Effects of sulfhydryl compounds on cell injuries caused by hypochlorous acid (HOCl) were studied in isolated rat cardiomyocytes by way of continuous measurement of intracellular Ca2+ using calcium sensitive fluorescent dye, fura-2. Ten minutes exposure of the cell to 100 microM HOCl induced a significant increase in the intracellular free calcium concentration ([Ca2+]i) from 90 +/- 20 nM to 266 +/- 74 nM (n = 44, mean +/- S.D.). This increase in [Ca2+]i was reversed by subsequent application of dithiothreitol (DTT) in a dose dependent manner; 30 microM DTT was found effective, while 2 mM DTT almost completely restored the [Ca2+]i to the control level. Similar to DTT, cysteine ethyl ester and cysteine methyl ester could also reverse the HOCl-induced rise of [Ca2+]i where the order of the potency was DTT > cysteine ethyl ester > cysteine methyl ester. In contrast, increase in [Ca2+]i induced by HOCl was not recovered by application of other sulfhydryl compounds such as cysteine and glutathione. Since HOCl administrated in the presence of cysteine failed to induce an increase in [Ca2+]i, we conclude that membrane permeable sulfhydryl compounds may reverse the increase in [Ca2+]i caused by HOCl.