Effects of porcine somatotropin on circulating testosterone concentrations in boars and mechanism of action.

Four experiments were conducted to elucidate the effects of porcine somatotropin (pST, growth hormone) on circulating testosterone concentrations in boars. In Exp. 1, jugular-cannulated obese boars were administered 4 mg recombinant (r) pST/d for 24 d before collection of samples at 15-min intervals over 6 h for measurement of pST, testosterone, and LH. Somatotropin treatment decreased plasma concentrations of testosterone and LH (P < .05). In Exp. 2, White composite boars were administered rpST with implants (4 mg/d) for 10 to 28, 16 to 28, or 22 to 28 wk of age. Reductions in testosterone concentrations were proportional to the length of time rpST was administered; boars treated for 18 wk had the lowest testosterone concentrations. Concentrations of LH declined throughout the study in rpST-implanted boars but remained static in untreated, control boars. In vitro production of testosterone and dehydroepiandrosterone/dehydroepiandrosterone sulfate was unaffected by rpST treatment, and hCG stimulation of in vitro androgen secretion was similar in rpST and control treatments. In Exp. 3, i.v. injection of pituitary pST (USDA-B1; 5 micrograms/kg BW) into jugular-cannulated White composite and Meishan boars resulted in an acute increase in circulating LH followed by an increase in testosterone concentrations, which then declined to below preinjection levels at 6 to 7 h after pST injection. Multiple injections, two or four per day, of 5 micrograms pST/kg BW resulted in decreased testosterone concentrations in White composite boars, whereas in Meishan boars testosterone concentrations were unaffected (Exp. 4). Concentrations of LH were not different from control values in either breed, but in both breeds, four injections of pST per day produced lower LH concentrations than did two injections per day (P < .01). Depression of circulating concentrations of androgens in boars requires extended periods of pST treatment.