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

The effects of insulin-like growth factor I (IGF-I) and insulin on the func tion of coho salmon gonadotropes in vitro were investigated. Dispersed pitu itary 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 in tracellular FSH content after GnRH treatment were measured. Incubation of p ituitary cells with ICF-I for 7 or 10 days increased GnRH-stimulated FSH re lease and remaining cell content, but did not affect basal release. To exam ine the specificity of the effects of IGF-I, we compared FSH release and ce ll content of FSH and LH after 10-day incubation with a range of concentrat ions of IGF-I or insulin. Incubation with physiological concentrations of I GF-I resulted in significantly higher GnRH-stimulated FSH release and remai ning cell content of FSH and LH. Conversely, supraphysiological concentrati ons of insulin were required to produce more moderate effects on gonadotrop in levels. These results suggest that elevation of gonadotropin levels by I CF-I may be one mechanism by which somatic growth and nutrition promote pub ertal development in salmon.