Decrease in oxygen cost of contractility during hypocapnic alkalosis in canine hearts

Ca2+ sensitization of contractile machinery could theoretically enhance the mechanoenergetics of the heart. We studied the effects of alkalosis with Ca2+ sensitization on mechanoenergetics within the framework of the relationships of left ventricular pressure-volume area (PVA; a measure of the total mechanical energy), myocardial oxygen consumption per beat (VO2), and the contractility index [E(max) (slope of end-systolic pressure-volume relation)] in 10 excised, cross-circulated canine hearts. Alkalosis was stably maintained without hypoxia (mean pH 7.66). Alkalosis increased E(max) without changing the slope of the VO2-PVA relation, a reflected contractile efficiency. The incremental ratio of unloaded VO2 to E(max) in alkalosis was significantly lower than that in Ca2+ sensitization (0.0012 +/- 0.0010 vs. 0.0062 +/- 0.0030 ml O2 . mmHg-1 . ml . beat-1 . 100 g LV-2; P < 0.01). Basal metabolism under KCl arrest was unchanged by alkalosis, indicating the decreased energy cost of the excitation-contraction coupling by alkalosis. Compared with the control, alkalosis increased E(max) during the Ca2+ infusion of various concentrations without any further increase in unloaded VO2. Thus we demonstrated a decreased oxygen cost of contractility during alkalosis, presumably due to Ca2+ sensitization.