Segmental differences in ion transport in rat cecum.

Ion-transport properties of the epithelium of the cecum, the biggest fermental chamber in non-ruminant species, are largely unknown. Recently, in Ussing chamber experiments, segmental differences in basal short-circuit current (Isc) in rat corpus ceci were observed. The oral segment usually exhibited a much lower or even negative basal Isc in comparison with the aboral segment. The aim of the present study was the closer characterization of these differences. Basal Isc was inhibited by bumetanide and tetrodotoxin in both segments, whereas indomethacin reduced basal Isc only in the aboral corpus. Amiloride did not inhibit basal Isc suggesting that spontaneous anion secretion (but not electrogenic Na+ absorption via ENaC) contributes to the baseline current. In both segments, mucosally applied K+ channel blockers increased Isc indicating a spontaneous K+ secretion. Basolateral depolarization was used to characterize the ion conductances in the apical membrane. When a Cl- gradient was applied, apical Cl- conductance stimulated by carbachol and by forskolin was revealed. When the Cl- gradient was omitted and instead a K+ gradient was used to drive currents across apical K+ channels, a Ba2+-sensititve K+ conductance was observed in both segments, and carbachol stimulated this conductance leading to a negative Isc. Conversely, forskolin induced a positive Isc under these conditions which was dependent on the presence of mucosal Na+ consistent with electrogenic Na+ absorption. This current was reduced by amiloride and several blockers of members of the TRP channel superfamily. These results indicate that similar transport mechanisms are involved in electrogenic ion transport across cecal oral and aboral segments, but with a higher spontaneous prostaglandin production in the aboral segment responsible for higher basal transport rates of both anions and cations.