Serum inhibin, FSH, LH and testosterone levels and testicular inhibin content in beef bulls from birth to puberty

Abstract

Developmental changes in testicular inhibin content and serum inhibin levels were observed with serum levels of gonadotrophins and testosterone in beef bulls up to 8 months of age. Twenty-two beef bulls (8 Japanese Black and 14 Japanese Shorthorn) were assigned to monthly groups from 1 to 8 months old ( n=2 for 1 and 8 months, and n=3 for 2–7 months). Inhibin activity in the testicular extracts was determined using an in vitro rat pituitary cell culture system. Serum levels of inhibin and other hormones were measured by radioimmunoassays. The concentrations of inhibin in the testis (U/g) were high during the first 3 months ( P < 0.05). They decreased temporarily between 4 and 6 months and then increased ( P < 0.05). Serum inhibin levels were high during the first 4 months and decreased after 5 months ( P < 0.05). Serum FSH levels were higher at 2 months than at any other month of age ( P < 0.05). Serum testosterone levels remained low during the first 3 months, increased at 4 months and continued to rise thereafter. A significant positive relationship was found between serum FSH and serum inhibin levels ( r=0.068; P < 0.001), whereas a negative relationship was found between serum testosterone and serum inhibin levels ( r = −0.58; P < 0.01). During the first 5 months of age, the concentrations of inhibin in the testis (U/g) correlated positively with serum FSH levels ( r=0.69; P < 0.01), while they correlated negatively with serum testosterone levels ( r=−0.85; P <0.001). These results indicate that the immature bovine testis produces and secretes high levels of inhibin, and that serum inhibin levels reflect this high inhibin production in the testis before the onset of puberty. After the onset of puberty, inhibin production and inhibin release into the circulation are no longer parallel, which is probably due to the formation of the blood-testis barrier. It is suggested that FSH is the principal regulator of inhibin production in the testis during the first 5 months of age in bulls. After the onset of puberty, testosterone might play a dominant role in suppression of FSH from the pituitary.