IDENTIFICATION OF RENAL GLUTAMINE TRANSPORTERS REGULATED BY METABOLIC ACIDOSIS

C. Moret, M. H. Dave, N. Schulz, F. Verrey, C. A. Wagner.
Institute of Physiology, University of Zurich, Switzerland.

The kidney plays a major role in regulating acid-base homeostasis by excreting acid-equivalents stemming from dietary intake and metabolism. The kidneys rapidly adapt to changes in metabolism and are able to increase urinary acid secretion by stimulating proton transport and by producing and excreting titratable acids. Titratable acids are comprised of phosphate, citrate or ammonia and provide for proton buffering in the urine. Thereby, titratable acids prevent the generation of a large pH gradient which would limit further proton secretion. The most abundant and important titratable acid, ammonia, is synthetized in the proximal tubule from glutamine which is taken up into cells by specialized amino acid transporters. During metabolic acidosis, glutamine uptake and subsequent ammonium synthesis are increased. Here we used a mouse model with induced metabolic acidosis (NH4Cl in drinking water) to identify glutamine transporters stimulated during metabolic acidosis in order to understand the molecular basis of this major adaptational mechanism contributing to the maintenance of acid-base homeostasis. Blood gas and urine analysis confirmed the induction of metabolic acidosis in mice treated with NH4Cl for 2 or 7 days as compared to a control group. An additional group was treated with NaHCO3 to induce a mild metabolic alkalosis. Real-time PCR for 16 amino acid transporters and GAPDH as control gene showed a strong induction of SLC38A3 (system N amino acid transporter, SN1) transporter mRNA in kidney, whereas all other mRNAs encoding glutamine transporters were unchanged or even reduced during metabolic acidosis. Also the mRNA level of PEPCK, an enzyme involved in renal gluconeogenesis and bicarbonate generation from glutamine during metabolic acidosis was elevated. Similar results were obtained with Western blotting confirming the results from real-time PCR. Our results suggest that SLC38A3 may serve a major role in the renal adaptation of acid secretion and thus plays an important role in the regulation of systemic acid-base balance.