Preservation of mitochondrial function may contribute to cardioprotective effects of Na+/Ca2+ exchanger inhibitors in ischaemic/reperfused rat hearts

Background and purpose: Na+/Ca2+ exchanger (NCX) inhibitors are known to attenuate myocardial reperfusion injury. However, the exact mechanisms for the cardioprotection remain unclear. The present study was undertaken to examine the mechanism underlying the cardioprotection by NCX inhibitors against ischaemia/reperfusion injury. Experimental approach: Isolated rat hearts were subjected to 35-min ischaemia/60-min reperfusion or 20-min ischaemia/ 60-min reperfusion. NCX inhibitors (3-30 mu M KB-R7943 (KBR) or 0.3-1 mu M SEA0400 (SEA)) were given for 5 min prior to ischaemia (pre-ischaemic treatment) or for 10 min after the onset of reperfusion (post-ischaemic treatment). Key results: With 35-min ischaemia/60-min reperfusion, pre- or post-ischaemic treatment with KBR or SEA neither enhanced post-ischaemic contractile recovery nor attenuated ischaemia- or reperfusion-induced Na+ accumulation and damage to mitochondrial respiratory function. With the milder model (20-min ischaemia/ reperfusion), pre- or post-ischaemic treatment with 10 mM KBR or 1 mM SEA significantly enhanced the post-ischaemic contractile recovery, associated with reductions in reperfusion-induced Ca2+ accumulation, damage to mitochondrial function, and decrease in myocardial high-energy phosphates. Furthermore, Na+ influx to mitochondria in vitro was enhanced by increased concentrations of NaCl. KBR (10 mM) and 1 mM SEA partially decreased the Na+ influx. Conclusions and implications: The NCX inhibitors exerted cardioprotective effects during relatively mild ischaemia. The mechanism may be attributable to prevention of mitochondrial damage, possibly mediated by attenuation of Na+ overload in cardiac mitochondria during ischaemia and/or Ca2+ overload via the reverse mode of NCX during reperfusion.