R56865 was previously characterized as an inhibitor of Na and Ca overload that has beneficial effects during ischemia and reperfusion. An isolated guinea-pig heart preparation was subjected to 60 minutes of ischemia and 60 minutes of reperfusion. R56865 was given before ischemia and with the onset of reperfusion, applying different dosing schedules, including an initial loading dose. R56865 below 0.1 µmol/l had no cardiodepressant effects in normoxic hearts and at 0.1 µmol/l reduced left ventricular pressure marginally. The onset of ischemic contracture was delayed only at this concentration. R56865 given before ischemia potently inhibits delayed sustained fibrillation occurring during reperfusion in the concentration range between 0.01 µmol/l and 0.1 µmol/l. Analysis of cellular Na+, K+, and Ca2+concentrations revealed that R56865 substantially improves the ionic homeostasis of myocardial cells. Most importantly, the compound also reduced the incidence of delayed sustained fibrillation when given at the onset of reperfusion. R56865 was most effective when fast equilibration of drug with tissue was achieved by giving an initial loading dose. In particular, the cellular Na+and Ca2+contents were improved using this dosing scheme. The results are compatible with the classification of R56865 as an inhibitor of Na+and Ca2+overl
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