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Outwardly rectifying swelling-activated chloride conductance (lCl,Swell) in rabbit heart plays a critical role in cardioprotection following ischemic preconditioning (IP). But the functional characterization and molecular basis of this chloride conductance in rabbit heart ventricular myoeytes is not clear. Candidate chloride channel clones (e.g. ClC-2, ClC-3, CIC-4 and CIC-5) were deter- mined using RT-PCR and West blot analysis.Whole cell ICl,Swell was recorded from isolated rabbit ventricular myoeytes using patch clamp techniques during hypo-osmotic stress. The inhibitory effects of 4,4′ isothiocyanato-2,2-disulfonic acid (DIDS), 5-nitro-2(3-phenylroylamino) benzoic acid (NPPB) and indanyloxyacetic acid 94 (IAA-94) on ICl,Swell were examined. The expected size of PCR products for ClC-2, ClC-3 and ClC-4 but not for ClC-5 was obtained. ClC-2 and ClC-3 expression was con- firmed by automated fluorescent DNA sequencing. RT-PCR and West blot showed that ClC-4 was expressed in abundance and ClC-2 was expressed at somewhat lower levels. The biological and pharmacological properties of ICl,Swell including outward rectification, activation due to cell volume change, sensitivity to DIDS, IAA-94 and NPPB were identical to those known properties of ICl,Swell in exogenously expressed systems and other mammals hearts. It was concluded that ClC-3 or ClC-4 might be responsible for the outwardly rectifying part of ICl,Swell and may be the molecular targets of cardioprotection associated with ischemic preconditioning or hypo-osmotic shock.