Glutamine or glutamate release by the liver constitutes a major mechanism for nitrogen salvage. R[acute]em[acute]esy, C., C. Moundras, C. Morand, and C. Demign[acute]e. Laboratoire des Maladies M[acute]etaboliques et des Micronutriments, I.N.R.A. de Clermont-Ferrand/Theix, 63122 St -Gen[grave]es-Champanelle, France, Tel : (33) 73 62 42 33, Fax : (33) 73 62 46 38
APStracts 3:0156G, 1996.
The aim of the present study was to investigate the mechanisms of nitrogen salvage by the liver when a diet is protein-deficient. For this purpose, rats were adapted either to a slightly deficient (11% casein) or moderately surfeit (22% casein) dietary protein level. Animals were sampled during the postprandial or the postabsorptive period, and fluxes across the digestive tract and liver were determined. During the postabsorptive period, there was a negative balance of glutamine across the digestive tract in both diet groups. During the postprandial period, the digestive balance of glutamine was still negative, in spite of a substantial supply of dietary glutamine and glutamate, suggesting that glutamine utilization is maximal during this period. In the present conditions, there was a net production of glutamate and glutamine by the liver with both diet groups, but glutamine release was 73% higher in rats fed the low -protein diet. In these last, due to the relatively low capacity of ureagenesis, nitrogen utilization was shifted towards glutamine synthesis: the overall uptake of amino acids by the liver was about 5.3 [mu]moles/min, and the net release of glutamine plus glutamate was about 2.9 [mu]moles/min (hence a 55% cycling, on a molar basis). This cycling was only 12% in rats adapted to the 22% casein diet. Nitrogen cycling, taking also into account liver ammonia uptake, showed parallel changes: 64% or 15% in rats adapted to the 11% or 22% casein diets, respectively. Besides glutamine delivery, glutamate was also released by the liver, representing a nitrogen source for extrasplanchnic tissues. With protein-deficient diets, hepatic glutamine delivery is mainly to fulfill substrate for intestinal metabolism, which represents a mechanism for nitrogen salvage. This shift of nitrogen metabolism from urea towards glutamine production may imply a glutamate transfer from periportal to glutamine -synthesizing perivenous hepatocytes. 

Received 19 April 1996; accepted in final form 28 July 1996.
APS Manuscript Number G145-6.
Article publication pending Am. J. Physiol. (Gastrointest. Liver
Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 29 August 1996