Electrogenic Cl(-) secretion does not occur in the ileum of the Australian common brushtail possum, Trichosurus vulpecula, due to low levels of expression of the NaK2Cl cotransporter, NKCC1

cAMP-dependent secretagogues stimulate the NaHCO3 cotransporter in the villous epithelium of the brushtail possum, Trichosurus vulpecula

In the ileum of the brushtail possum, Trichosurus vulpecula, fluid secretion appears to be driven by electrogenic HCO₃^- secretion. Consistent with this, the cystic fibrosis transmembrane conductance regulator is expressed in the apical membrane of the ileal epithelial cells and the pancreatic or secretory variant of the NaHCO₃ cotransporter in the basolateral membrane. This suggests that in the possum ileum, electrogenic HCO₃^- secretion is driven by basolateral NaHCO₃ cotransporter (NBC) activity. To determine if the NBC contributes to HCO₃^- secretion in the possum ileum, intracellular pH (pHi) measurements in isolated villi were used to demonstrate NBC activity in the ileal epithelial cells and investigate the effect of cAMP-dependent secretagogues. In CO₂/HCO₃^--free solutions, recovery of the epithelial cells from an acid load was Na⁺-dependent and ≈80% inhibited by ethyl-isopropyl-amiloride (EIPA, 10 µmol L^-1), indicative of the presence of an Na⁺/H⁺ exchanger, most likely NHE1. However, in the presence of CO₂/HCO₃^-, EIPA only inhibited ≈ 50% of the recovery, the remainder was inhibited by 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS, 500 µmol L^-1), indicative of NBC activity. Under steady-state conditions, NHE1 inhibition by EIPA had little effect on pHi in the presence or absence of secretagogues, but NBC inhibition with DIDS resulted in a rapid acidification of the cells, which was increased fivefold by secretagogues. These data demonstrate the functional activity of an NaHCO₃ cotransporter in the ileal epithelial cells. Furthermore, the stimulation of NBC activity by secretagogues is consistent with the involvement of an NaHCO₃ cotransporter in electrogenic HCO₃^- secretion.

CFTR is restricted to a small population of high expresser cells that provide a forskolin-sensitive transepithelial Cl- conductance in the proximal colon of the possum, Trichosurus vulpecula

The cystic fibrosis transmembrane conductance regulator (CFTR) is central to anion secretion in both the possum and eutherian small intestine. Here, we investigated its role in the possum proximal colon, which has novel transport properties compared with the eutherian proximal colon. Despite considerable CFTR expression, high doses of the CFTR activator forskolin (EC(50)≈10 μmol l(-1)) were required for a modest, CFTR-dependent increase in short-circuit current (I(sc)) in the proximal colon. Presumably, this is because CFTR is restricted to the apical membrane of a small population of CFTR high expresser (CHE) cells in the surface and upper crypt epithelium. Furthermore, although the forskolin-stimulated I(sc) was dependent on serosal Na(+), Cl(-) and HCO(3)(-), consistent with anion secretion, inhibition of the basolateral Na-K-2Cl(-) (NKCC1) or Na-HCO(3) (pNBCe1) cotransporters did not prevent it. Therefore, although NKCC1 and pNBCe1 are expressed in the colonic epithelium they do not appear to be expressed in CHE cells. At low doses (IC(50)≈1 μmol l(-1)), forskolin also decreased the transepithelial conductance (G(T)) of the colon through inhibition of a 4,4'-diisothiocyano-2,2'-stilbenedisulphonic acid-sensitive anion conductance in the basolateral membrane of the CHE cells. This conductance is arranged in series with CFTR in the CHE cells and, therefore, the CHE cells provide a transepithelial Cl(-) conductance for passive Cl(-) absorption across the epithelium. Inhibition of the basolateral Cl(-) conductance of the CHE cells by forskolin will inhibit Na(+) absorption by restricting the movement of its counter-ion Cl(-), assisting in the conversion of the tissue from an absorptive to a secretory state.

The distribution and expression of CFTR restricts electrogenic anion secretion to the ileum of the brushtail possum, Trichosurus vulpecula

