Factors affecting the secretion of immunoactive inhibin into testicular interstitial fluid in rats

ABSTRACT Immunoreactive inhibin was measured in testicular interstitial fluid (IF) from rats during sexual maturation or after impairment of spermatogenesis induced by ethane dimethanesulphonate (EDS), unilateral cryptorchidism or local heating (43 °C, 30 min) of the testes, to ascertain its usefulness as a marker of changing Sertoli cell function. Cultures of isolated seminiferous tubules were also studied. Inhibin was measured by a radioimmunoassay directed towards the first 26 amino acids of the N-terminus of the α-subunit, and the results confirmed for selected pools of IF by in-vitro bioassay using dispersed ovine pituitary cells. During puberty, IF levels of immunoactive inhibin fell by more than 90% (P<0·001) between 30 and 60 days of age, a decrease paralleled by the levels of androgen-binding protein (ABP), another Sertoli cell product secreted into IF. These changes also paralleled, but preceded, the fall (60%; P<0·001) in serum levels of FSH between 40 and 70 days, while the serum and IF levels of testosterone increased more than two-fold over this period. When adult rats were injected with EDS to destroy the Leydig cells, testosterone levels in IF and serum were undetectable at 3 and 7 days after treatment, were just detectable at 14 days and thereafter returned slowly towards normal by 42 days. The initial androgen withdrawal following EDS treatment caused a progressive reduction in testicular weight up to 21 days and this was accompanied by a significant increase in the serum levels of FSH and a two- to threefold increase in the IF levels of immunoactive inhibin (and also of ABP). Serum FSH and IF levels of immunoactive inhibin returned to within the normal range by 42 days when testosterone levels had normalized. In contrast, in two other experimental situations in which a marked decrease in testicular weight coupled with an increase in IF levels of ABP occurs, different results for the IF levels of immunoactive inhibin were obtained. Thus, in rats exposed to local heating of the testes, IF levels of immunoactive inhibin remained unchanged from control values at 21–40 days after treatment, a finding confirmed by bioassay results. In rats made unilaterally cryptorchid for 10 months, levels of immunoactive inhibin in IF were reduced by 60% (P<0·01) in the abdominal compared with the contralateral scrotal testis. These results suggest that (1) IF levels of immunoactive inhibin do not always change in parallel to the levels of ABP and may be a useful marker of changing Sertoli cell function, and (2) in at least two situations (puberty and after EDS treatment), there may be a positive relationship between the serum levels of FSH and the IF levels of immunoactive inhibin. This positive relationship was confirmed by in-vitro findings in which FSH and dibutyryl cyclic AMP (but not testosterone) were shown to stimulate immunoactive inhibin production by isolated rat seminiferous tubules during culture for 2–6 days. J. Endocr. (1988) 119, 315–326