Lack of Effect of Acute Enteral Arginine Infusion on Whole-Body and Intestinal Protein Metabolism in Humans

Abstract

Arginine is a conditionally essential amino acid and exerts anabolic effects. We studied the effects of enteral arginine on whole-body and duodenal protein metabolism. Eight healthy fasted volunteers received randomly a 5-hr enteral infusion of either arginine (Arg; 20 g) or an isonitrogenous amino acid mixture (AA) and an IV infusion of [13C]leucine. Duodenal biopsies were performed. Whole-body protein turnover and duodenal protein synthesis (FSR) were calculated from GC/MS-assessed enrichment. The mRNA levels for major components of proteolytic pathways, ubiquitin, cathepsin D, and m-calpain, were evaluated by RT-PCR. Results were compared using paired Wilcoxon test. Endogenous, oxidative, and nonoxidative leucine fluxes were not different after Arg and AA infusions, respectively. Duodenal mucosal protein FSR (71% +/- 26% vs 81% +/- 30%/day) and mRNA levels of ubiquitin, cathepsin D, and m-calpain were also similar after Arg and AA infusions. We conclude that in healthy subjects, arginine infusion exerts no effect on whole-body and duodenal protein metabolism. Whether arginine might specifically affect these parameters in catabolic or inflammatory situations remains to be determined.

Influence of leucine on protein metabolism, phosphokinase expression, and cell proliferation in human duodenum

“…The precursor pool used was the intracellular free amino acid pool. In our study, only one tissue sampling was performed after the continuous infusion of labeled amino acids to evaluate mucosal protein FSR as previously reported by us (3,7,10) and other groups (2,23,24). Even though this approach might underestimate modifications of FSR, as suggested by some authors (25,26), we thought that in healthy volunteers it was not possible for ethical reasons to perform 4 endoscopic procedures over 2 wk.…”

“…Glutamine, the preferential substrate of enterocytes, enhanced protein metabolism in Caco-2 cells (8) and in human duodenum (3). Arginine also stimulated protein synthesis in the small intestine of piglets with rotavirus-induced enteritis (9) but not in the duodenal mucosa of healthy volunteers (10). Surprisingly, leucine effects have been poorly documented in the intestine (11,12), whereas leucine improved protein metabolism in muscle by stimulating protein synthesis (13,14) and inhibiting proteasome-dependent proteolysis (15).…”

Citrulline stimulates muscle protein synthesis, by reallocating ATP consumption to muscle protein synthesis

“…Since also arginine (ARG) activates lysosome‐located mTORC1 via a specific receptor (CASTOR1), one may expect that conversion of CIT into ARG is involved in TORC1 activation. However, this hypothesis can be ruled out for two reasons: (i) in vitro and in vivo studies did not evidence any effect of ARG on muscle protein synthesis and (ii) CIT directly stimulates muscle protein synthesis in myotubes that do not quantitatively express argininosuccinate synthase nor argininosuccinate lyase (the two enzymes required for the transformation of CIT into ARG) …”

Pea Protein Reduce Serum Cholesterol Levels in Hypercholesterolemia Hamsters by Modulating Compositions Of Gut Microbiota And Metabolites

“…L-arginine supplementation has been reported to enhance whole-body insulin sensitivity and reduce plasma lipid and cholesterol levels [56,57]. In addition to the direct bene ts, accumulating gut luminal arginine was also closely associated with the circulating amino acid metabolism by interacting with gut microbiotas to product relevant metabolites [32,58]. However, the correlation between L-arginine metabolites and gut microbiota relevant to cholesterol metabolism was not as clear as the function of L-arginine itself based on the plasma absorption.…”