Cyanide inhibition of chloride conductance across toad skin

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 IC(50) was 180 +/- 12 microm for voltage-activated G(Cl) and 305 +/- 30 microm for the cAMP-inducted conductance. At [CN] <100 microm, the initial inhibition frequently declined partly in the continuous presence of CN(-). Inhibition was independent of the presence of Ca(2+). 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 microm 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.