Ae4 (Slc4a9) anion exchanger drives Cl- uptake-dependent fluid secretion by mouse submandibular gland acinar cells

Abstract

Transcellular Cl- movement across acinar cells is the ratelimiting step for salivary gland fluid secretion. Basolateral Nkcc1 Na+-K+-2Cl- cotransporters play a critical role in fluid secretion by promoting the intracellular accumulation of Cl-above its equilibrium potential. However, salivation is only partially abolished in the absence of Nkcc1 cotransporter activity, suggesting that another Cl- uptake pathway concentrates Cl-ions in acinar cells. To identify alternative molecular mechanisms, we studied mice lacking Ae2 and Ae4 Cl-/HCO3-exchangers. We found that salivation stimulated by muscarinic and β-adrenergic receptor agonists was normal in the submandibular glands of Ae2-/- mice. In contrast, saliva secretion was reduced by 35% in Ae4-/- mice. The decrease in salivation was not related to loss of Na+-K+-2Cl- cotransporter or Na+/H+exchanger activity in Ae4-/- mice but correlated with reduced Cl- uptake during β-adrenergic receptor activation of cAMP signaling. Direct measurements of Cl-/HCO3- exchanger activity revealed that HCO3--dependent Cl- uptake was reduced in the acinar cells of Ae2-/- and Ae4-/- mice. Moreover, Cl-/HCO3- exchanger activity was nearly abolished in double Ae4/3Ae2 knock-out mice, suggesting that most of the Cl-/HCO3-exchanger activity in submandibular acinar cells depends on Ae2 and Ae4 expression. In conclusion, both Ae2 and Ae4 anion exchangers are functionally expressed in submandibular acinar cells; however, only Ae4 expression appears to be important for cAMP-dependent regulation of fluid secretion.