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.