KAJIMURA, S.; UCHIDA, K.; YADA, T.; HIRANO, T.; GRAU, E.G.: Interaction between Insulin-like growth factors (IGF-I and IGF-II) and growth hormone in the euryhaline tilapia, Oreochromis mossambicus

Growth in vertebrates is controlled by the GH-IGF axis to an important degree. To better understand the interaction between IGFs and GH in teleosts, we studied the effects of recombinant bovine growth hormone (rbGH) on the IGF-I mRNA level in the tilapia, Oreochromis mossambicus. Significant growth-promoting effects were observed after 4 weekly injections of rbGH and also after a single injection of Posilac, a sustained-release formulation of rbGH. We found no significant correlation between plasma levels of rbGH and body weight or body length, except for a significant correlation between plasma rbGH and body length in fish injected with rbGH. RNase protection assay showed that IGF-I mRNA levels in liver were significantly increased by rbGH and by Posilac injection, correlating well with body weight and body length. These results indicate that the IGF-I mRNA level in liver is a better index for assessing body growth in tilapia than plasma GH. To examine the feedback mechanism of IGFs on GH in pituitary, in vitro effects of IGF-I and IGF-II on GH release and gene expression were examined. Pituitaries were incubated for 24 hours with human IGF-I or IGF-II at doses from 1-1000 ng/ml (0.13 nM-130 nM). IGF-I at concentrations higher than 10 ng/ml or IGF-II higher than 100 ng/ml significantly inhibited GH release after 8, 16 and 24 hours of incubation. GH mRNA levels were also significantly decreased by IGF-I at concentrations higher than 10 ng/ml or IGF-II higher than 100 ng/ml after 24 hours of incubation. These results support the idea of a negative feedback effect by IGFs on GH release and expression similar to that in other vertebrates.