Guanylin strongly stimulates rat duodenal HCO3- secretion: Proposed mechanism and comparison with other secretagogues

Background & Aims: Guanylin and heat-stable enterotoxin (STa) stimulate intestinal Cl- secretion via activation of the cystic fibrosis transmembrane regulator (CFTR)-encoded Cl- channel, it was speculated that CFTR activation also regulates electrogenic duodenal HCO3- secretion. Therefore, the effect of guanylin/STa and other secretagogues on rat duodenal HCO3- secretion was studied, Methods: The HCO3- secretory rate of in vitro rat proximal duodenum was determined by pH stat titration and paracellular permeability by H-3-mannitol fluxes, bidirectional Cl-36(-) fluxes were measured, and the short-circuit current (Isc) was recorded, Results: Luminal guanylin and STa concentration dependently stimulated the HCO3- secretory rate and Isc, Guanylin-stimulated HCO3- secretion was independent of luminal Cl, inhibited by the CI channel blocker 5-nitro-2-(3-phenylpropylamino)benzoate, and additive to the HCO3- secretory rate stimulated by glucagon and carbachol but not by the tested adenosine 3',5'-cyclic monophosphate (cAMP)-dependent agonists. The ratio of the HCO3- secretory rate/Isc stimulated by the tested guanosine 3',5'-cyclic monophosphate (cGMP)-dependent agonists was markedly higher than the cAMP-dependent agonists. Prostaglandin E(2) and 8-bromo-cAMP but not STa/guanylin also transiently increased paracellular permeability. Conclusions: Guanylin and STa stimulate electrogenic HCO3- secretion in rat duodenum, most likely vie CFTR Cl- channel activation, but the different relationship for HCO3- to Isc in cGMP- than in cAMP-stimulated anion secretion suggests a different cellular source and/or signaling pathways.