Proceedings of The Physiological Society

Europhysiology 2018 (London, UK) (2018) Proc Physiol Soc 41, PCA143

Poster Communications

Na+ and volume balance is maintained in AE4 (Slc4a9) knockout mice during Na+ depletion

M. Koch1, H. Vitzthum1, C. Hübner2, H. Ehmke1

1. Department of Cellular and Integrative Physiology, University Medical Center Hamburg-Eppendorf, Hamburg, Germany. 2. Institute of Human Genetics, Jena University Hospital, Friedrich Schiller University, Jena, Germany.

The renal Na+/HCO3- cotransporter AE4 (Slc4a9) is localized to the basolateral membrane of β-intercalated cells (β-ICs) in the renal collecting duct. It has been shown that β-ICs contribute not only substantially to acid-base homeostasis, but also to renal Na+ reabsorption during Na+ depletion. On the luminal side Na+ uptake is mediated by the parallel action of the Cl-/HCO3- exchanger pendrin (Slc26a4) and the Na+-driven Cl-/HCO3- exchanger NDCBE (Slc4a8). The Na+/HCO3- cotransporter AE4 participates in the basolateral Na+ extrusion in the ß-ICs, but it remained unclear whether AE4 is essential for the maintenance of Na+ and intravascular volume balance during Na+ depletion in vivo. We therefore analyzed urinary Na+ excretion, plasma volume, and activation of the renin-angiotensin-aldosterone system (RAAS) in AE4 wild-type (AE4+/+) and knockout mice (AE4-/-) during dietary Na+ and Cl- depletion for 10 days. As the basolateral AE4 cotransporter mediates Na+ and HCO3- reabsorption, we also analyzed Na+ and volume homeostasis after Na+ and HCO3- depleted diet. Both genotypes exhibited an equally and strongly reduced urinary Na+ excretion after changing from normal to Na+ and Cl- depleted diet. In addition, plasma volume and hematocrit of AE4-/- mice did not differ from those of AE4+/+ mice, and the RAAS was similarly activated in both genotypes after 10 days of Na+ and Cl- depletion. As we found no difference with regard to renal Na+ balance and plasma volume we examined, whether other renal Na+ transporters of the ASDN were upregulated during NaCl depletion to compensate for the AE4 deficiency. Therefore, the abundance and phosphorylation of the Na+/H+-exchanger NHE3 and the thiazide-sensitive NaCl cotransporter NCC were analyzed by western blotting. Also the protein abundance of the α-, β-, and γ-subunit of the epithelial Na+ channel ENaC was determined. No compensatory activation of these renal Na+ reabsorption pathway was found in AE4-/- mice. Analysis of the natriuretic response to the diuretics hydrochlorothiazide and amiloride revealed an equal activity of the NCC and even a lower activity of the epithelial Na+ channel ENaC in AE4-/- mice compared to AE4+/+ mice. Also after Na+ and HCO3- depletion no evidence for disturbance of Na+ and volume balance was found, as plasma Na+ concentration and hematocrit of AE4-/- mice did not differ from those of AE4+/+ mice. These findings show that the Na+/HCO3- cotransporter AE4 is not essential for maintaining Na+ and intravascular volume balance during Na+ depletion, independent of the accompanying ion. Our data suggest that AE4 deficiency can be compensated by another yet unidentified basolateral Na+ efflux pathway in ß-ICs, maintaining Na+ balance during Na+ and HCO3- reabsorption by the apical transporter NDCBE.

Where applicable, experiments conform with Society ethical requirements