The aim of the present study was to examine the mechanisms of Ca2+ overload-induced contractile dysfunction in rat hearts independent of ischemia and acidosis. Experiments were performed on 30 excised cross-circulated rat heart preparations. After hearts were exposed to high Ca2+, there was a contractile failure associated with a parallel downward shift of the linear relation between myocardial O(2) consumption per beat and systolic pressure-volume area (index of a total mechanical energy per beat) in left ventricles from all seven hearts that underwent the protocol. This result suggested a decrease in O(2) consumption for total Ca2+ handling in excitation-contraction coupling. In the hearts that underwent the high Ca2+ protocol and had contractile failure, we found marked proteolysis of a cytoskeleton protein, alpha-fodrin, whereas other proteins were unaffected. A calpain inhibitor suppressed the contractile failure by high Ca2+, the decrease in O(2) consumption for total Ca2+ handling, and membrane alpha-fodrin degradation. We conclude that the exposure to high Ca2+ may induce contractile dysfunction possibly by suppressing total Ca2+ handling in excitation-contraction coupling and degradation of membrane alpha-fodrin via activation of calpain.

Recently, we have consistently observed curved endsystolic pressure-volume relations (ESPVRs) of the left ventricle (LV) in rat blood-perfused [1][2][3] and crystalloid-perfused whole heart preparations [4] and in situ ejecting rat hearts [5,6], like those of the puppy LV [7] and adult canine LV in supernormal contractility [8,9]. These studies suggest more generality of the curvilinear than linear ESPVR in different animal species. Despite this curvilinearity, we have obtained a linear myocardial oxygen consumption per Japanese Journal of Physiology, 49, 513-520, 1999 Key words: excitation-contraction coupling, oxygen consumption, E max (end-systolic pressure-volume ratio), systolic pressure-volume area (PVA). Abstract:We have already reported the linear oxygen consumption per beat (VO 2 )-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 VO 2 intercept (PVA-independent VO 2 ) is primarily composed of VO 2 for Ca 2ϩ 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 VO 2 for Ca 2ϩ 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 VO 2 and LV contractility. We obtained a composite VO 2 -PVA relation line at a mid-range LV volume (mLVV) under gradually enhanced LV contractility by stepwise increased Ca 2ϩ infusion and thus the gradually increased PVA-independent VO 2 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 VO 2 and (eESP/ ESV) mLVV and the oxygen cost of LV contractility as the slope of this relation.

To gain insight into the pathogenesis of diabetic cardiomyopathy, we investigated cardiac function in terms of the coupling of left ventricular mechanical work and the energetics in Otsuka Long-Evans Tokushima Fatty rats, which are well known as a model of type 2 diabetes mellitus (DM). Neither left ventricular systolic function and mean coronary flow nor coronary flow reserve differed even in late DM rats. The amount of oxygen required for mechanical work and contraction was unaltered, although myosin isozyme was finally transformed from V(1) to V(3). The maximum pacing rate was decreased from 300 to 240 beats/min, and the left ventricular relaxation rate was significantly (P < 0.05) slower only in late DM rats, resulting in decreased oxygen consumption per minute for total Ca(2+) handling in excitation-contraction coupling mainly consumed by sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA2) without significant changes in basal metabolism or in mitochondrial oxidative phosphorylation. The protein level of SERCA2 in membranes was significantly (P < 0.001) lower in severe DM rats. We conclude that the only lusitropic dysfunction due to the depressed expression of SERCA2 is related to generating diabetic cardiomyopathy even in the present type 2 diabetic rats.

We hypothesized that cardiac dysfunction in hypothyroidism is mainly caused by the impairment of Ca2+ handling in excitation-contraction coupling. To prove this hypothesis, we investigated left ventricular (LV) mechanical work and energetics without interference of preload and afterload in an excised, blood-perfused whole heart preparation from hypothyroid rats. We found that LV inotropism and lusitropism were significantly depressed, and these depressions were causally related to decreased myocardial oxygen consumption for Ca2+ handling and for basal metabolism. The oxygen costs of LV contractility for Ca2+ and for dobutamine in the hypothyroid rats did not differ from those in age-matched normal rats. The expression of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA2) significantly decreased and that of phospholamban significantly increased. The present results revealed that changes in LV energetics associated with decreased mechanical work in hypothyroid rats are mainly caused by the impairment of Ca2+ uptake via SERCA2. We conclude that the impairment of Ca2+ uptake plays an important role in the pathogenesis of cardiac dysfunction in hypothyroidism.

Recently we have shown that the left ventricular end-systolic pressure-volume relation (ESPVR) of in situ rat hearts is an upward convex curve in contrast to the linear left ventricular ESPVR in dog and human hearts. Within the smaller left ventricular volume range, the left ventricular end-systolic pressure rose steeply with increases in left ventricular volume, but it gradually reached a plateau at the larger left ventricular volumes. In adult rat hearts, the myosin isozyme is V1, unlike V3 in dog and human hearts. To investigate whether myosin isozyme affects the curvilinearity of the left ventricular ESPVR, we evaluated the left ventricular ESPVR in hypothyroid rats in which the left ventricular myosin isozyme had been shifted to V3. In the hypothyroid rats, the left ventricular contractility was depressed and the ESPVR became closer to linear. However, after dobutamine administration the ESPVR returned to curvilinear. In nor-mal rats the curvilinearity of the left ventricular ESPVR was decreased by negative inotropic agents such as adrenergic blockers. These results indicate that the depressed left ventricular contractility in the hypothyroidism make ESPVR linear and that the enhanced left ventricular contractility from dobutamine make it curvilinear. We concluded that the curvilinearity of the rat left ventricular ESPVR is not determined by myosin isozyme per se, but by the left ventricular contractility.