Potassium restriction, high protein intake, and metabolic acidosis increase expression of the glutamine transporter SNAT3 (Slc38a3) in mouse kidney

Abstract

Kidneys produce ammonium to buffer and excrete acids through metabolism of glutamine. Expression of the glutamine transporter Slc38a3 (SNAT3) increases in kidney during metabolic acidosis (MA), suggesting a role during ammoniagenesis. Potassium depletion and high dietary protein intake are known to elevate renal ammonium excretion. In this study, we examined SNAT3, phosphate-dependent glutaminase (PDG), and phosphoenolpyruvate carboxykinase (PEPCK) regulation during a control (0.36%) or low-K(+) (0.02%) diet for 7 or 14 days or a control (20%) or high-protein (50%) diet for 7 days. MA was induced in control and low-K(+) groups by addition of NH(4)Cl. Urinary ammonium excretion increased during MA, after 14-day K(+) restriction alone, and during high protein intake. SNAT3, PDG, and PEPCK mRNA abundance were elevated during MA and after 14-day K(+) restriction but not during high protein intake. SNAT3 protein abundance was enhanced during MA (both control and low K(+)), after 14-day low-K(+) treatment alone, and during high protein intake. Seven-day dietary K(+) depletion alone had no effect. Immunohistochemistry showed SNAT3 staining in earlier parts of the proximal tubule during 14-day K(+) restriction with and without NH(4)Cl treatment and during high protein intake. In summary, SNAT3, PDG, and PEPCK mRNA expression were congruent with urinary ammonium excretion during MA. Chronic dietary K(+) restriction, high protein intake, and MA enhance ammoniagenesis, an effect that may involve enhanced SNAT3 mRNA and protein expression. Our data suggest that SNAT3 plays an important role as the glutamine uptake mechanism in ammoniagenesis under these conditions.

Loss of function mutation of the Slc38a3 glutamine transporter reveals its critical role for amino acid metabolism in the liver, brain, and kidney

“…Several members of the SLC38 family have been studied extensively in heterologous and other in vitro systems [18,20,23,34] but their function in vivo is mainly unknown. Here we report a severe phenotype of mice lacking Snat3 due to an early stop codon introduced by random ENUmutagenesis.…”

“…Alternatively, low glucose levels may trigger increased protein catabolism to maintain energy production and the elevated urea levels and In the kidney, glutamine is the primary metabolic source for the release of NH 3, which when excreted as NH 4 + helps to save CO 2 for de-novo synthesis of bicarbonate to restore systemic acid-base balance. Uptake of glutamine for ammoniagenesis has been attributed to Snat3 because Snat3 shows induction in parallel to increased ammoniagenesis [18,20,23]. Snat3-deficient mice showed a decrease in urinary ammonium excretion consistent with impaired ammoniagenesis.…”

“…In the liver, SNAT3 is likely to be a candidate for both periportal uptake and perivenous efflux of glutamine, which is essential for the detoxification of ammonia delivered via the portal blood and excreted as urea [6,21]. Furthermore, SNAT3 mRNA and protein expression in the kidney is increased in response to metabolic acidosis possibly providing glutamine for ammoniagenesis in the proximal tubule cell [18,20,[23][24]. Plasma glutamine levels decrease substantially during metabolic acidosis [25] and glutamine flow is redirected from the splanchnic bed to the kidney with SNAT3 proposed to deliver glutamine for renal ammoniagenesis.…”

Urea and Ammonia Metabolism and the Control of Renal Nitrogen Excretion

“…Dietary protein intake has important effects on renal ammonia metabolism. In general, high-protein diets, particularly if high in sulfur-containing amino acids, increase endogenous acid production, causing a parallel increase in ammonia excretion, whereas low-protein diets decrease ammonia excretion (75,76). Because ammonia nitrogen excretion changes parallel dietary nitrogen changes, net nitrogen balance does not change.…”

Differential axial localization along the mouse brain vascular tree of luminal sodium-dependent glutamine transporters Snat1 and Snat3

“…Polyclonal antisera against mouse Snat1, human SNAT3, and human LAT1 were raised in rabbits using synthetic peptides of the following epitopes: mouse Snat1: MMHFKSGLELTELQNMTVC (Abgent, Bioggio, Switzerland); human SNAT3: MEIPRQTEMVELVPNGKC (Pineda, Berlin, Germany) (Busque and Wagner, 2009;Moret et al, 2007); and human LAT1: CTVLCQKLMQVVPQET (Franca et al, 2005 …”