are modulated by metabolic conditions that develop during ischaemia. At pH 7.1 and 37°C rigor and myosin ATPase showed co-ordinated bell-shaped dependence on ATP concentration over 3–1000μm. Rigor, but not myosin ATPase, was inhibited by acidosis (pH 6.2), indicating reduced efficiency of cross-bridge cycling, while both parameters were stimulated by ADP (≤1 mm) and unaffected by inorganic phosphate (Pi, 30 mm), AMP, Mg2+, lactate or inhibition of adenylate kinase with diadenosine pentaphosphate. Combined acidosis and high ADP inhibited rigor, while Piattenuated the enhancement of rigor by ADP. Thus, rigor complex formation activates myosin ATPase in the intact myofilament array, modulated by ADP, Piand acidosis in the ranges that occur in ischaemia. There was no evidence that adenylate kinase might attenuate falling ATP/ADP ratio at the myofilaments. In combination these effects are sufficient to resolve the apparent discrepancy between ATP concentrations triggering rigor in actomyosin and onset of contracture in ischaemic myocardium. Since rigor contracture activates myosin ATPase it is likely to exacerbate ATP depletion and thereby limit vital cell functions. This positive feedback is consistent with the abrupt depletion of ATP observed in individual cardiomyocytes undergoing de-energization contracture.