CALCIUM-MEDIATED AGONISTS ACTIVATE AN INWARDLY RECTIFIED K-CELLS( CHANNEL IN COLONIC SECRETORY)

Single-channel recording techniques were used to identify and characte rize the K+ channel activated by Ca2+-mediated secretory agonists in T 84 cells. Carbachol (CCh; 100 muM) 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 C Ch and ionomycin (2 muM) 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 patche s in symmetric K+, permitting further characterization of channel prop erties. Partial substitution of bath or pipette K+ with Na+ gave a K+- to-Na+ selectivity ratio of 5.5:1. Channel activity increased with inc reasing bath Ca2+ concentration in the physiological range of 50-800 n M. Maximal channel activity occurred at intracellular pH 7.2 and decre ased at more acidic or alkaline pH values. Extracellular charybdotoxin (CTX; 50 nM) blocked inward but not outward currents. Extracellular t etraethylammonium (TEA; 10 mM) reduced single-channel amplitude at all voltages. No apparent block of the channel was observed with extracel lular Ba2+ (1 mM), apamin (1 muM), 4-aminopyridine (4-AP; 4 mM), quini ne (500 muM), or glyburide (10 muM). Cytosolic quinine and 4-AP blocke d both inward and outward currents, whereas Ba2+ blocked only outward currents. Apamin, CTX, TEA, and glyburide did not affect channel activ ity. The agonist activation and pharmacological profile of this inward ly rectified K+ channel indicate that it is responsible for the increa se in basolateral K+ conductance stimulated by Ca2+-mediated agonists in T84 cells.