Cyanide inhibition of chloride conductance across toad skin

The effect of cyanide (CN-) on voltage-activated or cAMP-induced passive ch loride conductance (G(Cl)) was analyzed in isolated toad skin. Comparativel y low concentrations of CN- inhibited G(Cl) almost completely and fully rev ersibly, regardless of whether it was applied from the mucosal or serosal s ide. The IC50 was 180 +/- 12 mu M for voltage-activated G(Cl) and 305 +/- 3 0 mu M for the cAMP-inducted conductance. At [CN] <100 mu M, the initial in hibition frequently declined partly in the continuous presence of CN-. Inhi bition was independent of the presence of Ca2+. Inhibition was stronger at more alkaline pH, which suggests that dissociated CN- is the effective inhi bitor. The onset of the inhibition of voltage-activated or cAMP-induced G(C l) by CN- occurred with half-times of 34 +/- 10 sec, whereas reversibility upon washout was twice as fast (18 +/- 7 sec). If [CN-] <200 mu M (w) over bar as 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 partial ly in the continuous presence of CN-. Dinitrophenol inhibited G(Cl) similar ly, 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 t he 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 s ite remain to be identified.