Poorly Selective Cation Channels in Apical Membranes of Epithelia

Summary

The apical membrane of frog skin contains two types of pathways which allow the passage of several monovalent cations in the absence of external Ca²⁺. Differences between the two pathways concern their open-close kinetics, selectivity, and the affinity for several blocking agents. Type S channels open and close relatively slowly, whereas type F channels display fast open-close kinetics. Both channel types allow the passage of Na⁺, K⁺, and Rb⁺ currents which are blocked by divalent cations and La³⁺ added to the extracellular side. Type F channels are permeable for Cs⁺ which is, however, excluded from type S channels. Shifts in open-close kinetics induced by Mg²⁺ occur at concentrations below 5 μM for type F channels, whereas more than a tenfold higher dose is required for the type S pathway. UO₂ ²⁺ concentrations up to 100 μM only occlude type S channels while 100 μM tetracaine selectively blocks type F channels.

Apical membranes of toad urinary bladder, cultured amphibian renal epithelia (A6), and toad colon contain only type F channels. In toad bladder and A6 cells volume expansion strongly activates this pathway. Macroscopic currents carried by Ba²⁺ and Ca²⁺ could be recorded after activation of toad bladders with oxytocin and treatment of the apical surface with nanomolar concentrations of Ag⁺, which seems to interact with a site located at the channel interior.