Glutamine metabolism by the intestinal tract

Postoperative alterations in interorgan glutamine exchange in enterectomized dogs

The effect of enterectomy on postoperative visceral organ glutamine exchange was studied in order to gain further understanding of the role of the intestinal tract in the altered glutamine metabolism that occurs following catabolic illness. In addition to studying glutamine, which transports 1/3 of whole blood amino acid nitrogen, we determined the fluxes of glutamate and alanine across the gastrointestinal tract, liver, and kidneys in 18 postoperative dogs. Arterial glutamine and glutamate were significantly higher in enterectomized animals than in controls. With enterectomy the gut became an organ of glutamine balance while in control dogs the GI tract consumed glutamine (0.11 +/- 0.04 vs 1.67 +/- 0.14 mumole/kg X min, P less than 0.001). The gut switched from an organ of glutamate release to one of net glutamate uptake following enterectomy and intestinal alanine release simultaneously fell by 50%. Simultaneously, the liver reduced its uptake of alanine and became an organ of glutamine release. Renal glutamine consumption was also diminished in enterectomy animals. The interorgan exchange of glutamine and other amino acids is altered by enterectomy. The increase in circulating glutamine levels in enterectomized animals suggests that the accelerated intestinal glutamine consumption that characterizes catabolic illnesses contributes to the low glutamine levels in these stress states. In addition, it becomes apparent that the gut is an important supplier of alanine to the liver, which supports gluconeogenesis. Metabolic adaptation and cooperation between organs is essential during organ absence or dysfunction if the organism is to survive critical illness.

Branched chain amino acid uptake and muscle free amino acid concentrations predict postoperative muscle nitrogen balance

Amino acid solutions rich in branched chain amino acids (BCAA) are commonly utilized both clinically and in experimental protocols in an attempt to reduce skeletal muscle and whole body protein catabolism. To investigate the effectiveness of BCAA infusion, amino acid formulas containing varying concentrations of BCAA were given during operation in this study to three groups of dogs undergoing a standard laparotomy and retroperitoneal dissection. A fourth group was given saline alone. With the use of previously described hindquarter flux techniques, individual and total amino acid nitrogen exchange rates were measured and utilized in estimating skeletal muscle protein catabolism. Intracellular free amino acid concentrations were measured in percutaneous muscle biopsy samples. Although there was no relationship with the rate of BCAA infusion, there was a significant correlation between the rate of BCAA uptake by muscle and diminished total nitrogen release from hindquarter skeletal muscle after operation. There was also a significant relationship between muscle nitrogen balance and the postoperative change in the muscle concentration of either total amino acids or the single amino acid glutamine. When combined in a single equation, BCAA uptake and the change in muscle free amino acid concentration predict skeletal muscle nitrogen release with an r = 0.86. Thus, the rate of BCAA uptake and the free glutamine or total amino acid concentration in muscle appear to be independent predictors of muscle nitrogen balance. The nitrogen-sparing effect of BCAA in skeletal muscle is unrelated to infusion concentration or rate of infusion.

Role of glutamine in adaptations in nitrogen metabolism during fasting

The present study examines the effect of glutamine infusion on the interorgan fluxes of glutamine, alanine, urea, and ammonia with progressive fasting. Experiments were carried out in two groups of conscious dogs with catheters previously implanted in an artery and in the hepatic, portal, and renal veins. Group I (n = 12) was fasted for 24 h, and group II (n = 10) was fasted for 96 h. On the day of the study, seven animals of group I and five of group II received a constant infusion of glutamine (3.0 mumol . kg-1 . min-1) for 1 h, and the rest (controls) received saline. Four-day fasting produced ketosis with a compensated metabolic acidosis. The demand for glutamine by the kidneys and gut increased, and the liver switched from net glutamine utilization to that of net production. The kidneys (33%) and gut (230%) increased their production of ammonia, while their release of alanine decreased. The estimated efflux of glutamine by skeletal muscle, however, did not change. All of the infused glutamine was cleared by the splanchnic and renal tissues. In group I, there was increased gut production of alanine, which was matched by increased hepatic alanine uptake and urea production. No such changes were observed in Group II. There was, however, an increase in hepatic uptake of ammonia. Finally, glutamine infusion did not alter glutamine or alanine output by skeletal muscle in group I, while it decreased efflux of alanine but not that of glutamine in group II. The data emphasize the complex interdependence of the liver, gut, kidneys, and skeletal muscle in nitrogen sparing.(ABSTRACT TRUNCATED AT 250 WORDS)