L-Glutamine in intestinal sodium absorption: lessons for physiology, pathobiology, and therapy for diarrhea

Effect of glutamine on water and sodium absorption in human jejunum at baseline and during PGE1-induced secretion

Glutamine, a major fuel for enterocytes, stimulates water and sodium absorption in animal models of secretory diarrhea, but data in humans are still limited. The aim of this study was to investigate the effect of glutamine on jejunal absorption during hypersecretion in humans. In six healthy adults, the effects of glutamine on jejunal absorption were assessed with a triple-lumen tube on two occasions, at baseline and during PGE1-induced hypersecretion (0.1 μg·kg−1·min−1) in a random order. Isoosmolar solutions containing polyethylene glycol 4000 as nonabsorbable marker were infused in the jejunum at 10 ml/min over 1-h periods: saline (sodium chloride 308 mmol/l), glucose-mannitol 45:45 mM, glucose 90 mM, alanine-glucose 45:45 mM, glutamine-glucose 45:45 mM, and glutamine 90 mM. Net absorptive and secretory fluxes were measured at steady state. At baseline, glutamine- and alanine-containing solutions induced a threefold increase of water and sodium absorption ( P < 0.05); 90 mM glutamine stimulated water absorption more than 90 mM glucose (3.6 ± 0.6 vs. 1.9 ± 0.3 ml·min−1·30 cm−1, P < 0.05). PGE1-induced hypersecretion was reduced ( P < 0.05) by solutions of alanine-glucose, glutamine-glucose, and glutamine 90 mM ( P < 0.05) and reversed to absorption by alanine-glucose and glutamine-glucose. Glutamine and alanine absorption was nearly complete and was not influenced by PGE1. In conclusion, glutamine stimulates water and electrolyte absorption in human jejunum, even during experimental hypersecretion. In addition to the metabolic effects of glutamine, these results support the evaluation of glutamine-containing solutions for the rehydration and the nutritional support of patients with secretory diarrhea.

Renal metabolism of amino acids: its role in interorgan amino acid exchange

The kidneys play a role in the synthesis and interorgan exchange of several amino acids. The quantitative importance of renal amino acid metabolism in the body is not, however, clear. We review here the role of the kidney in the interorgan exchange of amino acids, with emphasis on quantitative aspects. We reviewed relevant literature by using a computerized literature search (PubMed) and checking relevant references from the identified articles. Our own data are discussed in the context of the literature. The kidney takes up glutamine and metabolizes it to ammonia. This process is sensitive to pH and serves to maintain acid-base homeostasis and to excrete nitrogen. In this way, the metabolism of renal glutamine and ammonia is complementary to hepatic urea synthesis. Citrulline, derived from intestinal glutamine breakdown, is converted to arginine by the kidney. Renal phenylalanine uptake is followed by stoichiometric tyrosine release, and glycine uptake is accompanied by serine release. Certain administered oligopeptides (eg, glutamine dipeptides) are converted by the kidneys to their constituent components before they can be used in metabolic processes. The kidneys play an important role in the interorgan exchange of amino acids. Quantitatively, for several important amino acids, the kidneys are as important as the gut in intermediary metabolism. The kidneys may be crucial "mediators" of the beneficial effects of specialized, disease-specific feeding solutions such as those enriched in glutamine dipeptides.

A double-blind placebo-controlled pilot study of glutamine therapy for abnormal intestinal permeability in patients with AIDS

OBJECTIVES

Up to 20% of patients with AIDS have abnormal intestinal permeability (IP). Glutamine seems to play an important role in preventing the increase in IP and loss of intestinal mucosal mass associated with total parenteral nutrition, and may be superior to glucose for oral rehydration in the setting of intestinal infection. This study was designed to see if supplemental glutamine could alter the abnormal IP of AIDS.

METHODS

Randomly chosen patients with AIDS from the Jacobi Medical Center human immunodeficiency virus (HIV) clinic underwent IP testing using lactulose and mannitol. Those with abnormal IP were enrolled. Duodenal biopsies were performed with a Crosby capsule and the patients were randomized in a double-blind fashion to receive placebo or glutamine (4 g/day or 8 g/day) for 28 days, after which intestinal permeability tests and duodenal biopsies were repeated. Intestinal morphology was graded by ratio of villus height to crypt depth, and by degree of inflammation.

RESULTS

All patients complied with the therapy and there were no dropouts or reported side effects. The results showed less worsening of IP with the 4 g/day dose, compared with placebo. At the 8 g/day dose, there was stabilization of IP and improved absorption of mannitol. Intestinal morphology and inflammation did not change in any group.

CONCLUSIONS

These results, although not significant, suggest a trend towards improved IP and enhanced intestinal absorption with glutamine. Glutamine doses of at least 20 g/day may be necessary to improve IP. We recommend further studies at higher doses and for longer durations.

