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Frusemide can be used as an antiasthma drug and appears to inhibit the release (conditioned by activation of Cl -channels) of mast cell proinflammatory mediators. We studied the cause of the effects of frusemide, checking its action on Cl -channels.The patch-clamp technique was used to study single-channel currents, and differences in electrical potential of the cellular membrane of rat peritoneal mast cells were measured.In inside-out configuration, outwardly-rectifying Cl -channels were identified whose conductance was 2.4/1.7 pS at positive and negative voltages. The results of this study suggest that frusemide can inhibit mast cell Cl -channels only via an indirect mechanism, which probably involves an inhibition of a Na + -K + -2Cl -symport.

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