Vectorial Secretion of Inhibin by Immature Rat Sertoli Cells in Vitro: Reexamination of Previous Results*

Abstract

The vectorial secretion of immunoactive and bioactive inhibin by immature rat Sertoli cells (Sc) cultured in a two-compartment system was investigated using various culture supports. When Sc were cultured on Millipore-HA filters (used in all previous studies on vectorial secretion of inhibin), both immuno- and bioactive inhibin were found almost exclusively in the apical compartment, suggesting predominantly apical secretion of the glycoprotein. However, the cell-free Millipore-HA filters completely blocked the passage of Sc-conditioned medium (SCCM) inhibin, even after pretreatment with BSA and SCCM to saturate the protein-binding sites. On the other hand, polycarbonate Nucleopore filters or Millicell-CM membranes, both exhibiting extremely low protein-binding capacity, did not significantly block the passage of SCCM inhibin. When Sc were cultured on Nucleopore filters, the immunoactive inhibin was detected in both culture compartments; the basal compartment/apical compartment (BC/AC) ratio was about 1.5 (range, 1.2-1.9). The maximal effective dose of FSH or (Bu)2cAMP caused a 6- to 9-fold increase in the total (BC plus AC) secretion of immunoactive inhibin, but only a 60% increase in the secretion of bioactive inhibin, as evaluated by RIA and pituitary cell bioassay, respectively. The latter phenomenon was not accompanied by any significant change in the basal/apical distribution of either bioactive nor immunoactive inhibin. The presence of testosterone alone (10(-6) M) did not affect either total immunoactive inhibin secretion or its BC/AC ratio. The effects of the concomitant presence of FSH and testosterone did not differ significantly from those of FSH alone. Similarly to testosterone, the lack of any significant effect was observed for 17 beta-estradiol, dihydrotestosterone, androstenediol, and androstenedione regardless of the presence or absence of FSH. The striking dissimilarity of BC/AC ratios of inhibin noted in cultures maintained on Millipore-HA and Nucleopore filters was not due to differences in permeability barrier or Sc functional polarity. When cultured on either support, Sc monolayers developed comparable permeability barriers, as evaluated by measuring the passage of [3H]inulin and development of electrical resistance. The maximal electrical resistance (130-150 omega cm2) developed after 6-8 days of culture on either support. Also, total transferrin secretion and transferrin BC/AC ratio were similar on both supports, suggesting comparable cell numbers and functional polarities. These findings demonstrate that immature Sc in vitro secrete inhibin bidirectionally (BC/AC ratio, approximately 1.5); the polarity of secretion is unaffected by either FSH or various naturally occurring steroids, including testosterone.(ABSTRACT TRUNCATED AT 400 WORDS)