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) epical 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 Na-22(+) 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 similar to 7.35, acid incubation (pH similar to 7.10) up-regulated by 35-100% rBSC-1 transport activity in cells and membrane vesicles, and rBSC-1 protein and mRNA abundance. In contrast, acid incubation did not alter alkaline phosphatase and Na+/K+-ATPase enzyme activities or beta-actin protein abundance. After 3 h of in vivo chronic metabolic acidosis (CMA) rBSC-1 mRNA abundance increased in freshly harvested MTALs, which was accompanied after 1-6 days of CMA with enhanced rBSC-1 protein abundance, These results demonstrate that both in vivo and in vitro CMA stimulate rBSC-1 expression, which would contribute to the adaptive increase in MTAL absorption and urinary excretion of NH4+ in response to CMA.