Permissive Effect of Insulin-like Growth Factor I (IGF-I) on Gonadotropin Releasing-hormone Action on In Vitro Growth Hormone Release, in Rainbow Trout ( Oncorhynchus mykiss)

Abstract

The aim of the work was to study the influence of insulin-like growth factor I (IGF-I) on GnRH-induced GH release by cultured pituitary cells of normally growing rainbow trout ( Oncorhynchus mykiss), collected at different stages of gametogenesis. When pituitary cells were pre-incubated with human IGF-I (10 −8 M) for 48 hours they became responsive to sGnRH (10 −8 to 10 −6 M) in the subsequent 24-hour incubation period, depending on the sexual stage, while not IGF-I pre-incubated cells were always non-responsive to GnRH. The permissive effect of IGF-I was detected in immature fish or those at the beginning of the gametogenesis, but not in mature fish. IGF-I inhibition of GH release during the preincubation period varies also with the sexual stage and is greater in immature than in mature fish. The permissive effect of IGF-I seems specific to somatotropes since IGF-I does not modify GnRH action on GtH 2 release. This work suggests that GnRH action on GH release can vary for a particular fish species depending on the physiological status.