Hepatic Glutamine Metabolism under the Influence of the Portal Ammonia Concentration in the Perfused Rat Liver

Abstract

l ‐Glutamine, when added to the portal perfusate in a system of isolated perfused rat liver (non‐recirculating open system) at physiological concentration of 0.6 mM, does not lead to an extra production of urea and ammonia, but it does so when ammonium ions at physiological concentration of 0.3 mM are simultaneously present. This stimulatory effect of ammonium ions on glutaminase activity is readily reversible. The effect of portal ammonium ions is half‐maximal at 0.2–0.3 mM and maximal at about 0.6 mM. The maximal rate of production of urea‐N plus ammonia from glutamine is exhibited at 10–12 mM glutamine, amounting to about 2.6 μmolxmin −1 xg wet weight −1 . The modulatory effects of ammonium ions on the disposition of glutamine by the liver were demonstrated (a) by the extra release of nitrogen from glutamine, (b) by the release of 14 CO 2 from [U‐ 14 C]glutamine, and (c) by measurement of glutamine uptake. The simultaneous operation of glutamine synthetase and glutaminase activities in liver was demonstrated to exist (futile cycle), based on 14 CO 2 release from [U‐ 14 C]glutamine and the positive glutamine balance across the hepatic and portal veins, and further on the effects of methionine sulfoximine. This inhibitor of glutamine synthetase abolished net glutamine release and left the rate of 14 CO 2 relcase from [U‐ 14 C]glutamine unaltered. Moreover, there is an extra O 2 uptake by the liver upon glutamine addition which is not accounted for by extra urea synthesis; the extra O 2 uptake increases with the portal glutamine supply. An estimate of the rate of this futile cycle at 0.6 mM glutamine is given, approx. 0.1 μmol × min −1 × g wet wt liver −1 . The stimulatory effect of portal ammonium ions on hepatic glutamine breakdown is discussed in terms of an inter‐organ feed‐forward mechanism, e. g. between small intestine and liver during conditions of increased glutamine supply and, therefore, increased requirement of glutamine removal from the circulation.