Effect of frusemide on Cl- channel in rat peritoneal mast cells

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. In cell-attached configuration, the open probability (Po) of the channel increased with depolarization or with the presence of cyclic adenosine monophosphate (cAMP) in the incubation medium. Po increased with a rise of cytoplasmic free calcium concentration [Ca2+] and was inhibited by 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) and by 4-4'-diisothiocyanatoostilbene-2-2'-disulphonic acid (DIDS). These channels seem to be the main cause of mast cell Cl- conductance. Frusemide (10(-5) and 10(-3) M) did not affect Cl- channel activity when using excised patches. In cell-attached configuration experiments, the presence of frusemide (from 10(-5) to 10(-3) M) in the cell incubation medium, increasingly reduced Po (median inhibitory concentration (IC50) = 4.3 x 10(-7) M). In similar conditions, bumetanide also inhibited Po (IC50 = 5.7 x 10(-3) M). The results of this study suggest that frusemide can inhibit mast cell Cl- channels only via an indirect mechanisms, which probably involves an inhibition of a Na(+)-K(+)-2Cl- symport.