We examined whether the chloride channel blockers anthracene-9-carboxylic acid (9-AC) and 4-acetamide-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS) exert protective effects against myocardial ischemia-reperfusion damage. In isolated guinea pig ventricular cells, 9-AC (200 microM), but not SITS (100 microM), inhibited the chloride current induced by isoproterenol. Electrical and mechanical activities and intracellular pH of arterially perfused guinea pig right ventricular preparations were recorded with an intracellular microelectrode, a force transducer and a pH-sensitive fluorescent probe, respectively. The preparations were subjected to 30 min of no-flow ischemia, with or without 9-AC (100 microM) or SITS (10 microM), followed by reperfusion. No-flow ischemia produced decreases in action potential amplitude and duration, and the contractile force was completely abolished. Although the changes in electrical parameters were reversed upon reperfusion, the contractile force recovered only to about 50% of preischemic values. 9-AC and SITS had no inhibitory effect on contractile force under normal conditions and during ischemia but significantly improved the recovery of contractile force upon reperfusion to about 80% of preischemic values. Both 9-AC and SITS showed significant inhibition of the ischemia-induced abbreviation of action potential duration. Other parameters were not affected by 9-AC or SITS. During ischemia, intracellular pH showed a transient small increase followed by a sustained decrease, which was completely recovered upon reperfusion. The decrease in pH during ischemia was attenuated by 80% in SITS- but not 9-AC-treated preparations. Thus, we demonstrated that the chloride channel blockers 9-AC and SITS, which have no cardiosuppressive effects, exert protective effects against myocardial ischemia-reperfusion damage.