The bumetanide-resistant part of forskolin-induced anion secretion in rat colon.

In order to reveal the contribution of the Na(+)-K(+)-2 Cl(-)- cotransporter to forskolin-induced anion secretion, the inhibition of forskolin-stimulated short-circuit current (Isc) by bumetanide, furosemide, azosemide and piretanide was investigated. In the distal colon, all blockers inhibited the forskolin-stimulated Isc with a maximal efficiency of 70%. In contrast, in the proximal colon, bumetanide and furosemide inhibited only about 40% of the forskolin response, whereas piretanide and azosemide were ineffective. A similar result was observed, when Na+ was replaced by impermeant cations suggesting especially in the proximal colon a great part of forskolin-induced anion secretion to be independent of the Na(+)-K(+)-2 Cl(-)-cotransporter. In anion substitution experiments the forskolin-induced increase in Isc was reduced by 70-80%, if either Cl- or HCO3- were omitted from the buffer solution, whereas in the combined absence of both anions the response was nearly suppressed. Measurement of the mucosal alkalinization revealed that forskolin stimulated a HCO3- secretion, which was, however, too weak to explain the bumetanide-insensitive Isc induced by forskolin. Bumetanide inhibited the serosa-to-mucosa flux of Cl- (JsmCl) stimulated by forskolin; an effect, which was strongly enhanced by subsequent administration of the anion exchange inhibitor, SITS. These data suggest that the bumetanide-resistant part of the forskolin-induced Isc is mainly mediated by a basolateral anion exchanger, probably a Cl(-)-HCO3- exchanger, which contributes to forskolin-evoked Cl- secretion, and in addition by a small HCO3- secretion stimulated by the drug.