Highly Polarized Secretion of Inhibin by Sertoli Cellsin Vitro*

Abstract

We have studied the regulation of inhibin secretion by rat Sertoli cells grown on extracellular matrix-impregnated porous filters in a twin chamber assembly. Previous studies have established that rat Sertoli cells cultured under these conditions reproduce the morphological and functional polarization observed in the Sertoli cell in situ. Sertoli cells isolated from 18- to 22-day-old Wistar rats were cultured for up to 8 days with daily changes of fully defined supplemented Eagle's Minimum Essential Medium (MEM). Rat inhibin was measured by RIA and pituitary cell bioassay, and transferrin by RIA. Inhibin measured by immunoassay or bioassay was always readily detectable in the upper, but not the lower, chamber. Inhibin secretion into the upper chamber exhibited a dose-dependent stimulation of up to 3.7-fold by ovine FSH, with a medium effective dose of 2.2 micrograms/liter and a constant bio- to immunoreactive ratio (3.6 +/- 0.4). Apically directed secretion accounted for over 80% of inhibit output under basal conditions and over 94% with FSH stimulation. Insulin also stimulated upper chamber inhibin secretion at a high dose (5 mg/liter) but not at lower doses or in conjunction with FSH exposure of Sertoli cells. Testosterone augmented FSH-induced stimulation of inhibin secretion, but was ineffective without FSH exposure. In contrast to inhibin secretion, for which FSH is the principal regulator, transferrin secretion by Sertoli cells is more evenly bidirectional (overall mean upper to lower chamber ratio of 1.5) and requires exposure to other stimuli (insulin, retinoic acid, and testosterone) in addition to FSH to achieve maximal secretion. Both submaximal and maximal FSH stimulation of inhibin output were augmented by a phosphodiesterase inhibitor, isobutylmethylxanthine, and these effects were fully reproduced by forskolin, which suggests the involvement of cAMP in the vectorial secretion of inhibin. The marked polarization of Sertoli cell inhibin secretion in vitro could not be explained by restricted transmembrane passage of inhibin. It is, therefore, suggested that the bulk of inhibin secretion by the immature rat Sertoli cell in vivo may be directed primarily into the seminiferous tubular lumen. Thus, in addition to its role in endocrine negative feedback signaling to the pituitary, inhibin may also have important functions in seminiferous tubular function and the support of spermatogenesis.