Threonine influences the absorption capacity and brush-border enzyme gene expression in the intestine of juvenile blunt snout bream (Megalobrama amblycephala)

Abstract

A 9-week experiment was conducted to investigate the effects of dietary threonine levels on the enzyme activities, plasma ammonia content and intestinal brush-border enzyme gene expressions in juvenile blunt snout bream. Five isonitrogenous and isoenergetic semi-purified diets (34% crude protein) were formulated to contain graded levels of dietary threonine (0.6 (un-supplemented control), 1.1, 1.6, 2.1 and 2.6% in dry matter). The diets were supplemented with 0, 0.5, 1.0, 1.5 or 2.0% l-crystalline threonine. l-Amino acid mixtures were prepared taking into account the amount of amino acids contributed by fish meal, casein and gelatin. Juvenile fish (initial weight, 3.01±0.01g) were assigned into triplicate groups and fed with their respective diet to near satiation four times per day. The results indicated that the activities of alkaline phosphatase (AKP), sodium/potassium-ATPase transporter (Na+/K+-ATPase) and γ-glutamyl transpeptidase (γ-GT) in the three intestinal segments rose as dietary threonine levels increased and reached a pick value at 1.58% dietary threonine and thereafter decreased. The highest glutamate-oxaloacetate transaminase (GOT) activity in hepatopancreas was found in the group fed with high (2.58%) threonine diet (P<0.05). Glutamate-pyruvate transaminase (GPT) activity in hepatopancreas and plasma ammonia content (PAC) exhibited significant (P<0.05) “V-shaped” trends with increasing dietary threonine levels and the lowest values were found at 1.58% threonine, while plasma alkaline phosphatase (ALP) didn't significantly respond to threonine supplementation. Dietary threonine regulated the gene expressions of brush-border enzymes (AKP, Na+/K+-ATPase and γ-GT) in the intestinal segments, which may explain further that threonine enhanced the absorptive function of fish via regulating brush-border enzyme synthesis and secretion. Statement of relevanceBlunt snout bream (Megalobrama amblycephala) is one of the major aquaculture species in China with extremely high economic value. In the last decade, its production in China has been rapidly increased and reached approximately 0.7million tons in 2012. It is also introduced to North America (northern Canada to southern Mexico), Africa and Eurasia. Recently, many studies have been reported regarding nutrient requirement of blunt snout bream. This study is part of our previous studies that investigated the effects of threonine on the growth, feed utilization, digestive capacity, immune function, and gene expressions of digestive enzymes, signaling molecules related to protein synthesis, immune and cytokines in blunt snout bream. Animal growth depends on the nutrient digestion and absorption, which is governed by the activities of enzymes including brush-border enzymes. In fish species the activities of brush-border enzymes appear to be strongly correlated with the composition of diet. Among indispensible amino acids, threonine has been proved to be an indispensible amino acid for optimal growth of fish, and it is extracted in greater proportion by the small intestine in animals, suggesting that threonine is involved in intestinal functionality and maintenance. However, there is no report regarding the effects of threonine on the activities of brush-border enzymes in blunt snout bream. To date, there is no report regarding the relationship between dietary threonine and the gene expressions of brush-border enzymes in fish. Indeed, this study was conducted to test the following hypothesis: firstly, an adequate level of threonine may play an important role in absorptive function by enhancing the activities of brush-border enzymes in blunt snout bream; secondly, threonine may regulate the brush-border enzyme gene expressions in the intestinal segments of blunt snout bream, which may explain further the role of threonine in absorptive function of this fish species.