Characteristics of glutamine metabolism by rat kidney tubules: a carbon and nitrogen balance

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Abstract
The metabolism of glutamine by a suspension of rat kidney tubules was studied in vitro. The influence of duration of incubation, glutamine concentration and metabolic state of the donor animals was investigated. The relative importance of glucose synthesis, amino acid production, and oxidation to CO2 was estimated by drawing a complete balance of the nitrogens and the C chains of the extracted glutamine. The initial (1st 15 min) rate of glutamine utilization was significantly greater than the subsequent rate due to an initial, but transient, extracellular accumulation of glutamate. This phenomenon was suppressed when a small amount of glutamate was added to the incubation medium. Glucose production constitutes the major fate for glutamine metabolism. No net oxidation of glutamine could be detected with 1 mM glutamine during the first 30 min. Glutamine oxidation becomes significant after prolonged incubation (16% at 120 min). The metabolic fate of glutamine differs when 5 or 10 mM are presented to the tubules, glutamate production and oxidation to CO2 becoming more important. Metabolic acidosis or a 48-h fast increases glutamine extraction and enhances its utilization towards glucose synthesis while they depress glutamate accumulation and oxidation to CO2. Metabolic alkalosis has the opposite effect. The metabolism of glutamine in vitro is dependent on the experimental conditions. Total oxidation to CO2 is not a major fate for glutamine at physiological concentration and is not enhanced by acidosis in the rat kidney in vitro.