The apolipoprotein E knockout (apoE) mouse, when fed a high-fat diet, is a well established model of atherosclerosis and coronary disease(1); however the associated changes in cardiac excitation-contraction coupling (ECC) have not been fully characterised. The present study was to investigate ECC in this model. Ventricular myocytes were isolated from the hearts of male apoE mice which had been fed an atherogenic high-fat diet for 6 months, and from wild-type (WT) mice fed a normal rodent diet for 6 months. Intracellular Ca was recorded using fluo-3 in conjunction with confocal microscopy during electrical stimulation (0.5 Hz) and during application of caffeine (20 mmol/L) to release Ca from the sarcoplasmic reticulum (SR). L-type Ca current was recorded using the whole-cell patch clamp technique. ICa density was significantly smaller in apoE than in WT myocytes (at 0 mV: -5.31±0.63 vs. -9.50±1.33 pA/pF; mean ± SEM; p=0.0115; n=11/12; unpaired t-test), and the half-time of inactivation was longer (19.09±2.80 vs. 10.05±1.30 ms; p=0.0189), although the time to peak was not significantly different. Systolic Ca transient amplitude was also smaller in apoE than in WT myocytes (F/F0: 0.95±0.06 vs. 3.89±0.45; p<0.0001; n=9/7) as was its rate of decline (rate constant (k): 6.15±0.34 vs. 10.92±1.68 s-1; p=0.0361), although there was no difference in time to peak. Ca transient amplitude/ICa was not significantly different in apoE and WT myocytes (15.17±1.05 vs. 14.12±0.98 pF.pA-1 respectively; ns). The amplitude of the caffeine-induced Ca transient was not significantly different in myocytes from apoE and WT mice, so that systolic Ca transient amplitude/caffeine-induced Ca transient amplitude was smaller (69±3%) in apoE myocytes than in WT myocytes (100±3%; p=0.0004; n=9/7). However the decline of the caffeine-induced Ca transient was slower in apoE myocytes (k: 0.38±0.05 vs. 0.53±0.04 s-1; p=0.0256; n=8/11) suggesting that Ca removal by non-SR pathways is slower in these cells. Inhibition of NCX using NiCl (10 mmol/L) had less effect on the rate of decline of the caffeine-induced transient in apoE myocytes (k: 0.38±0.05 to 0.34±0.08 s-1; 11% decrease; ns; n=8/6) than in WT myocytes (0.53±0.03 to 0.17±0.05 s-1; 68% decrease; p<0.0001; n=11/8), suggesting that Ca efflux via NCX is reduced in apoE myocytes. Calculating the percentage contribution of the SR and NCX to Ca removal as described previously(2) suggests that 95% of Ca is removed by SR and 3% by NCX in WT myocytes, compared to 94% by the SR and 0.4% by NCX in apoE myocytes. Thus estimated SR Ca content and gain of the Ca release process appear to be unchanged in apoE myocytes, suggesting that the smaller Ca transient results from decreased ICa, which is accompanied by a decreased contribution of NCX to Ca extrusion.