INSULIN-LIKE GROWTH-FACTOR-I STIMULATES GONADOTROPIN PRODUCTION FROM EEL PITUITARY-CELLS - A POSSIBLE METABOLIC SIGNAL FOR INDUCTION OF PUBERTY

Insulin-like growth factor (IGF)-I has been suggested as a potential s ignal linking growth and puberty in mammals. Using the juvenile Europe an eel as a model, we employed a ion-term, serum-free primary culture of pituitary cells to study the direct effect of IGF-I on gonadotrophi n (GtH-II=LH) production. IGF-I increased both cell con tent and relea se of GtH-II in a time- and dose-dependent manner. IGF-I and IGF-II ha d similar potencies but insulin was 100-fold less effective, suggestin g the implication of an IGF type 1 receptor. Other growth and metaboli c factors, such as basic fibroblast growth factor and thyroid hormones , had no effect on GtH-II production. IGF-I did not significantly incr ease the number oi GtH-II immunoreactive cells, indicating that its st imulatory effect on GtH-II production does not result from gonadotroph proliferation. Comparison of IGF-I and somatostatin (SRIH-14) effects showed that both factors inhibited growth hormone (GH) release but on ly IGF-I stimulated GtH-II production by eel pituitary cells. This ind icates that the effect of IGF-I on gonadotrophs is not mediated by the reduction oi GH released by somatotrophs into the culture medium. Thi s study demonstrates a specific stimulatory effect of IGF-I on eel GtH -II production, played out directly at the pituitary level. These data obtained in a primitive teleost suggest that the role oi IGF-I as a l ink between body growth and puberty may have been established early in the evolution of vertebrates.