Effects of amiloride on metabolism and contractility during reoxygenation in perfused rat hearts

Myocardial recovery after hypoxia may be determined not only by the extent of metabolic depression during the hypoxic period but also by changes in cation contents as well. Calcium overload during reoxygenation, mediated in part by Na-Ca exchange and supported by the rise in cell sodium during hypoxia, may be one factor. The effects of amiloride (0.1 mM), a diuretic that inhibits Na⁺-H⁺ and Na-Ca exchanges in cardiac sarcolemma and mitochondria preparations, were studied during hypoxia-reoxygenation in the isovolumic, isolated rat heart. During hypoxia, cell sodium, measured using potassium ethylenediamine tetraacetate cobaltate as an extracellular marker, increased in amiloride and amiloride-free hearts, but there was no increase in cell calcium (3.3 ± 0.3 vs. 3.6 ± 0.9 μmol/g dry wt; p = NS). Amiloride did not alter developed pressure (DP), end-diastolic pressure (EDP), pH, or integrated areas of adenosine triphosphate (ATP) and phosphocreatine (PCr) (determined by phosphorus-31-nuclear magnetic resonance copy) during hypoxia or normal perfusion conditions. Forty minutes after reoxygenation, however, cell calcium was significantly lower in the amiloride (5.1 ± 1.3 μmol/g dry wt) than in the amiloride-free group (10.4 ± 1.8 μmol/g dry wt; p<0.001), and there was improved recovery of DP (percent of initial) (72 ± 12% vs. 41 ± 12%; p<0.001), PCr (99 ± 9% vs. 70 ± 14%; p<0.001), and pH (7.17 ± 0.17 vs. 6.88 ± 0.16; p<0.001) in the amiloride group. To determine whether this dose of amiloride inhibits the manifestations of sodium-mediated calcium gain in the same model during normoxia, the metabolic and functional sequelae of lithium-substituted low sodium (50 mM) perfusion were studied. Amiloride significantly limited the manifestations of sodium-mediated calcium gain as indexed (all expressed as percent of control) by a lower peak DP (221 ± 25% vs. 284 ± 20%) at 3 minutes, improved preservation of PCr (85 ± 10% vs. 68 ± 9%) and ATP (104 ± 12% vs. 84 ± 9%), lower rise in inorganic phosphate (201 ± 74% vs. 332 ± 106%), and a smaller fall in intracellular pH (7.01 ± 0.04 vs. 6.70 ± 0.15, p<0.05) for all metabolic parameters during a 20-minute period. These findings indicate that at a dose shown to alter the manifestations of sodium-mediated calcium gain in the isolated rat heart, amiloride limits cellular calcium gain during reoxygenation after hypoxia and significantly improves mechanical and metabolic recovery, even though it does not alter the decline in metabolic parameters during hypoxia.