EFFECTS OF AGE, SEASON, AND FERTILITY STATUS ON PLASMA AND INTRATESTICULAR IMMUNOREACTIVE (IR) INHIBIN CONCENTRATIONS IN STALLIONS

The nature of the relationship between inhibin and reproductive functi on in the stallion is yet to be elucidated. Blood and testes from 51 l ight horse stallions ranging in age from 2 mo to 25 years were collect ed during the breeding and nonbreeding seasons to study the effects of testicular maturation, aging, season, and fertility status on periphe ral and intratesticular concentrations of Ir inhibin and other reprodu ctive hormones. Of the 51 stallions, 12 age-matched stallions (6 ferti le, 3 subfertile, and 3 infertile) were used in the fertility study. B lood samples were taken before castration and plasma stored at -20 deg rees C for analysis of Ir inhibin, luteinizing hormone (LH), follicle stimulating hormone (FSH), testosterone (T), estradiol (E-2), and estr ogen conjugates (EC) by radioimmunoassay (RIA). Testes were homogenize d and testicular extracts prepared and frozen at -70 degrees C for ana lysis of Ir inhibin, T, E-2, and EC by RIA. Plasma concentrations of I r inhibin, LH, FSH, T, E-2, and EC and intratesticular concentrations of Ir inhibin, T, E-2, and EC increased with age (P < 0.01). The most dramatic effect appeared to be during testicular maturation. An aging effect was not observed in adult stallions. A seasonal effect was not detected for any of the plasma hormones, whereas for the intratesticul ar hormones the only change noted was an increase in T in the nonbreed ing season (P < 0.05). Plasma Ir inhibin, E-2, and EC were lower (P < 0.01) and gonadotropins higher (P < 0.05) in infertile stallions. Plas ma T levels did not change. Intratesticular Ir inhibin concentrations tended to be lower (P < 0.1) in subfertile stallions and significantly lower (P < 0.01) in infertile stallions, whereas intratesticular ster oid levels were not different among the three groups. In conclusion, p lasma and intratesticular Ir inhibin concentrations seem to be affecte d by testicular maturation and fertility status. (C) Elsevier Science Inc. 1998.