DISSOCIATION OF INCREASES IN PLASMA INSULIN-LIKE GROWTH-FACTOR-I AND TESTOSTERONE DURING THE ONSET OF PUBERTY IN BULLS

The present study was conducted to examine the relationship between pl asma concentrations of testosterone, insulin-like growth factor I (IGF -I) and IGF-binding proteins (IGFBPs) during puberty, in male calves t reated with GnRH or testosterone propionate. Twelve male Holstein calv es (10 weeks old) were assigned to the control group (n = 6), the GnRH -treated group (n = 3) or the testosterone-treated group (n = 3). For 8 weeks, the GnRH-treated group received a single i.v. injection of Gn RH (0.5 mu g kg(-1) body mass) each day while the testosterone-treated group received an i.m. injection of testosterone propionate (0.5 mg k g(-1) body mass) twice a day. The calves were studied until they were 200 days old. Hormone treatments were stopped one month after puberty was reached in the control group. Blood samples were collected every 3 0 min for 8 h every third day. Hormone concentrations were determined by radioimmunoassay. Western ligand blotting and immunoblotting, using monoclonal antibodies against IGFBP-2, and IGFBP-3, were used to char acterize the IGF-binding proteins. In the control group, puberty occur red at about 120 days of age and was associated with an increase in co ncentrations of testosterone, IGF-I and IGFBP-3 and a decrease in conc entration of IGFBP-2. In the GnRH-treated group, plasma testosterone r emained low until 8 weeks after establishment of puberty in the contro l group (4 weeks after the end of treatment). In the testosterone-trea ted group, testosterone was high during the treatment period and then decreased to prepubertal values when treatment was stopped. Testostero ne values increased again to reach postpubertal values 5 weeks after t he end of hormone treatment. Nevertheless, independent of testosterone status, the profile of IGF-I and the IGFBPs in the GnRH- and testoste rone-treated groups were parallel to that reported for the control gro up with the transition from prepubertal to adult values at about 120 d ays of age. In conclusion, concentrations of testosterone, IGF-I and I GFBP-3 increase together, but probably independently, during the onset of puberty in male calves.