Tubular CO₂ Production from Glutamine in the Rat: Segmental Profile and Modulation

Abstract

The present study was designed to test whether tubular carbon dioxide production from the carbon skeleton of uniformly ¹⁴C-labelled glutamine exhibits quantitative and qualitative segmental heterogeneity. Our results show that CO₂ production from glutamine in the proximal convoluted tubule (PCT) was dependent on substrate concentrations and is saturable at 10^-4M of glutamine. Glutamine oxidation was demonstrable in all nephron segments examined. The PCT is the quantitatively predominant site of glutamine oxidation. Intermediate nephron segments, however, such as the thick ascending limb (MAL) and the distal convoluted tubule possess a significant capacity for glutamine oxidation, particularly when examined in terms of tubular protein content. Modulation of glutamine oxidation by extracellular pH was segment specific. Stimulation by acidosis and inhibition by alkalosis were observed in the PCT while carbon dioxide production from glutamine in the MAL was pH-insensitive. Glutamine oxidation was closely linked to sodium transport and greatly decreased by inhibition of Na-K-ATPase. In both the PCT and MAL, glutamine oxidation was inhibited by high extracellular potassium concentrations and in the PCT enhanced by extracellular hypokalemia. N-Ethyl maleiamide, an inhibitor of proton ATPase, led to almost complete cessation of CO₂ production from the substrate in both PCT and MAL. Acetazolamide, an inhibitor of carbonic anhydrase, led to a partial reduction in carbon dioxide formation in the PCT, but did not affect glutamine oxidation in the MAL. We conclude that segmental qualitative heterogeneity characterizes oxidation of the carbon skeleton of glutamine with proximal segments showing the predictable effects of pH changes and carbonic anhydrase inhibition. The MAL appears to be nonmodulating.

Complexity of glutamine metabolism in kidney tubules from fed and fasted rats

“…Thus glutamine may well be utilized by the terminal portion of the proximal tubule. In agreement with this view is the demonstration that the straight portion of the rat proximal tubule produced substantial amounts of 14 CO 2 from [U-14 C]glutamine [59]. Thus, taking into account the distribution along the rat nephron of the mitochondrial and cytosolic enzymes involved in glutamine degradation [57], it appears that all proximal tubular segments are able to metabolize glutamine carbon into glutamate, glucose and CO 2 .…”

Enzymatic Suppression of the Membrane Conductance Associated with the Glutamine Transporter SNAT3 Expressed in Xenopus Oocytes by Carbonic Anhydrase II

“…Uptake, deamination, and oxidation of glutamine are supported by system N transporters, glutaminase activity, and entry of carbon skeleton into the tricarboxic cycle, respectively. Interestingly, blocking CA activity with acetazolamide CO 2 reduces ammonia production and hence the acid/base balance (Mujais and Zahid, 1992). During acidosis, glutamine reuptake is increased by SNAT3, which helps to reduce the acid overload by outward transport of H + (George and Solomon, 1981;Wiklund, 1996).…”

The sodium-bicarbonate cotransporter NBCe1 supports glutamine efflux via SNAT3 (SLC38A3) co-expressed in Xenopus oocytes

“…These two membrane transporters are coexpressed in a variety of cell types, including kidney epithelial cells [8] and glial cells in the brain [9]. In the kidney, glutamine uptake is required for ammoniagenesis and HCO À 3 production [10], which is greatly increased during chronic acidosis [11], whereas NBCe1 supports the salvage of base equivalents across the basolateral membrane [12]. In the brain, SNAT3 contributes to the glutamine efflux from astrocytes [9]; glutamine is then taken up by neurons for re-synthesis of glutamate (and γ-aminobutyric acid, GABA) mediated by phosphateactivated glutaminase (PAG), completing the glutamate/ glutamine shuttle [13].…”