Effects of insulin-like growth factors (IGF-I and -II) on growth hormone and prolactin release and gene expression in euryhaline tilapia, Oreochromis mossambicus

Abstract

We investigated in vitro effects of insulin-like growth factors (IGF-I and -II) on growth hormone (GH) and prolactin (PRL) release and gene expression in euryhaline tilapia, Oreochromis mossambicus. Pituitaries were removed from freshwater-acclimated adult males and incubated for 2–24 h in the presence of human IGF-I or -II at doses ranging from 1–1000 ng/ml (0.13–130 nM). IGF-I at concentrations higher than 10 ng/ml and IGF-II higher than 100 ng/ml significantly inhibited GH release after 8, 16, and 24 h. No effect of IGFs was seen during the first 4 h of incubation. IGFs at the same concentrations also significantly attenuated GH gene expression after 24 h, although no effect was seen at 2 h. By contrast, PRL 188 release was stimulated significantly and in a dose-related manner by IGF-I at concentrations higher than 10 ng/ml and by IGF-II at concentrations higher than 100 ng/ml within 2 h. No stimulation was observed after 4 h. Similarly, both IGFs at concentrations higher than 10 ng/ml increased PRL 177 release within 2 h. However, no significant effect of IGF-I or -II was observed on mRNA levels of both PRLs after 2 and 24 h at all concentrations examined. These results clearly indicate differential regulation of GH and PRL release and synthesis by IGFs in the tilapia pituitary, i.e., rapid-acting, stimulatory effects of IGFs on PRL release and slow-acting, inhibitory effects on GH release and synthesis.