Effects of SM-20550, a selective Na+-H+ exchange inhibitor, on the ion transport of myocardial mitochondria.

The effect of a novel Na+-H+ exchange inhibitor, SM-20550 [N-(aminoiminomethyl)-1,4-dimethyl-1H-indole-2-carboxamide methanesulfonate] (SM) on the ion transport of myocardial mitochondria was studied using ion fluorometry and superfusion techniques. Isolated mitochondria from the guinea-pig heart were pre-loaded with fluoroprobes of either BCECF AM for H+, SBFI AM for Na+ or fura-2 AM for Ca2+. Initially, the treated mitochondria were superfused with a normal medium (MOPS-buffer, pH 7.4, 24 degrees C), subsequently fluorometric experiments on the Na+, H+, Ca2+ mobilization across the mitochondrial membrane were performed. The intramitochondrial pH (pHm) was increased by the superfusion of Na+ at physiological cytosolic concentrations of 10 mM, indicating the existence of a Na+-H+ exchange in mitochondrial membranes. The Na+ induced elevation of pH was dose-dependently inhibited by SM 1 microM (delta pHm; 45% as drug-free 100%), and 10 microM (delta pHm; 70%), as observed in our experiments with the myocardial sarcolemmal membrane. The selective Na+-H+ exchange inhibitor SM reduced such pHm elevations more markedly than that of EIPA [5-(N-ethyl-N-isopropyl) amiloride]. The Na+-H+ exchange inhibitors, SM and EIPA suppressed the intramitochondrial Ca2+ elevation ([Ca2+]m) brought on by external Ca2+ concentration changes: The pretreatment with SM 1 microM, 10 microM and EIPA 10 microM reduced the [Ca2+]m influx by 28.3, 56.5 and 63%, respectively. Additionally, the [Ca2+]m elevation induced by acidification of the perfusate was reduced by the prior infusion of SM and EIPA. Pretreatment of mitochondria with SM or EIPA which had beneficial effects on the left ventricular developed pressure (LVDP) in the ischemia-reperfusion injury of Langendorff hearts, reduced the intramitochondrial Na+ and pHm levels, indicating interplay of the inhibitory mechanism of Ca2+-uptake into mitochondria coupled with Na+-H+ exchange. These findings suggested that protective effects of Na+-H+ exchange inhibitors on reperfused myocardium are due in part to the Ca2+-paradox at the mitochondria level.