title: Poorly selective cation channels in apical membranes of epithelia.
abstract: 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 microM for type F channels, whereas more than a tenfold higher dose is required for the type S pathway. UO2(2+) concentrations up to 100 microM only occlude type S channels while 100 microM 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.
authors.affiliation: Laboratory for Physiology, KU Leuven, Belgium.
authors.forename: W
authors.initials: W
authors.lastname: Van Driessche
authors.forename: L
authors.initials: L
authors.lastname: Desmedt
authors.forename: P
authors.initials: P
authors.lastname: De Smet
authors.forename: J
authors.initials: J
authors.lastname: Simaels
mesh_terms: D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D002024:Bufo marinus; D002118:Calcium / Q000494:pharmacology; D002413:Cations, Divalent / Q000494:pharmacology; D002414:Cations, Monovalent / Q000378:metabolism; D002462:Cell Membrane / Q000187:drug effects / Q000502:physiology*; D003106:Colon / Q000502:physiology; D004848:Epithelium / Q000502:physiology; D066298:In Vitro Techniques; D007473:Ion Channels / Q000037:antagonists & inhibitors / Q000187:drug effects / Q000502:physiology*; D007668:Kidney / Q000502:physiology*; D008274:Magnesium / Q000494:pharmacology; D009130:Muscle, Smooth / Q000502:physiology; D010121:Oxytocin / Q000494:pharmacology; D011896:Rana temporaria; D012879:Skin Physiological Phenomena; D013748:Tetracaine / Q000494:pharmacology; D001743:Urinary Bladder / Q000502:physiology*; D014982:Xenopus laevis
chemical_list: D002413:Cations, Divalent; D002414:Cations, Monovalent; D007473:Ion Channels; D013748:Tetracaine; D010121:Oxytocin; D000584:Amiloride; D008274:Magnesium; D002118:Calcium