Abstract
This study aimed to investigate the effects of threonine (Thr) on the digestive and absorptive ability, proliferation and differentiation of enterocytes, and gene expression of juvenile Jian carp (Cyprinus carpio var. Jian). First, seven isonitrogenous diets containing graded levels of Thr (7.4–25.2 g/kg diet) were fed to the fishes for 60 days. Second, enterocyte proliferation and differentiation were assayed by culturing enterocytes with graded levels of Thr (0–275 mg/l) in vitro. Finally, enterocytes were cultured with 0 and 205 mg/l Thr to determine protein synthesis. The percent weight gain (PWG), specific growth rate, feed intake, feed efficiency, protein retention value, activities of trypsin, lipase and amylase, weights and protein contents of hepatopancreas and intestine, folds heights, activities of alkaline phosphatase (AKP), γ- glutamyl transpeptidase and Na+/K+-ATPase in all intestinal segments, glutamate-oxaloacetate transaminase (GOT) and glutamate-pyruvate transaminase (GPT) activities in hepatopancreas, and 4E-BP2 gene expression in muscle, hepatopancreas and intestinal segments were significantly enhanced by Thr (p<0.05). However, the plasma ammonia concentration and TOR gene expression decreased (p<0.05). In vitro, Thr supplement significantly increased cell numbers, protein content, the activities of GOT, GPT, AKP and Na+/K+-ATPase, and protein synthesis rate of enterocytes, and decreased LDH activity and ammonia content in cell medium (p<0.05). In conclusion, Thr improved growth, digestive and absorptive capacity, enterocyte proliferation and differentiation, and protein synthesis and regulated TOR and 4E-BP2 gene expression in juvenile Jian carp. The dietary Thr requirement of juvenile Jian carp was 16.25 g/kg diet (51.3 g/kg protein) based on quadratic regression analysis of PWG.
Highlights
Fish growth is related to the capacity of fish digestive system to break down and assimilate nutrients, which partly depends on digestive and brush border enzymes activities [1]
percent weight gain (PWG), Specific growth rate (SGR), feed intake (FI), fish final body protein and lipid contents, and Protein retention value (PRV) significantly improved with increased dietary Thr level up to 15.7 g/kg diet, and decreased (p,0.001); whereas, the moisture and ash contents showed the opposite pattern with the lowest in fish fed with 15.7 g Thr/kg diet
The Feed efficiency (FE) and Protein efficiency ratio (PER) were improved by Thr and the highest in the group fed with 22.3 g Thr/kg diet (FE, p = 0.045; PER, p = 0.072)
Summary
Fish growth is related to the capacity of fish digestive system to break down and assimilate nutrients, which partly depends on digestive and brush border enzymes activities [1]. Intestinal brush border enzymes, such as alkaline phosphatase (AKP) [3], c-glutamyl transpeptidase (c-GT) [4] and Na+/K+ATPase [5], are involved in nutrient absorption in fish. The intestine plays a central role in digesting and absorbing nutrients [6]. The fish intestinal epithelium, a site for nutrients uptake, is expanded by folding. Gut folds is regarded as a sign of absorption ability in fish [7]. The understanding of the development of digestive organs and the activity of digestive enzymes in fish is gradually improving [1]. There are few studies concerning the factors that influence the digestive and absorptive functions of fish
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