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.

Abnormal intracellular Ca 2؉ cycling plays an important role in cardiac dysfunction and ventricular arrhythmias in the setting of heart failure and transient cardiac ischemia followed by reperfusion (I͞R). We hypothesized that overexpression of the sarcoplasmic reticulum Ca 2؉ ATPase pump (SERCA2a) may improve both contractile dysfunction and ventricular arrhythmias. Continuous ECG recordings were obtained in 46 conscious rats after adenoviral gene transfer of either SERCA2a or the reporter gene ␤-galactosidase (␤gal) or parvalbumin (PV), as early as 48 h before and 48 h after 30 min ligation of the left anterior descending artery by using an implantable telemetry system. Sham-operated animals were used for comparison for hemodynamic measurements, whereas within-animal baseline was used for electrocardiographic and echocardiographic parameters. All episodes of nonsustained ventricular tachycardia (VT) and ventricular fibrillation (VF) were counted, and their durations were summed by telemetry. I͞R decreased regional cardiac wall thickening as well as the maximal rate of left ventricular pressure rise (؉dP͞dt) and ventricular pressure fall (؊dP͞dt). SERCA2a restored regional wall thickening and ؉dP͞dt and ؊dP͞dt to levels seen preoperatively. Regionalwall motion and anterior-wall thickening were improved in the SERCA2a animals, as assessed by echocardiography and piezoelectric crystals. To assess whether these effects are SERCA2a specific, we overexpressed a skeletal-muscle protein, PV, to examine whether Ca 2؉ buffering alone can mitigate ventricular arrhythmias. During the first hour after I͞R, the rate of nonsustained VT plus VF was 16 ؎ 5 episodes per h (n ‫؍‬ 6) in the Ad.␤gal group, 22 ؎ 6 in the Ad.PV group, and 4 ؎ 2 (n ‫؍‬ 6, P < 0.01) in the Ad.SERCA2a group. The decrease in VT plus VF in the Ad.SERCA2a group was consistent throughout the 48 h of monitoring. These results show that improving intracellular Ca 2؉ handling by overexpression of SERCA2a restores contractile function and reduces ventricular arrhythmias during I͞R.

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.

The aim of this study was to examine whether short- and long-term gene transfer of Ca(2+) handling proteins restore left ventricular (LV) mechanoenergetics in aortic banding-induced failing hearts. Aortic-banded rats received recombinant adenoviruses carrying sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2a) (Banding+SERCA), parvalbumin (Banding+Parv) or beta-galactosidase (Banding+betagal), or an adeno-associated virus carrying SERCA2a (Banding+AAV.SERCA) by a catheter-based technique. LV mechanoenergetic function was measured in cross-circulated hearts. "Banding", "Banding+betagal" and "Banding+saline" groups showed lower end-systolic pressure at 0.1 ml intraballoon water (ESP(0.1)), higher end-diastolic pressure at 0.1 ml intraballoon water (EDP(0.1)) and slower LV relaxation rate, compared with "Normal" and "Sham". However, "Banding+SERCA" and "Banding+Parv" showed high ESP(0.1), low EDP(0.1) and fast LV relaxation rate. In "Banding", "Banding+betagal" and "Banding+saline", slope of relation between cardiac oxygen consumption and systolic pressure-volume area, O(2) cost of total mechanical energy, was twice higher than normal value, whereas slope in "Baning+SERCA" and "Banding+Parv" was similar to normal value. Furthermore, O(2) cost of LV contractility in the 3 control banding groups was approximately 3 times higher than normal value, whereas O(2) cost of contractility in "Banding+SERCA", "Banding+AAV.SERCA" and "Banding+Parv" was as low as normal value. Thus, high O(2) costs of total mechanical energy and of LV contractility in failing hearts indicate energy wasting both in chemomechanical energy transduction and in calcium handling. Improved calcium handling by both short- and long-term overexpression of SERCA2a and parvalbumin transforms the inefficient energy utilization into a more efficient state. Therefore enhancement of calcium handling either by resequestration into the SR or by intracellular buffering improves not only mechanical but energetic function in failing hearts.