FLICKERY BLOCK OF SINGLE CFTR CHLORIDE CHANNELS BY INTRACELLULAR ANIONS AND OSMOLYTES

Cystic fibrosis transmembrane conductance regulator (CFTR) is a phosph orylation- and nucleotide-dependent chloride channel. Single CFTR curr ents recorded on cell show slight outward rectification, which has pre viously been suggested to be due to an asymmetrical chloride ion gradi ent or to a specific interaction between permeant intracellular anions and the channel. Using a single-channel recording from Chinese hamste r ovary cells stably expressing CFTR, we have found that both the spar ingly permeant anion glutamate and the impermeant anion gluconate caus e a rapid, voltage-dependent block of CFTR channels when applied to th e intercellular, but not the extracellular, face of excised patches. B oth the affinity and the voltage dependence of block were affected by the extracellular chloride concentration in a manner consistent with c hloride ions being able to repel these blocking ions from the pore. Th ese results are discussed in terms of previous models of CFTR current outward rectification, and it is suggested that this rectification may result from a combination of asymmetrical chloride concentrations and voltage-dependent block of the channel by large cytoplasmic anions. I n addition, we find that CFTR conductance is decreased by high concent rations of intracellular sucrose, sorbitol, and urea in a manner consi stent with a rapid block of the channel by these molecules.