Evaluation of the role of germ cells in regulating the route of secretion of immunoactive inhibin from the rat testis

ABSTRACT During normal sexual maturation of the male rat there is a progressive change in the route of secretion of inhibin by the Sertoli cell, from a predominantly basal route of secretion in prepuberty to a predominantly apical route of secretion in adulthood. This change may be monitored by comparing the levels of inhibin in testicular (TV), spermatic and peripheral (PV) venous blood and the levels in testicular interstitial fluid (IF). This study has assessed the role of germ cells in effecting this change by assessing (a) the effect of total germ cell depletion by X-irradiation of the males in utero, and (b) the effect of selective germ cell depletion in adulthood using the testicular toxicant, methoxyacetic acid (MAA). Female rats were X-irradiated on day 20 of gestation to produce male offspring whose testes were germ-cell deficient. Blood and IF samples were collected from groups of these offspring and age-matched controls at 35 and 100 days of age. In blood and IF samples, inhibin concentrations were significantly higher at 35 days of age than at 100 days. The absence of germ cells in X-irradiated animals did not affect the age-related fall in inhibin levels, nor the change in the predominant route of secretion of inhibin from the testis into blood. Testosterone was almost undetectable in 35-day-old controls, but was raised significantly by 100 days of age. In X-irradiated animals, testosterone levels were increased significantly at 35 days of age, and the levels in most samples were increased even more substantially by 100 days of age. However, PV levels of testosterone in 100-day-old X-irradiated animals were significantly lower than in controls. LH and FSH levels were raised in X-irradiated animals compared with their age-matched controls, but FSH levels in X-irradiated animals still fell with age, as in the controls. The role of specific germ cell types in regulating the route of secretion of inhibin from the normal adult testis was studied after depletion (80–100%) of pachytene and later spermatocytes by a single oral administration of MAA (650 mg/kg) to adult rats. At 3 days after MAA treatment, coincident with the loss of pachytene spermatocytes, plasma inhibin levels were increased significantly in blood and IF samples, and this was associated with a dramatic change in the route of secretion of inhibin from the testis, with increased secretion of this peptide via the base of the Sertoli cell into IF and TV blood. However, previous studies suggest that this may be a consequence of direct stimulation by MAA, rather than the absence of pachytene spermatocytes. By 21 days after MAA treatment, when late-stage spermatids are absent, plasma inhibin levels were reduced significantly compared with controls, although the route of secretion of inhibin from the testis was comparable with that of controls. By 42 days, when a normal germ cell complement has been restored, plasma concentrations and the route of secretion of inhibin from the testis were similar to controls. It is concluded that: (1) the presence of germ cells is not necessary for the maturational changes in the rate and route of secretion of inhibin by the Sertoli cell; these changes are most likely a consequence of formation of the blood–testis barrier, (2) in the normal adult testis, MAA-induced depletion of the most mature germ cell types affects the rate, but not the route, of inhibin secretion, whilst depletion of pachytene spermatocytes affects both parameters; the latter may indicate an early effect of MAA on the functional competence of the blood–testis barrier. Journal of Endocrinology (1992) 132, 439–448