Effect of age upon glutamine synthesis and glutamine amide exchange of rat liver, kidney and testis.
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The relative roles of the three sodium-dependent transport systems: A, ASC and N in the uptake of [3H]Gln, and the compatibility of the uptake characteristics with the expression of mRNAs coding for the Gln transporting molecules, were examined in primary cultures of astrocytes and neurons derived from mouse cerebellum, a glutaminergic system-enriched structure, and in cerebral cortex. Gln uptake activity (Vmax) was higher in cerebellar astrocytes or neurons than in their cerebral cortical counterparts. The N-methylamino-isobutyric acid (MeAiB)- and pH-sensitive, system A-mediated component of the uptake, and the uptake of [14C]MeAiB itself, was much more active in neurons than in astrocytes derived from either region. Also, the expression of mRNA for GlnT (SAT1), a system A isoform specific for Gln, was only expressed in neurons derived from both structures, while an alanine (Ala)-preferring system A transporter, SAT2, was expressed in neurons and astrocytes from either region. System ASC-mediated Gln uptake and expression of ASCT2 mRNA were in both structures more pronounced in astrocytes than in neurons, consistent with the postulated role of ASCT2 in the efflux of de novo synthesized Gln from astrocytes. System N-mediated (threonine+MeAiB-inhibitable) Gln uptake showed comparable activities in all four types of cells, which is compatible with the ubiquitous expression of NAT2 mRNA-a mouse brain-specific N-system isoform.
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The effect of enteral Gln on protein and Gln metabolism was investigated during experimental hypercortisolemia. Four groups of subjects that had received corticosteroids and either enteral Gln (0.5 g x kg(-1) x day(-1) for 2 days) or isonitrogenous Ala-Gly were studied in a fasted or in a fed state. In either state, enteral Gln, compared with Ala-Gly, induced no statistically significant change in the endogenous rate of Leu appearance, an index of proteolysis, Leu oxidation, and nonoxidative Leu disposal, an index of protein synthesis, as studied by kinetics of [1-(13)C]Leu. Similar data were obtained from kinetics of [(2)H(5)]Phe, resulting in an unchanged protein balance in both cases. In contrast, enteral Gln significantly decreased the endogenous rate of Gln appearance and Gln de novo synthesis in the fed state (P < 0.05) as estimated by the kinetics of [(15)N]Gln. This decrease in Gln de novo synthesis induced by Gln could contribute to spare amino acids in hypercatabolic patients.
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