POSITIVE INOTROPISM IN HYPOTHERMIA PARTIALLY DEPENDS ON AN INCREASE IN MAXIMAL CA2+-ACTIVATED FORCE

We investigated the contribution of maximal Ca2+-activated force to the positive inotropism induced by mild hypothermia. Phosphorus-31 nuclear magnetic resonance spectroscopy revealed that neither energy-related phosphorus compounds in myocardium nor intracellular pH was responsible for the change in contractility. Maximal Ca2+-activated pressure (MCAP), the intact-heart correlate of maximal Ca2+-activated force, was determined in isolated perfused rabbit hearts by measuring isovolumic left ventricular pressure during tetani at extracellular Ca2+ concentrations greater-than-or-equal-to 10 mM. Tetani were elicited by rapid pacing after exposure to ryanodine. MCAP increased by 2.17 +/- 0.28% (means +/- SE, P < 0.001, n = 19) for each degree of myocardial cooling between 30 and 38-degrees-C. Our results indicate that a primary change in myofilament Ca2+ responsiveness underlies the positive inotropism in hypothermia. The increase in maximal Ca2+-activated force may explain the observation of positive inotropism without an upward shift in the relation between oxygen consumption and pressure-volume area, as previously reported for cooled whole hearts.