Cotransporter in the Kidney by Metabolic Acidosis and Dehydration in Rats

The functional role and mechanisms of regulation of the Na 1 /K 1 /2Cl 2 -cotransporter NKCC1 in the kidney have not yet been clarified. NKCC1 mRNA and protein expression in control rats, rats with dehydration (2 d), and rats with metabolic acidosis (NH4Cl in the food for 6 to 7 d) was examined using reverse transcription-PCR and Western blot- ting. In contrast to the abundant NKCC1 mRNA expression in the terminal inner medullary collecting ducts in mice, expres- sion was found to be most abundant in the outer medullary collecting ducts (OMCD) in rats. Dehydration and metabolic acidosis increased NKCC1 mRNA expression three- to five- fold not only in the OMCD but also in the cortical collecting ducts and inner medullary collecting ducts. Dehydration and metabolic acidosis increased NKCC1 protein expression two- fold in the membrane fraction from the outer medulla. NKCC1 protein expression was observed not in the microdissected medullary thick ascending limbs but in the OMCD, and it was stimulated twofold by dehydration and metabolic acidosis. Incubation of OMCD in low-pH medium increased NKCC1 mRNA expression. In summary, NKCC1 mRNA and protein expression is upregulated with dehydration and metabolic ac- idosis. NKCC1 may play an important role in adaptation to these physiologic conditions. Low pH and possibly hyperto- nicity stimulate NKCC1 mRNA expression in OMCD.