Ionic dependency, sensitivity to transport inhibitors and signal transduction of propionate-induced anion secretion in rat caecum.

BACKGROUND

Although the caecum is the most important fermentation chamber in non-ruminant animals, the epithelial responses evoked by microbially produced short-chain fatty acids such as propionate are not well understood.

METHODS

Short-circuit currents (Isc) across rat caecum were measured in Ussing chamber experiments, where anionic conditions, transport mechanisms and underlying signaling pathways regarding propionate-induced Isc (IProp) were analyzed. These findings were complemented with immunofluorescence and RT-PCR.

MAIN

RESULTS

IProp was dependent on the simultaneous presence of Cl- and HCO3-. This contrasts with the Isc evoked by the cAMP- or Ca2+-dependent secretagogues, forskolin and carbachol, where replenishment of Cl- was sufficient to induce a strong increase in Isc. Blocking the cystic fibrosis transmembrane regulator (CFTR) with CFTRinh-172 inhibited IProp in the oral caecum, whereas in the aboral caecum the additional presence of 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS), an inhibitor of anion exchangers, was necessary to reduce IProp significantly. Since propionate can activate free-fatty acid receptors type 2 (coupled to Gq and Gi proteins) and 3 (coupled to Gi proteins), downstream signaling was further elucidated. Inhibition of Gqα, phospholipase C, inositol-3,4,5-trisphosphate receptors, or phosphatidyl inositol kinases reduced IProp significantly. Kinases of the 'with no lysine' family (WNK), known to mediate the switch between Cl- to HCO3- secretion in other cells, are also involved in propionate-induced anion secretion, since IProp was reduced by panWNK and WNK1 inhibition.

CONCLUSION

Propionate stimulates anion secretion, probably in order to buffer the caecal luminal content, by a mechanism exhibiting a unique anion dependency different from that of other secretagogues.