THE BASOLATERAL CA2-DEPENDENT K+ CHANNEL IN RAT COLONIC CRYPT CELLS()

Previous studies have indicated that a 16-pS K+ channel (KCca) in the basolateral membrane is responsible for the acetylcholine-induced whol e-cell K+ conductance in these cells. In the present study we have exa mined this channel in excised inside-out patches of the basolateral me mbrane. Over a wide voltage range this channel showed inward rectifica tion. The Ca2+ sensitivity was very marked, with a Hill coefficient of three and with half-maximal activation at 330 nmol/l. After several m inutes most channels showed a slow run-down. Channel activity could be refreshed by addition of ATP (1 mmol/l) to the bath solution. The non -metabolizable derivative 5'-adenylylimidodiphosphate (AMP-PNP) had no such effect. In contrast, it inhibited channel activity by some 50%. ATP and its derivatives had no effect on the Ca2+ sensitivity. Channel s activated by ATP were subsequently studied in the presence of alkali ne (10 kU/l) or acidic (1 kU/l) phosphatase. Both phosphatases reduced channel activity significantly. These data suggest that the 16-pS Kchannel is directly controlled by cytosolic Ca2+. This regulatory step is probably distal to an activation produced by protein-kinase-C-depe ndent phosphorylation. As is the case for several other K+ channels, h igh concentrations of non-metabolizable ATP analogues inhibit this cha nnel.