In eutherian mammals, fluid secretion is essential for intestinal function. This is driven by electrogenic Cl(-) secretion, which involves a NaK2Cl cotransporter (NKCC1) in the enterocyte basolateral membrane and the cystic fibrosis transmembrane conductance regulator (CFTR) in the apical membrane. However, in the possum ileum, NKCC1 expression is low and secretagogues stimulate electrogenic HCO(3)(-) secretion driven by a basolateral NaHCO(3) cotransporter (pNBCe1). Here we investigated whether electrogenic anion secretion occurs in possum duodenum and jejunum and determined the role of CFTR in possum intestinal anion secretion. Prostaglandin E(2) (PGE(2)) and forskolin stimulated a large increase in ileal short-circuit current (I(sc)), consistent with electrogenic HCO(3)(-) secretion, but had little effect on the duodenal and jejunal I(sc). Furthermore, 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) and N-(2-naphthalenyl)-[(3,5-dibromo-2,4-dihydroxyphenyl)methylene]glycine hydrazide (GlyH101) inhibited cloned possum CFTR in cultured cells and the PGE(2)-stimulated ileal I(sc), implicating CFTR in ileal HCO(3)(-) secretion. Consistent with this, CFTR is expressed in the apical membrane of ileal crypt and lower villous cells, which also express pNBCe1 in the basolateral membrane. In contrast, duodenal and jejunal CFTR expression is low relative to the ileum. Jejunal pNBCe1 expression is also low, whereas duodenal and ileal pNBCe1 expression are comparable. All regions have low NKCC1 expression. These results indicate that cAMP-dependent electrogenic Cl(-) secretion does not occur in the possum small intestine because of the absence of CFTR and NKCC1. Furthermore, CFTR functions as the apical anion conductance associated with HCO(3)(-) secretion and its distribution limits electrogenic HCO(3)(-) secretion to the ileum.

Molecular and functional characterization of the cystic fibrosis transmembrane conductance regulator from the Australian common brushtail possum, Trichosurus vulpecula

Unlike eutherian mammals, the colon of the Australian common brushtail possum, Trichosurus vulpecula, a metatherian mammal, is incapable of electrogenic Cl(-) secretion and has elevated levels of electrogenic Na(+) absorption, while the ileum secretes HCO (3) (-) rather than Cl(-). In eutherian mammals, the cystic fibrosis transmembrane conductance regulator (CFTR) is essential for both Cl(-) and HCO (3) (-) secretion and the regulation of Na(+) absorption. Therefore, we have sequenced possum (p)CFTR, described its distribution and characterized the properties of cloned pCFTR expressed in Fischer rat thyroid (FRT) cells. pCFTR (GenBank accession No. AY916796) has a 1,478 amino acid open reading frame, which has >90% identity with CFTR from other marsupials and >80% identity with non-rodent eutherian mammals. In pCFTR, there is a high level of conservation of the transmembrane and nucleotide binding domains although, with the exception of other marsupials, there is considerable divergence from other species in the R domain. FRT cells transfected with pCFTR express mature CFTR protein which functions as a small Cl(-) channel activated by cAMP-dependent phosphorylation. In whole-cell recordings it has a linear, time and voltage-independent conductance, with a selectivity sequence P(Br) > P(Cl) > P(I) > P(HCO)(3) >> P(Gluconate). pCFTR transcript is present in a range of epithelia, including the ileum and the colon. The presence of pCFTR in the ileum and its measured HCO (3) (-) permeability suggest that it may be involved in ileal HCO (3) (-) secretion. Why the possum colon does not secrete Cl(-) and has elevated electrogenic Na(+) absorption, despite the apparent expression of CFTR, remains to be determined.

Secretagogues stimulate electrogenic HCO3- secretion in the ileum of the brushtail possum, Trichosurus vulpecula: evidence for the role of a Na+/HCO3- cotransporter

Fluid secretion is essential for intestinal function and, in eutherian mammals, is driven by electrogenic Cl(-) transport, which is dependent upon a bumetanide-sensitive, basolateral Na(+)/K(+)/2 Cl(-) cotransporter, NKCC1. However, ileal secretion in the brushtail possum, a marsupial, involves a fundamentally different process, since NKCC1 expression is low in this tissue and the secretagogue-induced short circuit current (I(sc)) is insensitive to bumetanide. In view of these differences we have investigated the basis of the secretory response of the possum ileum. In the Ussing chamber the secretory I(sc) is independent of Cl(-) but dependent upon Na(+) and serosal HCO(3)(-)/CO(2), suggesting that secretagogues stimulate electrogenic HCO(3)(-) secretion. In agreement with this, serosal DIDS (4,4'-diisothiocyano-stilbene-2,2'-disulfonate; 1 mmol l(-1)) inhibited the secretory response. However, acetazolamide (1 mmol l(-1)) and serosal amiloride (1 mmol l(-1)) had little effect, indicating that HCO(3)(-) secretion is driven by HCO(3)(-) transport from the serosal solution into the cell, rather than hydration of CO(2) by carbonic anhydrase. Consistent with this the pancreatic variant of the electrogenic Na(+)/HCO(3)(-) cotransporter (pNBC) is highly expressed in the ileal epithelium and is located in the basolateral membrane of the epithelial cells, predominantly in the mid region of the villi, with lower levels of expression in the crypts and no expression in the villous tips. We conclude that the secretory response of the possum ileum involves electrogenic HCO(3)(-) secretion driven by a basolateral pNBC and that the ileal HCO(3)(-) secretion is associated with a specialised function of the possum ileum, most probably related to hindgut fermentation.