Ca2+ influx via the Na+/Ca2+ exchanger (NCX) may lead to Ca2+ overload and myocardial injury in ischemia-reperfusion. Direct evidence that increased cytoplasmic Ca2+ concentration ([Ca2+]i) is mediated by the reverse mode of the NCX is limited, so in the present study the [Ca2+]i dynamics and left ventricular pressure were monitored in perfused beating hearts. The effects of KB-R7943 (KBR), a selective inhibitor of the NCX in the reverse mode, were analyzed during low-Na+ exposure and ischemia-reperfusion. Hearts from Sprague-Dawley rats were retrogradely perfused and loaded with 4 micromol/L fura-2 to measure the fluorescence ratio as an index of [Ca2+]i. To evaluate KBR effects on the reverse mode exchanger, the increase in [Ca2+]i induced by low-Na+ exposure (Na+: 30 mmol/L, 10 mmol/L caffeine pre-treatment) was measured with and without 10 micromol/L KBR (n=5). In another series, the hearts were subjected to 10 min of low-flow ischemia with pacing, followed by reperfusion in the absence (n=6) or in the presence of 10 micromol/L KBR (n=6). Background autofluorescence was subtracted to estimate the ratio in the ischemia-reperfusion protocol. KBR significantly suppressed the increase in [Ca2+]i induced by low-Na+ (40.2 +/- 11.2% of control condition, p=0.014), as well as on increase in diastolic [Ca2+]i during ischemia (% increase from pre-ischemia in [Ca2+]i at 10 min: KBR, 17.9 +/- 6.4%; no KBR, 44.4 +/- 7.7%; p=0.024). After reperfusion, diastolic [Ca2+]i normalized more rapidly in KBR-treated hearts (% increase at 1 min: KBR, 4.5 +/- 7.0%; no KBR, 39.8 +/- 12.2%; p=0.03). Treatment with KBR also accelerated recovery of the rate-pressure product on reperfusion (1 min: KBR, 8,944 +/- 1,554 min(-1) mmHg; no KBR, 4,970 +/- 1,325; p<0.05). Thus, inhibition of the reverse mode exchanger by KBR reduced ischemic Ca2+ overload and possibly improved functional myocardial recovery during reperfusion in a whole heart model.