Enhanced formation of a HCO3- transport metabolon in exocrine cells of Nhe1-/- mice

Abstract

Cl- influx across the basolateral membrane is a limiting step in fluid production in exocrine cells and often involves functionally linked Cl-/HCO3- (Ae) and Na+/H+ (Nhe) exchange mechanisms. The dependence of this major Cl- uptake pathway on Na+/H+ exchanger expression was examined in the parotid acinar cells of Nhe1-/- and Nhe2-/- mice, both of which exhibited impaired fluid secretion. No change in Cl-/HCO 3- exchanger activity was detected in Nhe2-deficient mice. Conversely, Cl-/HCO3- exchanger activity increased nearly 4-fold in Nhe1-deficient mice, despite only minimal or any change in mRNA and protein levels of the anion exchanger Ae2. Acetazolamide completely blocked the increase in Cl-/HCO3- exchanger activity in Nhe1-null mice suggesting that increased anion exchange required carbonic anhydrase activity. Indeed, the parotid glands of Nhe1 -/- mice expressed higher levels of carbonic anhydrase 2 (Car2) polypeptide. Moreover, the enhanced Cl-/HCO3- exchange activity was accompanied by an increased abundance of Car2·Ae2 complexes in the parotid plasma membranes of Nhe1-/- mice. Anion exchanger activity was also significantly reduced in Car2-deficient mice, consistent with an important role of a putative Car2·Ae2 HCO 3- transport metabolon in parotid exocrine cell function. Increased abundance of this HCO3- transport metabolon is likely one of the multiple compensatory changes in the exocrine parotid gland of Nhe1-/- mice that together attenuate the severity of in vivo electrolyte and acid-base balance perturbations. © 2007 by The American Society for Biochemistry and Molecular Biology, Inc.