No dependency of a new index for oxygen cost of left ventricular contractility on heart rates in the blood-perfused excised rat heart

We have reported the linear relation of myocardial oxygen consumption per b eat (VO2) and systolic pressure-volume area (PVA) in the left ventricle of the cross-circulated rat heart. The VO2 intercept (PVA-independent VO2) is primarily composed of VO2 for Ca2+ handling in excitation-contraction coupl ing and basal metabolism. Recently, we proposed a new index for oxygen cost of contractility obtainable as a slope of a linear relation between PVA-in dependent VO2 and left ventricular contractility. This index indicates the Ca2+ handling VO2 per unit contractility change. However, a dependency of t his index on heart rate has not yet been investigated. The aim of the prese nt study was to investigate the dependency of oxygen cost of contractility on heart rate. This is a critical point to compare this cost under differen t heart rates. At first we found no differences of VO2-PVA relations at 240 and 300 beats/min (bpm). Therefore, after control VO2-PVA relation at 300 bpm, we gradually enhanced left ventricular contractility by Ca2+ at a midr ange left ventricular volume and obtained the gradually increased PVA-indep endent VO2 At each contractility level, the pacing rate was alternately cha nged at 240 and 300 bpm. We obtained the two composite VO2-PVA relation lin es and found no significant differences between the slopes of PVA-independe nt VO2 and left ventricular contractility relations at 240 and 300 bpm. The present results indicated no dependency of oxygen cost of left ventricular contractility on heart rates within 240-300 bpm. Based on this fact, we co ncluded that even under the different pacing rates within 240-300 bpm, this oxygen cost is valid for assessing cardiac mechanoenergetics, especially t he economy of total Ca2+ handling in E-C coupling.