Effect of intravenous glutamine on duodenal mucosa protein synthesis in healthy growing dogs

Abstract

To determine whether glutamine acutely stimulates protein synthesis in the duodenal mucosa, five healthy growing dogs underwent endoscopic biopsies of duodenal mucosa at the end of three 4-h primed, continuous intravenous infusions ofl-[1-13C]leucine on three separate days, while receiving intravenous infusion of 1) saline, 2)l-glutamine (800 μmol ⋅ kg−1 ⋅ h−1), and 3) isonitrogenous amounts of glycine. The three infusions were performed after 24 h of fasting, a week apart from each other and in a randomized order. Glutamine infusion induced a doubling in plasma glutamine level, and glycine caused a >10-fold rise in plasma glycine level. During intravenous infusions of [13C]leucine, the plasma leucine labeling attained a plateau value between 3.22 and 3.68 mole % excess (MPE) and [13C]ketoisocaproate ([13C]KIC) of 2.91–2.84 MPE; there were no significant differences between glutamine, glycine, and saline infusion days. Plasma leucine appearance rate was 354 ± 33 (SE), 414 ± 28, and 351 ± 35 μmol ⋅ kg−1 ⋅ h−1(not significant) during glycine, saline, and glutamine infusion, respectively. The fractional synthetic rate (FSR) of duodenal mucosa protein was calculated from the rise in protein-bound [13C]leucine enrichment in the biopsy sample, divided by time and with either plasma [13C]KIC or tissue free [13C]leucine as precursor pool enrichment. Regardless of the precursor pool used in calculations, duodenal protein FSR failed to rise significantly during glutamine infusion (65 ± 11%/day) compared either with saline (84 ± 18%/day) or glycine infusion days (80 ± 15%/day). We conclude that 1) plasma [13C]KIC and tissue free [13C]leucine can be used interchangeably as precursor pools to calculate gut protein FSR; and 2) short intravenous infusion of glutamine does not acutely stimulate duodenal protein synthesis in well-nourished, growing dogs.