Effect of chronic exposure to a gonadotrophin-releasing hormone agonist on in vitro responsiveness of ovine pituitary cells to inhibin, activin and oestradiol.

To investigate the extent to which the direct actions of inhibin, activin and oestradiol on pituitary output of FSH and LH are dependent on the presence of functional gonadotrophin-releasing hormone (GnRH) receptors, we have compared the effects of these agents on cultured ovine pituitary cells derived from control and GnRH agonist-suppressed ewes. Chronic treatment with GnRH agonist reduced plasma LH and FSH levels (P < 0.01) and abolished GnRH-induced release of LH and FSH both in vivo and in vitro. As expected, basal LH release and LH cell content in vitro were drastically reduced in GnRH agonist-suppressed cells (P < 0.001). However, basal FSH release and FSH cell content were approximately twofold higher than in control cells (P < 0.001). Irrespective of whether the cells had been desensitized to GnRH, inhibin and oestradiol were both found to suppress basal FSH release and FSH cell content in a dose-dependent fashion (P < 0.001). Although inhibin had no effect on basal release of LH from control cells, it markedly enhanced GnRH-induced release (P < 0.001). In contrast, inhibin increased (P < 0.001) basal LH release from GnRH agonist-suppressed cells (which were unresponsive to the GnRH challenge). Inhibin had no overall effect on total LH content/well for either control or GnRH agonist-suppressed cells. Treatment with oestradiol, on the other hand, reduced total LH content/well, an effect which was more pronounced with GnRH agonist-suppressed cells (-44%; P < 0.001) than with control cells (-14%, P < 0.01). Whereas in control cells activin had no significant effect on any aspect of FSH production examined, in GnRH agonist-treated cells activin enhanced basal FSH release, residual cell content and total FSH content/well (P < 0.001). Altering GnRH receptor status also modified the LH response to activin. With control cells activin increased basal release (P < 0.001), decreased GnRH-induced release (P < 0.001) and increased total LH content/well (P < 0.001). With GnRH agonist-treated cells, however, activin had a uniform inhibitory effect on each aspect of LH production examined (P < 0.001 in each case). It was concluded that desensitization of ovine gonadotrophs to GnRH by chronic agonist treatment results in a paradoxical enhancement of FSH output in vitro but has little effect on the responsiveness of the cells (in terms of gonadotrophin release and content) to either inhibin or oestradiol. In contrast, GnRH agonist treatment leads to qualitative changes in cellular responsiveness to activin.