EVIDENCE AGAINST DIRECT ACTIVATION OF CHLORIDE SECRETION BY CARBACHOL IN THE RAT DISTAL COLON

Carbachol (5 x 10(-5) mol.l(-1)) induced a biphasic increase in short-circuit current (Isc) consisting of an initial peak phase followed by a long-lasting plateau. Complete dependence on the presence of Cl- ions and sensitivity to bumetanide confirmed that carbachol induces Cl- secretion. The plateau phase was blocked by indomethacin, and both the plateau and the peak phase were suppressed in the combined presence of indomethacin and tetrodotoxin. Inhibition of the carbachol response could be overcome by agonists of the cAMP pathway like prostaglandin E(2), forskolin or 8-(4-chlorophenylthio)-adenosine-3',5'-cyclic monophosphate. The increase in Isc was inhibited by a blocker of cAMP-activated Cl- channels, glibenclamide, but was resistant to an inhibitor of Ca2+-activated Cl- channels, 4,4-'diisothiocyanato-stilbene-2,2'-disulfonic acid (DIDS). The K+ channel blockers Ba2+ and charybdotoxin inhibited the first and suppressed the second phase of the carbachol response, whereas a less specific K+ channel blocker, quinine, suppressed both phases. These results suggest that the dominant effect of carbachol in the intact colonic mucosa is an opening of Ca2+-dependent, charybdotoxin- and Ba2+-sensitive K+ channels, which leads to hyperpolarization of the epithelial cells. This stimulates Cl- secretion only if there are spontaneously open apical Cl- channels which are basically stimulated by a continuous release of neurotransmitters and prostaglandins. Only during the first phase of the carbachol effect is there indirect evidence for activation of a Cl- conductance synergistically with the cAMP pathway as shown by the increase in tissue conductance resistant to K+ channel blockers.