SMALL-CONDUCTANCE CA2-DEPENDENT K+ CHANNELS ACTIVATED BY ATP IN MURINE COLONIC SMOOTH-MUSCLE()

The patch-clamp technique was used to determine the ionic conductances activated by ATP in murine colonic smooth muscle cells. Extracellular ATP,UTP, and 2-methylthioadenosine 5'-triphosphate (2-MeS-ATP) increa sed outward currents in cells with amphotericin B-perforated patches. ATP (0.5-1 mM) did not affect whole cell currents of cells dialyzed wi th solutions containing ethylene glycol-bis(beta-aminoethyl ether)-N,N ,N',N'-tetraacetic acid. Apamin(3 x 10(-7) M) reduced the outward curr ent activated by ATP by 32 +/- 5%. Single channel recordings from cell -attached patches showed that ATP, UTP, and 2-MeS-ATP increased the op en probability df small-conductance, Ca2+-dependent K+ channels with a slope conductance of 5.3 +/- 0.02 pS. Caffeine (500 mu M) enhanced th e open probability of the small-conductance K+ channels, and ATP had n o effect after caffeine. Pyridoxal phosphate 6-azophenyl-2',4'-disulfo nic acid tetrasodium (PPADS, 10(-4) M), a nonselective P-2 receptor an tagonist, prevented the increase in open probability caused by ATP and 2-MeS-ATP. PPADS had no effect on the response to caffeine. ATP-induc ed hyperpolarization in the murine colon may be mediated by P-2y-induc ed release of Ca2+ from intracellular stores and activation of the 5.3 -pS Ca2+-activated K+ channels.