Characterisation of HCO3- secretion in the small intestine of the brushtail possum, Trichosurus vulpecula

Abstract

The Australian common brushtail possum, (Trichosurus vulpecula), is the most significant mammalian pest in New Zealand. The damage done by wild possums has a major impact on New Zealand flora and fauna through destruction of habitats, competitive encroachment on native ecosystems, and as a vector for bovine tuberculosis. The intestinal physiology of the possum, a metatherian mammal, differs markedly to that of eutherian (placental) mammals. This will potentially affect the efficacy of oral toxins designed for use in the control of the possum, but in addition, could potentially provide targets for the development of a possum-specific toxin. In eutherian mammals intestinal fluid secretion is driven by electrogenic Cl- secretion, with secondary HCO3- secretion a vital component of intestinal function. However, electrogenic Cl- secretion does not occur in the ileum of the brushtail possum, instead electrogenic HCO3- secretion appears to provide the driving force for fluid secretion. The Ussing short-circuit (ISC) technique revealed the duodenum and jejunum both lack the electrogenic response to the cAMP dependent secretagogues prostaglandin E2 (PGE2) (∆ISC =2±5 µA.cm-2, and ∆ISC =14±5 uA.cm-2, respectively) seen in the possum ileum (94±7 µA.cm-2). Similar responses were also seen with direct activation of adenylyl cyclase by the agonist forskolin. This is due to an absence of NaK2Cl cotransporter (NKCC1) in the small intestine, with low CFTR expression in the duodenum and jejunum compared with ileum. High levels of NaHCO3 cotransporter (NBC) expression in the ileum and duodenum indicate the potetnial for HCO3- secretion in these tissues, with the combination of CFTR and pNBC expression in the ileum accounting for electrogenic HCO3- secretion which only occurs in the distal region of the small intestine. In addition, high levels of transcript for the Cl-/HCO3- exchangers SLC26A3 (DRA) and -A6 (PAT-1) also occur in the duodenum and ileum. In the ileum immunohistochemistry localised NBC to the basolateral membrane of the villous and crypt regions and DRA and PAT-1 to the apical membrane of the villous and crypts, consistent with a proposed role for electroneutral Cl-/HCO3- exchange. However, In the duodenum NBC was predominantly restricted to the villi tip, while both PAT-1 and DRA were largely identified in intracellular vesicle-like structures, which suggests the possibility of membrane insertion and removal as a mechanism of transporter regulation in this tissue. The pH stat technique measurement of serosal-to-mucosal HCO3- flux (JHCO3sm) in the ileum and comparison to the change in current resulting from PGE2 addition, demonstrated that mucosal [HCO3-] was found to have a marked effect on the ΔISC, the electrogenic pathway being greatly enhanced under physiological conditions of high mucosal [HCO3-], and reduced in zero mucosal HCO3-. The results of this study show the possum intestinal physiology is markedly different to that of other eutherians. Unlike eutherian intestine, Cl- secretion does not occur in the possum and HCO3- is the dominant secretory anion, resulting in a high luminal [HCO3-] and pH compared to mammalian intestine.