Zeta-crystallin mediates the acid pH-induced increase of BSC1 cotransporter mRNA stability.

The Na+/K+/2Cl- cotransporter (BSC1/NKCC2) is the major transporter mediating sodium chloride and ammonium absorption in the medullary thick ascending limb. A loss-of-function mutation of BSC1 is responsible for Bartter's syndrome. We previously showed both in vivo and in vitro that acidosis increases the expression and activity of BSC1 and that acid pH enhances the stability of BSC1 mRNA by mechanisms involving its 3'-untranslated region (UTR).

Recent concepts concerning the renal handling of NH3/NH4+.

To be appropriately excreted in urine, NH4+ , the major component of urinary acid excretion, must be synthesized by proximal tubular cells, secreted into the proximal tubular fluid, reabsorbed by the medullary thick ascending limb (MTAL) to accumulate in the medullary interstitium, and finally be secreted in the medullary collecting ducts. Each of the various steps of this particular renal pathway is highly regulated, and the control of gene expression explains how the renal handling of NH 4 + becomes fully adapted to chronic acid-base changes. Several targets have been identified to account for the adaptation of renal NH 4 + synthesis and transport in response to an acid load.

Renal handling of NH3/NH4+: recent concepts.

To be appropriately excreted in urine, NH4+, the major component of urinary acid excretion, must be synthesized by proximal tubular cells, secreted into the proximal tubular fluid, reabsorbed by the medullary thick ascending limb (MTAL) to be accumulated in the medullary interstitium, and finally secreted in medullary collecting ducts. Several targets have been identified to account at the gene expression level for the adaptation of renal NH4+ synthesis and transport in response to a chronic acid load. These targets are the key enzymes of ammoniagenesis (mitochondrial glutaminase and glutamate dehydrogenase) and gluconeogenesis (phosphoenolpyruvate carboxykinase) and the Na+/H+(NH4+) exchanger NHE3 in the proximal tubule, the apical Na+-K+(NH4+)-2Cl- cotransporter of the MTAL, the basolateral Na+-K+(NH4+)-2Cl- cotransporter, and likely the epithelial Rh B and C glycoproteins in the collecting ducts.

Regulation by glucocorticoids of expression and activity of rBSC1, the Na+-K+(NH4+)-2Cl- cotransporter of medullary thick ascending limb.

To assess whether glucocorticoids regulate rBSC1, the apical Na(+)-K(+)(NH(4)(+))-2Cl(-) cotransporter of kidney medullary thick ascending limb (MTAL), studies were performed in normal rats, adrenalectomized (ADX) rats, and ADX rats infused with dexamethasone for 6 days. The effects of dexamethasone on rBSC1 were also studied in vitro using isolated rat MTAL segments. Cotransport activity was estimated by intracellular pH measurements; rBSC1 protein was quantified in MTAL crude membranes by immunoblotting analysis, and mRNA was quantified by quantitative reverse transcription-polymerase chain reaction.

Stimulation by in vivo and in vitro metabolic acidosis of expression of rBSC-1, the Na+-K+(NH4+)-2Cl- cotransporter of the rat medullary thick ascending limb.

To assess whether metabolic acidosis per se regulates rBSC-1, the rat medullary thick ascending limb (MTAL) apical Na+-K+(NH4+)-2Cl- cotransporter, rat MTALs were incubated for 16 h in an acid 1:1 mixture of Ham's nutrient mixture F-12 and Dulbecco's modified Eagle's medium. Cotransport activity was estimated in intact cells and membrane vesicles by intracellular pH and 22Na+ uptake measurements, respectively; rBSC-1 protein was quantified by immunoblotting analysis and mRNA by quantitative reverse transcription-polymerase chain reaction. As compared with incubation at pH approximately 7.