CA2-LINE HT29-CL.16E( AND CAMP ACTIVATE DIFFERENT K+ CONDUCTANCES IN THE HUMAN INTESTINAL GOBLET CELL)

The mechanism of regulated Cl- secretion was evaluated in the mucin-se creting cell line HT29-Cl.16E by transepithelial electrophysiology and fura 2 measurements of cytosolic Ca2+. Carbachol by itself was a weak secretagogue, but augmented adenosine 3',5'-cyclic monophosphate (cAM P)-mediated secretion more than twofold, consistent with activation of a rate-limiting K+ conductance. To characterize this conductance, mon olayers were apically permeabilized with amphotericin B. At least two types of K+ conductances were identified. One type was activated by el evated cytosolic cAMP levels and inhibited by Ba2+ (inhibitor constant 0.3 mM) in the basolateral solution but was not affected by quinidine or elevated cytosolic Ca2+. The other type was activated by carbachol via cytosolic Ca2+ and was partially inhibited by quinidine (60% inhi bition by 2.5 mM quinidine) but was not affected by Ba2+ up to 1 mM. B oth conductances appear to be involved in active, transepithelial Cl- secretion in intact monolayers but under different conditions because 1) the cAMP-stimulated short-circuit current (I-sc) can be partially i nhibited by 1 mM Ba2+ (50%) but not quinidine, 2) the Ba2+ inhibition does not affect the carbachol-induced increase in I-sc in cells with e levated cAMP levels, and 3) the carbachol-dependent I-sc can be inhibi ted by quinidine. Therefore, the contribution of the cAMP-dependent K conductance appears important for maintaining the membrane potential and therewith Cl- secretion when cAMP is the only messenger for secret ion signals, whereas the Ca2+-dependent K+ conductance is responsible for the carbachol-stimulated increase in I-sc.