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Protective Effect of SM-20550, a Selective Na+-H+ Exchange Inhibitor, on Ischemia-Reperfusion-Injured Hearts

The protective effects of Na+-H+ exchange inhibitors SM-20550 (SM) and 5-(N-ethyl-N-isopropyl)-amiloride (EIPA) against ischemia-reperfusion injury were investigated in guinea pig Langendorff hearts. The changes in intracellular pH (pHi), high-energy phosphates, and biologic intracellular active ions ([Na+]i and [Ca2+]i) were regarded using the 31P-NMR and specific fluorescent signals from the heart tissues together with simultaneous recordings of the left ventricular developed pressure (LVDP). The recovery rate of LVDP from ischemia (40 min) by reperfusion was 36.8% in the control experiments, whereas in the presence of SM 10−7 M, a gradual increase to 75.9% (55.5% with 10−8 M), in contrast to EIPA (10−7 M), 47.5% was observed. SM 10−7 M restored the ATP level by 70% in 40-min reperfusion, which was already higher than the control in the latter half (20-40 min) of the ischemic period. The recovery rate of phosphocreatine by pretreatment of the heart with SM 10−7 M was 75% in 40 min reperfusion. The pHi estimated from Pi/phosphocreatine chemical shift became highly acidic in ischemic heart so that SM 10−7 M caused slight but significant pHi reduction from control pHi of 5.89 to 5.75. The level returned to pHi at around 7.38 during 30-40 min reperfusion, and the recovery was significantly greater than the control pHi of 7.24. The fura-2 Ca2+ or SBFI-Na+ signals during Langendorff ischemia heart increased, and rapidly returned to the control level after the reperfusion. SM suppressed the [Na+]i or [Ca2+]i elevation induced in the late stage during ischemia, resulting in LVDP restoration after reperfusion; Diastolic Ca2+ in the end period of ischemia, SM 10−7 M 194% versus drug-free 220.7%, Na+: SM 10−7 M 121.6% versus drug-free 128.0%. The present results suggest that the selective Na+-H+ exchange inhibitor SM is promising as a potent and specific protective agent against ischemia-reperfusion injuries with Ca2+ overload induced via Na+-H+, Na+-Ca2+ exchange.