Abstract
Adiponectin plays critical roles in the regulation of energy metabolism in mammals. However, its roles in fish remain to be evaluated. In this study, we examined the tissue distribution characteristics of two adiponectin genes, namely, adipoqa and adipoqb, at transcriptional level, and detected the changes of their mRNA expressions during oral glucose tolerance test (OGTT). Furthermore, we prepared recombinant proteins of grass carp adiponectin A and adiponectin B and studied their roles in the regulation of glucose metabolism in the hepatopancreas of grass carp in vivo. The results showed that adipoqa mRNA was mainly expressed in the heart of grass carp, followed by red muscle, white muscle, and adipose tissue, while adipoqb mRNA was highly expressed in adipose tissue, red muscle, and white muscle. The mRNA levels of adipoqa and adipoqb in the white muscle and heart were significantly decreased at 1 h post OGTT, and their expressions in the heart were also suppressed at 3 h, whereas the transcriptional levels of adipoqa and adipoqb in the red muscle were elevated significantly at 12 h after glucose load. Intraperitoneal injection of adiponectin A or adiponectin B reduced basal serum glucose levels of grass carp and caused an increase of glycogen content in the hepatopancreas. Adiponectin A upregulated the expression of enzyme genes related to glycolysis (gk and pk) and glycogen synthesis (gys) in the hepatopancreas, and adiponectin B promoted the expression of pk and gys, as well as g6pase in the glycogenetic pathway. Besides, adiponectin A and adiponectin B have opposite regulatory effects on glut2 mRNA expression. These results suggested that adiponectin A and adiponectin B promoted glycolysis, gluconeogenesis, and glycogen synthesis, increased glycogen content of hepatopancreas, and reduced basal blood glucose level, and they may have an antagonistic regulatory mechanism in regulating glucose uptake.
Highlights
Carbohydrates are important economical energy nutrient in the feed of commercial fish
Grass carp adiponectin B clustered with its counterparts of other teleosts, which was separated from that of birds and mammals, and adiponectin A from grass carp clustered with zebrafish adiponectin A with high bootstrap values
We found that adipoqa and adipoqb mRNA levels in the heart and white muscle of grass carp decreased at the early stage of oral glucose tolerance test (OGTT)
Summary
Carbohydrates are important economical energy nutrient in the feed of commercial fish. Fish are limited in their ability to use carbohydrates, and they are prone to persistent hyperglycemia after carbohydrate intake [2]. The utilization of carbohydrates and glucose metabolism regulation in fish have become one of the hot topics in the fish nutrition research in the past years. The regulation of glucose homeostasis in fish is a complex process, which is affected by multiple factors, and the endocrine system plays a key role in this process. The variability in physiological function of some endocrine factors may contribute to glucose intolerance in fish. Insulin classically promotes glucose disposal in mammals, whereas it has the opposite effect in some fish species, e.g., it inhibits glucose disposal in rainbow trout (Oncorhynchus mykiss) [5]
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