Calcium-mediated agonists activate an inwardly rectified K+ channel in colonic secretory cells.
Single-channel recording techniques were used to identify and characterize the K+ channel activated by Ca(2+)-mediated secretory agonists in T84 cells. Carbachol (CCh; 100 microM) and taurodeoxycholate (TDC; 0.75 mM) stimulated oscillatory outward K+ currents. With K gluconate in bath and pipette, cell-attached single-channel K+ currents stimulated by CCh and ionomycin (2 microM) were inwardly rectified and reversed at 0 mV. The single-channel chord conductance was 32 pS at -90 mV and 14 pS at +90 mV. Similar properties were observed in excised inside-out patches in symmetric K+, permitting further characterization of channel properties. Partial substitution of bath or pipette K+ with Na+ gave a K(+)-to-Na+ selectivity ratio of 5.5:1. Channel activity increased with increasing bath Ca2+ concentration in the physiological range of 50-800 nM. Maximal channel activity occurred at intracellular pH 7.2 and decreased at more acidic or alkaline pH values. Extracellular charybdotoxin (CTX; 50 nM) blocked inward but not outward currents. Extracellular tetraethylammonium (TEA; 10 mM) reduced single-channel amplitude at all voltages. No apparent block of the channel was observed with extracellular Ba2+ (1 mM), apamin (1 microM), 4-aminopyridine (4-AP; 4 mM), quinine (500 microM), or glyburide (10 microM). Cytosolic quinine and 4-AP blocked both inward and outward currents, whereas Ba2+ blocked only outward currents. Apamin, CTX, TEA, and glyburide did not affect channel activity. The agonist activation and pharmacological profile of this inwardly rectified K+ channel indicate that it is responsible for the increase in basolateral K+ conductance stimulated by Ca(2+)-mediated agonists in T84 cells.
Devor
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Department of Physiology and Biophysics, University of Alabama at Birmingham 35294.
Frizzell
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D015761:4-Aminopyridine / Q000494:pharmacology; D000818:Animals; D001030:Apamin / Q000494:pharmacology; D001464:Barium / Q000494:pharmacology; D002118:Calcium / Q000502:physiology*; D002460:Cell Line; D018999:Charybdotoxin; D003106:Colon / Q000187:drug effects / Q000502:physiology*; D003600:Cytosol / Q000502:physiology; D005905:Glyburide / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D015221:Potassium Channels / Q000187:drug effects / Q000502:physiology*; D011803:Quinine / Q000494:pharmacology; D012604:Scorpion Venoms / Q000494:pharmacology; D012964:Sodium / Q000494:pharmacology; D019789:Tetraethylammonium; D013757:Tetraethylammonium Compounds / Q000494:pharmacology; D013997:Time Factors
D015221:Potassium Channels; D012604:Scorpion Venoms; D013757:Tetraethylammonium Compounds; D018999:Charybdotoxin; D001030:Apamin; D001464:Barium; D019789:Tetraethylammonium; D012964:Sodium; D011803:Quinine; D015761:4-Aminopyridine; D005905:Glyburide; D002118:Calcium