Spontaneous Ca²⁺ release from the sarcoplasmic reticulum (SR) can occur in ventricular cardiomyocytes in situations of high intracellular Ca²⁺ concentrations. Spontaneous SR Ca²⁺ release induced by β-adrenergic stimulation has been linked to contractile dysfunction and potentially lethal arrhythmias¹. K201 (1,4-benzothiazepine derivative) alters various intracellular Ca²⁺ handling pathways resulting in reduced incidence of spontaneous Ca²⁺ release in cardiomyocytes². While the cardiac-related effects of K201 have been studied using isolated cell preparations the effects on the whole heart remain unclear. Under terminal anaesthesia hearts were removed from adult New Zealand White rabbits and cannulated onto a working heart system. Preload (10cmH2O) and afterload (75cmH2O) were kept constant. Hearts were subjected to increased intracellular Ca²⁺ load by increasing [Ca²⁺]_o from 2.5mM to 4.5mM, followed 5min later by addition of 150nM isoproterenol (ISO) (n=6) ± 1µM K201 (n=5). In a subset of experiments (n=5) hearts were exposed to increasing concentrations of K201 (0.3, 1 and 3µM) at 2.5mM [Ca²⁺]_o. Mechanical and electrical function was assessed by insertion of a left ventricular 3-French pressure-volume catheter and a bipolar electrogram respectively. Initially, switching to high [Ca²⁺]+ISO significantly increased mean heart rate (HR) (206.9±10.1 vs. 251.7±24.9bpm), developed pressure (DP) (80.4±1.98 vs. 97.4±3.84mmHg) and dP/dt_max (1669±109 vs. 3530±183 mmHg.s^-1; 2.5mM vs. 4.5mM [Ca²⁺]+ ISO). After 50s, ectopic electrical events were observed, increasing in frequency from 0.06±0.02.s^-1 (50s after ISO) to 0.62±0.22.s^-1 (300s post ISO). HR, DP and dP/dt_max decreased (239±16.8bpm, 75.35±5.19mmHg, and 2697±199mmHg.s^-1) until cessation of aortic flow (AF) 632±70.3s post ISO. In contrast, hearts perfused with K201 maintained AF and mechanical function for a longer period (2545.2±585.25s, ttest P<0.05). HR and coronary flow (CF) were not significantly different between groups. At 2.5mM [Ca²⁺]_o increasing doses of K201 led to a significant decline in DP [90.5±2.7 (control), 88.1±2.6 (0.3µM), 82.6±2.6 (1µM), 78.7±2.3 (3µM)], and stroke volume (SV) [943±127 (control), 891±131 (0.3µM), 826±120 (1µM), 616±81.3µL (3µM)]. HR decreased significantly in the presence of 1 and 3µM K201 [197.9±5.4 (steady state) 193.6±5.2 (0.3µM), 180.6±4.9 (1µM), 169.8±7.4bpm (3µM)] but not in 0.3µM (197.9±5.4 vs. 193.6±5.2; control vs. 0.3µM K201). Cessation of AF had occurred in all hearts 3min after application of 3µM K201 indicating a narrow therapeutic index. This data demonstrates that perfusion with high [Ca²⁺]_o and ISO leads to severely impaired contractility and abnormal electrical activity. K201 preserved mechanical function of working hearts during exposure to high[Ca²⁺]_o+ISO, but caused a dose-dependent decrease in mechanical function when perfused with 2.5mM [Ca²⁺] alone.