Free dietary glutamine improves intestinal morphology and increases enterocyte migration rates, but has limited effects on plasma amino acid profile and growth performance of channel catfish Ictalurus punctatus

The present experiment was conducted to investigate the effects of graded amounts of dietary GLN on the intestinal morphology, plasma amino acids and growth performance of juvenile channel catfish. Six experimental diets (280 g kg(-1) crude protein, 80 g kg(-1) lipid) were composed of casein, gelatin and crystalline amino acids and supplemented with 0, 5, 10, 15, 20 or 30 g free glutamine kg(-1). These diets were fed to triplicate groups of fish (average initial weight of 6.1) for 10 weeks after which blood and intestinal samples were collected. Intestinal microstructures were positively affected by dietary GLN at the highest levels of supplementation (20 and 30 g kg(-1)), significantly (P<0.05) increasing enterocyte and microvilli height in anterior, mid and posterior intestinal sections. Moreover, a subsequent experiment revealed that GLN supplementation significantly (P<0.05) increased the enterocyte migration rate in all sampled intestinal segments. In contrast, immunohistochemical staining of proliferating cells with mouse anti-proliferating cell nuclear antigen (PCNA), revealed an increased number of proliferating cells only at the distal intestine. Plasma amino acid levels 15 h postprandial showed significantly (P<0.05) higher levels of asparagine, serine, glycine and threonine in fish fed the diet supplemented with 20 g GLN kg(-1) diet. Despite the consistent trend of higher values seen in fish fed the diet supplemented with 20 g GLN kg(-1), there were no significant (P>0.05) differences in growth-related parameters among treatments. The present results demonstrate an efficient utilization of free GLN by intestinal mucosal cells of channel catfish, which resulted in enhancement of the enteric microstructure along with increased migration rates of enterocytes and modified plasma amino acid profiles. However, these results were not translated into improved growth at the end of the 10-week feeding trial. (C) 2012 Elsevier B. V. All rights reserved.