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Mechanoenergetics of rat left ventricles in in situ and excised blood-perfused hearts and in unloaded rat left ventricular slices.

We investigated rat left ventricular (LV) mechanoenergetics in three different preparations. We obtained an upward convex curvilinear end-systolic pressure-volume relation (ESPVR) regardless of left ventricular (LV) contractility in in situ and excised cross-circulated rat hearts. We also obtained a linear myocardial O2 consumption per beat (VO2)-end-systolic pressure-volume area (PVA; a measure of ventricular total mechanical energy) relation regardless of the curvilinear LV ESPVR in the excised cross-circulated heart. The slope and the VO2 intercept of the VO2-PVA relation was similar to those in other species. The basal metabolism obtained by KCl arrest was higher than those in other species. In the whole heart preparation, the VO2 intercept may include O2 consumption for residual crossbridge cycling as well as O2 consumption for excitation-contraction (E-C) coupling and basal metabolism. Therefore, we established a new system for measuring myocardial O2 consumption of mechanically unloaded (zero PVA) rat LV slices. The increment in O2 consumption by stimulation of the slices represents O2 consumption for E-C coupling (but not for crossbridge cycling). Most O2 consumption for E-C coupling seems consist of O2 consumption for the sarcoplasmic reticulum Ca2+ pump.