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

Abstract

We have already reported the linear oxygen consumption per beat (VO2)- systolic pressure-volume area (PVA) relation from the curved left ventricular (LV) end-systolic pressure-volume relation (ESPVR) in the cross-circulated rat heart. The VO2 intercept (PVA-independent VO2) is primarily composed of VO2 for Ca2+ handling in excitation-contraction (E-C) coupling and basal metabolism. 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 and LV contractility. We obtained a composite VO2-PVA relation line at a mid-range LV volume (mLVV) under gradually enhanced LV contractility by stepwise 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 ratio; 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 assessing rat LV mechanoenergetics. Since the experimentally obtained PVA(mLVV) was not triangular due to the curved ESPVR, we propose an equivalent ESP-V ratio at a mLVV, (eESP/ ESV)(mLVV), as a LV contractility index. This index was calculated as an ESP-V ratio of the specific virtual triangular PVA(mLVV) that is energetically equivalent to the real PVA(mLVV). The present approach enabled us to obtain a linear relation between PVA- independent VO2 and (eESP/ESV)(mLVV) and the oxygen cost of LV contractility as the slope of this relation.