Insulin-like growth factor I increases follicle-stimulating hormone (FSH) content and gonadotropin-releasing hormone-stimulated FSH release from coho salmon pituitary cells in vitro

The effects of insulin-like growth factor I (IGF-I) and insulin on the function of coho salmon gonadotropes in vitro were investigated. Dispersed pituitary cells from immature coho salmon (Oncorhynchus kisutch) were incubated with ICF-I for 1, 3, 7, or 10 days, then incubated with salmon GnRH for an additional 24 h. Medium FSH content before and after GnRH treatment and intracellular FSH content after GnRH treatment were measured. Incubation of pituitary cells with ICF-I for 7 or 10 days increased GnRH-stimulated FSH release and remaining cell content, but did not affect basal release. To examine the specificity of the effects of IGF-I, we compared FSH release and cell content of FSH and LH after 10-day incubation with a range of concentrations of IGF-I or insulin. Incubation with physiological concentrations of IGF-I resulted in significantly higher GnRH-stimulated FSH release and remaining cell content of FSH and LH. Conversely, supraphysiological concentrations of insulin were required to produce more moderate effects on gonadotropin levels. These results suggest that elevation of gonadotropin levels by ICF-I may be one mechanism by which somatic growth and nutrition promote pubertal development in salmon.