Testicular and hormonal changes in stallions with thermally induced testicular degeneration.

The scrota of three Pony stallions and one miniature horse were insulated for 36 h. Plasma testosterone concentrations decreased gradually and were significantly (P < 0.05) lower than pretreatment values at 16, 24, 30, 38 and 44 h after onset of scrotal insulation. Plasma LH and oestradiol concentrations were significantly (P < 0.05) decreased 18 h, and 24 and 26 h, after onset of scrotal insulation, respectively. Plasma FSH concentrations were significantly (P < 0.05) decreased 4 days after the insulation was removed. Decreases in the potential daily sperm output per Pony for early primary spermatocytes (34% decrease) and late primary spermatocytes (60% decrease) were observed in the testes of Pony stallions castrated 7 days after the insulation was removed. Decreases in the potential daily sperm output of all germ cell types were observed in the testes of the miniature horse castrated 24 days after the insulation was removed. Testosterone propionate in corn oil (5 mg in 1.05 ml testis) was injected into the vaginal space surrounding the testes of two Pony stallions 2 days before scrotal insulation and of one stallion whose scrotum was not insulated. The percentage of sperm of normal morphology in ejaculates decreased significantly (P < 0.05; n=3) 15-26 days after onset of insulation compared with pretreatment values. Circulating testosterone concentrations were maintained at pretreatment concentrations for 18 days after the scrotum was insulated, but this did not prevent deterioration in semen quality. Circulating LH and oestradiol concentrations decreased significantly by 2 days after injection of testosterone (P < 0.05). LH concentrations were decreased significantly (P < 0.05) from pretreatment values for 18 days after scrotal insulation and oestradiol concentrations were decreased significantly (P < 0.05) for 26 days. In conclusion, insulation of Pony scrota for 36 h reduced the yield of germ cells during spermatogenesis. A reduction in circulating testosterone concentrations occurred by 16 h after onset of scrotal insulation, which is probably the result of Leydig cell impairment. However, testosterone replacement therapy did not prevent deterioration of semen quality, which indicates that the primary cause of germ cell degeneration after thermal injury to the testes may not be impaired Leydig cell production of testosterone.