Postnatal expression of transport proteins involved in acid-base transport in mouse kidney.

The kidney plays a major role in maintaining and controlling systemic acid-base homeostasis by reabsorbing bicarbonate and secreting protons and acid-equivalents, respectively. During postnatal kidney development and adaptation to changing diets, plasma bicarbonate levels are increasing, the capacity for urinary acidification maturates, and the final morphology and distribution of intercalated cells is achieved. In adult kidney, at least two types of intercalated cells (IC) are found along the collecting duct characterised either by the expression of AE1 (type A IC) or pendrin (non-type A IC) where non-type A IC are found only in the convoluted distal tubule, connecting tubule and cortical collecting duct. Here we investigated in mouse kidney the relative mRNA abundance, protein expression levels and distribution of several proteins involved in renal acid-base transport, namely, the Na(+)/HCO(3)(-) cotransporter NBC1 (SLC4A4), the Na(+)/H(+)-exchanger NHE3 (SLC9A3), two subunits of the vacuolar H(+)-ATPase [ATP6V0A4 (a4), ATP6V1B1 (B1)], the Cl(-)/HCO(3)(-) exchangers AE1 (SLC4A1) and pendrin (SLC26A4). Relative mRNA abundance of all transport proteins was lowest at day 3 after birth and increased thereafter in parallel with protein levels. The numbers of type A and non-type A IC in the cortical collecting duct (CCD) increased from day 3 to days 18 and 24, whereas the number of IC in the CCD with apical staining for the vacuolar H(+)-ATPase subunits a4 and B1 decreased from day 3 to days 18 and 24, respectively. In addition, cells with characteristics of non-type A IC (pendrin expression, basolateral expression of vacuolar H(+)-ATPase subunits) were found in the inner and outer medulla 3 days after birth but were absent from the medulla of 24-day-old mice. Taken together, these results demonstrate massive changes in mRNA and protein expression levels of several acid-base transporters during postnatal kidney maturation and also show changes in intercalated cell phenotype in the medulla during these processes.