To examine the role of Na+/H+ exchange in cardiac injury, we compared the effect of amiloride (174 microM) with the markedly more specific and potent inhibitor 5-(N,N-hexamethylene) amiloride (HMA, 1 microM) against cardiac injury produced by reperfusion, reoxygenation, and the calcium paradox. Reperfusion after 15-min ischemia resulted in a 55 +/- 4% recovery in contractility, whereas in the presence of amiloride or HMA, recovery was increased to 82 +/- 5.8 and 72 +/- 7.8%, respectively (p < 0.05 from control), with HMA showing particular efficacy in accelerating recovery. The rapid restoration of function with HMA was also evident in hearts reoxygenated for 1 min after 12-min hypoxia (control 35 +/- 3.2%, HMA 66 +/- 4.1%, p < 0.05) although the protective effect gradually reversed with continued reoxygenation. On the other hand, with addition of amiloride, the protective effect persisted so that after 30-min reoxygenation values were significantly higher than control (65 +/- 4.1 vs. 47 +/- 3.1%, p < 0.05). Resting tension increases after either reperfusion or reoxygenation were moderate: 124 +/- 8 and 119 +/- 6%, respectively (p > 0.05), but no increases were observed with amiloride or HMA. Bepridil (10 microM), a purported Na+/Ca2+ exchange inhibitor, exerted a salutary effect against reperfusion dysfunction identical to that of amiloride and HMA, whereas in reoxygenated hearts the effects were identical to those observed with HMA. The protective effects of the drugs were not related to improved energy metabolic status. None of the pharmacologic interventions exerted beneficial effects against the calcium paradox.(ABSTRACT TRUNCATED AT 250 WORDS)