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NBCn1 is a basolateral cotransporter in rat kidney inner medullary collecting ducts

The Water and Salt Research Center, University of Aarhus, DK-8000 Aarhus C, Denmark

Submitted 23 December 2002 ; accepted in final form 8 January 2004

Primary cultures of rat inner medullary collecting duct (IMCD) cells Na⁺ dependently import across the basolateral membrane through an undefined transport protein. We used RT-PCR, immunoblotting, and immunohistochemistry to identify candidate proteins for this basolateral cotransport. The mRNA encoding the electroneutral cotransporter NBCn1 was detected as the only cotransporter in the rat inner medulla (IM) among the five characterized Na⁺-dependent transporters. The mRNA of a yet uncharacterized transporter-like protein, BTR1, was also present in the IM, but its expression in microdissected tubules seemed restricted to the thin limbs of Henle's loop. Immunoblotting confirmed the presence of NBCn1 as an 180-kDa protein of the rat IM. Anti-NBCn1 immunolabeling was confined to the basolateral plasma membrane domain of IMCD cells in the papillary two-thirds of the IM. Consistent with the presence of NBCn1, IMCD cells possessed stilbene-insensitive, Na⁺- and -dependent pH recovery after acidification, as assessed by fluorescence microscopy using a pH-sensitive intracellular dye. In furosemide-induced alkalotic rats, NBCn1 protein abundance was decreased in both the IM and inner stripe of outer medulla (ISOM) as determined by immunoblotting and immunohistochemistry. In contrast, NBCn1 abundance in the IM and ISOM was unchanged in NaHCO₃-loaded animals, and the NBCn1 abundance increased only in the ISOM after NH₄Cl loading. In conclusion, NBCn1 is a basolateral cotransporter of IMCD cells and is differentially regulated in IMCD and medullary thick ascending limb.

acid-base balance; hydrogen-ATPase; bicarbonate transport; bicarbonate metabolism; immunohistochemistry; intracellular pH; 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein

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