The effect of cyanide (CN−) on voltage-activated or cAMP-induced passive chloride conductance (G Cl ) was analyzed in isolated toad skin. Comparatively low concentrations of CN− inhibited G Cl almost completely and fully reversibly, regardless of whether it was applied from the mucosal or serosal side. The IC50 was 180 ± 12 μm for voltage-activated G Cl and 305 ± 30 μm for the cAMP-inducted conductance. At [CN] <100 μm, the initial inhibition frequently declined partly in the continuous presence of CN−. Inhibition was independent of the presence of Ca2+. Inhibition was stronger at more alkaline pH, which suggests that dissociated CN− is the effective inhibitor. The onset of the inhibition of voltage-activated or cAMP-induced G Cl by CN− occurred with half-times of 34 ± 10 sec, whereas reversibility upon washout was twice as fast (18 ± 7 sec). If [CN−] <200 μm was applied under inactivating conditions (serosa −30 mV), the reduction of G Cl was stronger upon subsequent voltage-activation than under steady-state activated conditions. This effect was essentially complete less than 30 sec after apical addition of CN−, but G t recovered thereafter partially in the continuous presence of CN−. Dinitrophenol inhibited G Cl similarly, while omission of oxygen did not affect it. These observations, as well as the time course of inhibition and the full reversibility, suggest that interference of CN− with oxidative phosphorylation and subsequent metabolic depletion is not the reason for the inhibition of G Cl . We propose that the inhibition is directly on G Cl , presumably by competition with Cl− at a rate-limiting site in the pathway. Location and molecular nature of this site remain to be identified.
The Journal of Membrane Biology – Springer Journals
Published: Jan 15, 2000
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