Detrimental effects on villus form during conventional oral rehydration therapy for diarrhoea in calves; alleviation by a nutrient oral rehydration solution containing glutamine

This paper examines the possibility that treatment of diarrhoea with conventional oral rehydration solutions (ORSs) may be detrimental to villus structure by imposing nutrient deprivation and that such detrimental effects may be reduced or avoided by using a nutrient ORS. A conventional WHO-type ORS (W) was compared with two nutrient solutions (N and G) both containing high glucose concentrations and the latter containing glutamine; their effects on enteric structure were assessed by morphometric analysis of samples obtained from diarrhoeic calves after 96 h treatment. Comparisons were also made with samples from controls and diarrhoeic calves at the stage where oral rehydration would have begun in the treated groups. As in our previous ORS studies, diarrhoea was induced with enterotoxigenic Escherichia coli (09:K30:K99). We measured villus length and width, crypt depth and width and calculated villus surface area in proximal, mid and distal small intestine (PSI, MSI, DSI), using standard morphometric techniques. Proximal and distal spiral colon samples (PC, DC) were examined for crypt depth and width; mitoses per crypt were counted in samples from all regions. Non-diarrhoeic calves showed the expected gradient of villus length through PSI, MSI and DSI, hence data for each region are normalized as a percentage of the control value for that region. PSI showed the greatest loss of villus length and surface area (50%) with diarrhoea. In MSI and DSI the villus loss was greater with solution W and N or G, as were increased mitoses and crypt depth. Crypt depth and mitoses also increased in the colon with solution W. Colonic crypt width increased with diarrhoea and conventional oral rehydration but less so with G; there is reason to believe that such changes have functional significance. Crypt changes in colon, MSI and DSI were least with solution G. The changes developing in diarrhoeic calves prior to treatment were thus less apparent in those treated with a nutritional ORS, particularly if it contained glutamine.

Effect of mucosal amino acids on SCC and Na and Cl fluxes in the porcine small intestine

Most amino acids are, like glucose, co-transported with sodium and are thereby potential additives to oral rehydration solutions for the treatment of diarrhea. In this study the effects of mucosal amino acids on short-circuit current and Na and Cl fluxes in three segments of the porcine small intestine were studied. L-alanine, L-leucine, L-lysine, L-proline, L-phenylalanine and L-glutamine were added (chamber concentration of each amino acid was 20 mmol x l-1) to the mucosal side of stripped proximal, mid, and distal small intestine sheets mounted in Ussing chambers in glucose containing (16 mmol x l-1) bathing medium; electrical parameters and unidirectional fluxes were measured. The amino acids induced a significant elevation in mucosa to serosa flux and unaltered serosa to mucosa flux of Na in all three segments, resulting in significant elevation in net Na absorption in proximal (2.4 +/- 0.3 microEq x cm-2 x hr-1 to 3.5 +/- 0.3 microEq x cm-2 x hr-1, P < 0.05), mid (5.9 +/- 0.5 microEq x cm-2 x hr-1 to 8.1 +/- 0.7 microEq x cm-2 x hr-1, P < 0.05) and distal (5.9 +/- 0.6 microEq x cm-2 x hr-1 to 8.3 +/- 0.7 microEq x cm-2 x hr-1, P < 0.05) small intestine. The mucosa to serosa Cl flux was significantly elevated in all three segments (P < 0.01) by amino acids abolishing the net Cl secretion in proximal (2.5 +/- 0.4 microEq x cm-2 x hr-1 to 0.8 +/- 0.6 microEq x cm-2 x hr-1, P < 0.01) and mid (2.2 +/- 0.4 microEq x cm-2 x hr-1 to 0.1 +/- 0.9 microEq x cm-2 x hr-1, P < 0.01) small intestine. The changes in SCC were carried by the electrogen Na absorption. In conclusion, the amino acids, in addition to glucose, stimulate Na absorption by about 1.0 microEq x cm-2 x hr-1 in the proximal part, and 2.5 microEq x cm-2 x hr-1 in the mid and distal part of the small intestine.

The pathogenesis of cryptosporidiosis

Human infection with the protozoan parasite Cryptosporidium parvum has recently emerged as a global public health problem. Although infection is unrelenting in patients classically regarded as immunocompromised, a tantalizing observation is that infection with this parasite results in both acute self-limited as well as chronic diarrhea in young children. Recent data have begun to elucidate multiple potential mechanisms by which parasitism of the intestinal epithelium may yield an intestinal secretory response. However, a central issue for future studies is to understand how Cryptosporidium infection in young children results in such a broad spectrum of clinical presentation. An answer to this question is likely to result through a dual understanding of how systemic or enteric immunity impacts on intestinal secretory responses and how intra-cellular parasitism alters intestinal epithelial cell function and signals the submucosal intestinal compartment. The virulence factors of Cryptosporidium mediating these events need to be identified. Douglas Clark and Cynthia Sears here review the current understanding of the pathogenesis of intestinal secretion in response to Cryptosporidium infection, and discuss key questions requiring additional study.