Age-Dependent Changes in Physiological Action, Content, and Immunostaining of Inhibin in Male Rats*

We examined the role of endogenous inhibin in modulating FSH secretion in male rats during the infantile (days 10 to 21), juvenile (days 22 to 35), and pubertal (days 36 to 90) periods by 1) neutralization of endogenous inhibin using specific antibodies, 2) measurement of plasma and testicular levels of immunoreactive inhibin, and 3) immunohistochemical detection of testicular inhibin. In all studies, we used an antiserum against the first 26 N-terminal amino acids of the alpha-chain of porcine inhibin (anti-alpha-inhibin). Plasma immunoreactive inhibin levels were highest in young rats (8-15 days old), then decreased steadily with age. In addition, iv injection of the inhibin-alpha antiserum caused significant (P less than or equal to 0.01 or P less than or equal to 0.05) elevations in plasma FSH levels in male rats aged 10-24 days, whereas no significant (P greater than 0.05) changes occurred in older animals. In the testes, immunoreactive inhibin levels, expressed as femtomoles per mg wet weight, also declined with age. Inhibin immunostaining was most prominent in the Sertoli cells, with the greatest staining in the testes of 8 to 15-day-old rats. These results suggest that endogenous inhibin plays a physiological role in suppressing FSH secretion in infantile male rats, at an age when Sertoli cells contain the largest amount of this protein.