Sub‐chronic exposure to ammonia inhibits the growth of juvenile Wuchang bream (Megalobrama amblycephala) mainly by downregulation of growth hormone/insulin‐like growth factor axis

Abstract In this study, healthy Wuchang bream ( Megalobrama amblycephala ) juveniles were exposed to 0, 5, 10, 20 and 30 mg/L total ammonia nitrogen for 30 days to elucidate toxic effects and mechanisms of ammonia on growth performance involved with the regulation of growth hormone/insulin‐like growth factor (GH/IGF) and hypothalamic–pituitary‐thyroid (HPT) axes. Our results showed that the increasing total ammonia nitrogen concentrations caused dose‐depend decreases in the weight gain and specific growth rate but increases in the food conversion ratio and mortality in juvenile bream, indicating growth inhibitory effects induced by ammonia. Concurrently, GH, IGF‐1 at protein and mRNA levels were significantly decreased in ammonia exposure groups ( p < .05), while serum thyroid stimulating hormone, free thyroxine, free triiodothyronine levels were significantly reduced only in fish exposed to higher concentrations of 20 and 30 mg/L ammonia ( p < .05), suggesting that ammonia exposure could perturb both GH/IGF‐axis and HPT‐axis functions. Furthermore, transcriptional levels of extracellular regulated protein kinases 2 ( erk2 ), phosphatidylinositol 3‐kinase ( pi3k ), protein kinase B ( akt ), target of rapamycin ( tom ) and ribosomal protein S6 kinase‐polypeptide 1( s6k1 ) in the dorsal muscle were significantly down‐regulated in the fish exposed to ammonia ( p < .05). This fact indicated that MAPK/ERK pathway and PI3K/AKT pathway should be responsible for the growth inhibition. Combining the results of spearman correlation coefficient, it should be noted that the GH/IGF axis played a more important role in regulating the growth than the HPT axis in Wuchang bream under persistent ammonia stress.