Maximal Ca2+-activated force and myofilament Ca2+ sensitivity in intact mammalian hearts. Differential effects of inorganic phosphate and hydrogen ions.

Myofilament Ca2+ sensitivity and maximal Ca2+-activated force are fundamental properties of the contractile proteins in the heart. Although these properties can be evaluated directly in skinned preparations, they have remained elusive in intact tissue. A novel approach is described that allows maximal Ca2+-activated force to be measured and myofilament Ca2+ sensitivity to be deduced from isovolumic pressure in intact perfused ferret hearts. Phosphorus nuclear magnetic resonance spectra are obtained sequentially to measure the intracellular inorganic phosphate (Pi) and hydrogen ion (H+) concentrations. After a period of perfusion with oxygenated, HEPES-buffered Tyrode solution, hypoxia is induced as a means of elevating [Pi]. The decline in twitch pressure can then be related to the measured increase in [Pi]. After recovery, hearts are perfused with ryanodine to enable tetanization and the measurement of maximal Ca2+-activated pressure. Hypoxia is induced once again, and maximal pressure is correlated with [Pi]. We then compare the relations between [Pi] and maximal pressure on the one hand, and [Pi] and twitch pressure on the other. If the two relations differ only by a constant scaling factor, then the decline in twitch pressure can be attributed solely to a decline in maximal pressure, with no change in myofilament sensitivity. We obtained such a result during hypoxia, which indicated that Pi accumulation decreases maximal force but does not change myofilament sensitivity. We compared these results with acidosis (induced by bubbling with 5% CO2). In contrast with Pi, the accumulation of H+ decreases twitch force primarily by shifting myofilament Ca2+ sensitivity. This approach in intact tissue has strengths and limitations complementary to those of skinned muscle experiments.