New index for oxygen cost of contractility from curved end-systolic pressure-volume relations in cross-circulated rat hearts

We have already reported the linear oxygen, consumption per beat (VO2)-syst olic pressure-volume area (PVA) relation from the curved left ventricular ( LV) end-systolic pressure-volume relation (ESPVR) in the cross-circulated r at heart. The VO2 intercept (PVA-independent VO2) is primarily composed of VO2 for Ca2+ handling in excitation-contraction (E-C) coupling and basal me tabolism. The aim of the present study was to obtain the oxygen cost of LV contractility that indicates VO2 for Ca2+ handling in E-C coupling per unit LV contractility change in the rat heart. Oxygen cost of LV contractility is obtainable as a slope of a linear relation between PVA-independent VO2 a nd LV contractility. We obtained a composite VO2-PVA relation line at a mid -range LV volume (mLVV) under gradually enhanced LV contractility by stepwi se increased Ca2+ infusion and thus the gradually increased PVA-independent VO2 values. As a LV contractility index, we could not use E-max (ESP-V rat io; ESP/ESV) for the linear ESPVR because of the curved ESPVR in the rat LV , A PVA at a mLVV (PVA(mLVV)) has been proposed as a good index for assessi ng rat LV mechanoenergetics, Since the experimentally obtained PVA(mLVV) wa s not triangular due to the curved ESPVR, we propose an equivalent ESP-V ra tio at a mLVV, (eESP/ESV)(mLVV), as a LV contractility index. This index wa s calculated as an ESP-V ratio of the specific virtual triangular PVA,LV, t hat is energetically equivalent to the real PVA(mLVV). The present approach enabled us to obtain a linear relation between PVA-independent VO2 and (eE SP/ESV)(mLVV) and the oxygen cost of LV contractility as the slope of this relation.