Brush Border Transport of Glutamine and Other Substrates During Sepsis and Endotoxemia

Abstract

The effects of severe infection on luminal transport of amino acids and glucose by the small intestine were investigated. Studies were done in endotoxin-treated rats and in septic patients who underwent resection of otherwise normal small bowel. In rats the kinetics of the brush border glutamine transporter and the glutaminase enzyme were examined. In patients the effects of severe infection on the transport of glutamine, alanine, leucine, and glucose were studied. Transport was measured using small intestinal brush border membrane vesicles that were prepared by Mg⁺⁺ aggregation/differential centrifugation. Uptake of radiolabeled substrate was measured using a rapid mixing/filtration technique. Vesicles demonstrated 15-fold enrichments of enzyme markers, classic overshoots, transport into an osmotically active space, and similar 2-hour equilibrium values. The sodium-dependent pathway accounted for nearly 90% of total carrier-mediated transport. Kinetic studies on rat jejunal glutaminase indicated a decrease in activity as early as 2 hours after endotoxin secondary to a decrease in enzyme affinity for glutamine (K_m =. 2.23 ± 0.20 mmol/L [millimolar] in controls versus 4.55 ± 0.67 in endotoxin, p < 0.03), rather than a change in V_max. By 12 hours the decrease in glutaminase activity was due to a decrease in V_max (222 ± 36 nmol/mg protein/min in controls versus 96 ± 16 in endotoxin, p < 0.03) rather than a significant change in K_m. Transport data indicated a decrease in sodium-dependent jejunal glutamine uptake 12 hours after endotoxin secondary to a 35% reduction in maximal transport velocity (V_max = 325 ± 12 pmol/mg protein/10 sec in controls versus 214 ± 8 in endotoxin, p < 0.0001) with no change in K_m (carrier affinity). Sodium-dependent glutamine transport was also decreased in septic patients, both in the jejunum (V_max for control jejunum = 786 ± 96 pmol/mg protein/10 sec versus 417 ± 43 for septic jejunum, p < 0.01) and in the ileum (V_max of control ileum = 1126 ± 66 pmol/mg protein/10 sec versus 415 ± 24 in septic ileum, p < 0.001) The rate of jejunal transport of alanine, leucine, and glucose was also decreased in septic patients by 30% to 50% (p < 0.01). These data suggest that there is a generalized down-regulation of sodium-dependent carrier-mediated substrate transport across the brush border during severe infection, which probably occurs secondary to a decrease in transporter synthesis or an increase in the rate of carrier degradation. The decrease in the number of active brush border carriers for glutamine and the decrease in glutaminase activity, in conjunction with diminished consumption of circulating glutamine demonstrated during sepsis in previous studies, may result in inadequate amounts of this key small intestinal substrate.