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) increased outward currents in cells with amphotericin B-perforated patches. ATP (0.5-1 mM) did not affect whole cell currents of cells dialyzed with solutions containing ethylene glycol-bis(β-aminoethyl ether)-N,N,N',N'-tetraacetic acid. Apamin (3 x 10^-7 M) reduced the outward current activated by ATP by 32 ± 5%. Single channel recordings from cell-attached patches showed that ATP, UTP, and 2-MeS-ATP increased the open probability of small-conductance, Ca²⁺- dependent K⁺ channels with a slope conductance of 5.3 ± 0.02 pS. Caffeine (500 μM) enhanced the open probability of the small-conductance K⁺ channels, and ATP had no effect after caffeine. Pyridoxal phosphate 6- azophenyl-2',4'-disulfonic acid tetrasodium (PPADS, 10^-4 M), a nonselective P₂ receptor antagonist, prevented the increase in open probability caused by ATP and 2-MeS-ATP. PPADS had no effect on the response to caffeine. ATP- induced hyperpolarization in the murine colon may be mediated by P(2y)- induced release of Ca²⁺ from intracellular stores and activation of the 5.3-pS Ca²⁺-activated K⁺ channels.