title: Blocking agents of Ca2+-activated K+ channels in cultured medullary thick ascending limb cells.
abstract: Ca2+-activated K+ channels with estimated single channel conductances of 127 +/- 2 pS were identified in the apical cell membrane of clone A3 of cultured medullary thick ascending limb (MTAL) cells. Both Ba2+ and the scorpion toxin, charybdotoxin (CTX), are slow blockers of the channels. An application of 0.1 microM Ba2+ to the intracellular face caused a 50% reduction in fractional open time (fv). Ba2+ block is both concentration and voltage dependent. Concentrations of CTX as low as 2 nM in the extracellular solution caused a significant reduction in fv. Tetraethylammonium (TEA) and quinine are fast blockers of Ca2+-activated K+ channels in MTAL cells. TEA, 400 microM, in the extracellular solution caused a voltage-dependent reduction in channel amplitude, whereas it takes 10 mM in the intracellular solution to reduce channel amplitude by 30%. Micromolar amounts of quinine applied to the intracellular face caused the channels to flicker rapidly between open and blocked states. These results suggest that K+ channels in MTAL cells are homologous to those found in muscle cells, and that these blocking agents may be used to probe the nature of K+ conductances in several nephron segments.
mesh_terms: D000818:Animals; D001464:Barium / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology*; D002460:Cell Line; D002462:Cell Membrane / Q000502:physiology; D018999:Charybdotoxin; D004553:Electric Conductivity; D007473:Ion Channels / Q000187:drug effects* / Q000502:physiology; D007684:Kidney Tubules / Q000502:physiology*; D008138:Loop of Henle / Q000502:physiology*; D011188:Potassium / Q000378:metabolism*; D011803:Quinine / Q000494:pharmacology; D011817:Rabbits; D012604:Scorpion Venoms / Q000494:pharmacology; D019789:Tetraethylammonium; D013757:Tetraethylammonium Compounds / Q000494:pharmacology
chemical_list: D007473:Ion Channels; D012604:Scorpion Venoms; D013757:Tetraethylammonium Compounds; D018999:Charybdotoxin; D001464:Barium; D019789:Tetraethylammonium; D011803:Quinine; D011188:Potassium; D002118:Calcium