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Acid pH Increases the Stability of BSC1/NKCC2 mRNA in the Medullary Thick Ascending Limb

INSERM U.426, Institut Fédératif Régional Claude Bernard, Faculté de Médecine Xavier Bichat, Université Paris 7, Paris, France

Corresponding to Dr. Maurice Bichara, INSERM U.426. Faculté de Médecine Xavier Bichat, 16 rue Henri Huchard, 75870 Paris cédex 18, France. Phone: +33-1-44-85-62-81; Fax: +33-1-42-28-15-64;

ABSTRACT. Chronic metabolic acidosis enhances the ability of the medullary thick ascending limb (MTAL) to absorb NH₄⁺ at least in part by stimulating the mRNA and protein expression of BSC1/NKCC2, the MTAL apical Na⁺-K⁺(NH₄⁺)-2Cl^- co-transporter. For assessing the mechanism by which an acid pH enhances the BSC1 mRNA abundance, MTAL were harvested from adrenalectomized rats and incubated in control (pH 7.35) and acid (pH 7.10) 1:1 mixtures of Ham’s nutrient mixture F-12 and DME. rBSC1 mRNA abundance and gene transcription rate were quantified by quantitative reverse transcription–PCR and run-off assay, respectively. Acid incubation enhanced mRNA abundance within 4 h in whole cell (P < 0.02) but not in nucleus. BSC1 gene transcription rate was not affected by acid incubation. In contrast, under conditions in which gene transcription was blocked, rBSC1 mRNA decreased within 6 h by 38 ± 11% in control but only by 15 ± 15% in acid medium (P < 0.02), which represented an increase in the BSC1 mRNA half-life from approximately 7 to approximately 17 h. Furthermore, in a mouse TAL cell line, acid incubation for 16 h significantly increased (P < 0.02) the amount of BSC1 mRNA in cells transfected with the full-length mBSC1 cDNA but not in cells transfected with a mBSC1 cDNA lacking the 3'-UTR. These results demonstrate that acid pH enhances the stability of BSC1 mRNA probably by activating pathways that act on the AU-rich 3'-UTR of BSC1 mRNA, which contributes to the renal response to metabolic acidosis. E-mail: bichara@bichat.inserm.fr

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