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1 Institute of Anatomy, University of Aarhus, Aarhus, Denmark; The Water and Salt Research Center, University of Aarhus, Aarhus, Denmark
2 Department of Physiology, University of Aarhus, Aarhus, Denmark; The Water and Salt Research Center, University of Aarhus, Aarhus, Denmark
* To whom correspondence should be addressed. E-mail: sn{at}ana.au.dk.
Primary cultures of rat inner medullary collecting duct (IMCD) cells Na+ dependently import HCO3- across the basolateral membrane through an undefined transport protein. We used reverse transcription - polymerase chain reaction (RT-PCR), immunoblotting and immunohistochemistry to identify candidate proteins for this basolateral Na+:HCO3- transport. The mRNA encoding the electroneutral NBCn1 was detected as the only Na+:HCO3- transporter in rat inner medulla among the five characterized Na+ dependent HCO3- transporters. The mRNA of a yet uncharacterized transporter-like protein, BTR1, was also present in inner medulla, but its expression in micro-dissected tubules seemed restricted to the thin limbs of Henle's loop. Immunoblotting confirmed the presence of NBCn1 as a ~180 kDa protein of the rat inner medulla. Anti-NBCn1 immunolabeling was confined to the basolateral plasma membrane domain of IMCD cells in the papillary 2/3 of inner medulla. Consistent with the presence of NBCn1, IMCD cells possessed stilbene insensitive, Na+ and HCO3 -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 inner medulla (IM) and inner stripe of outer medulla (ISOM) as determined by immunoblotting and immunocytochemistry. In contrast, NBCn1 abundance in IM and ISOM was unchanged in NaHCO3 loaded animals, and the NBCn1 abundance increased only in ISOM after NH4Cl loading. In conclusion, NBCn1 is a basolateral Na+:HCO3- cotransporter of IMCD cells and is differentially regulated in IMCD and mTAL.
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