Pituitary gonadotrophin-releasing hormone receptor up-regulation in vitro: dependence on calcium and microtubule function

Abstract

Exogenous cyclic adenosine nucleotides increase gonadotrophin-releasing hormone (GnRH) receptors in intact cultured rat pituitary cells in a similar manner to that observed with GnRH itself. In this study the calcium and microtubule dependency of GnRH receptor up-regulation was examined in vitro. Treatment of pituitary cells in Ca²⁺ and serum-containing media with either GnRH (1 nmol/l), K⁺ (58 mmol/l) or dibutyryl cyclic AMP (dbcAMP; 1 mmol/l) for 7–10 h routinely resulted in a 50–100% increase in GnRH receptors. Incubation of pituitary cells with the calcium channel blocker verapamil, for 7 h, or the calcium chelator EGTA, for 10 h, had no effect on basal receptor levels but prevented the increase in GnRH receptors stimulated by either GnRH, K⁺ or dbcAMP. Luteinizing hormone release measured with the same stimulators over a 3-h period was prevented by both verapamil and EGTA. Calcium ionophore (A23187) increased GnRH receptors by 40–60% at low concentrations (10 and 100 nmol/l) while higher concentrations (10 and 100 μmol/l) reduced receptor levels. Luteinizing hormone release was not increased by receptor-stimulating concentrations of A23187, but was by higher concentrations (10 μmol/l). None of these pretreatments, for up to 10 h, impaired the subsequent LH response of the cells to increasing doses of GnRH. Vinblastine (1 μmol/l did not affect basal receptor levels but markedly reduced the increase in GnRH receptors stimulated by GnRH, K⁺ and dbcAMP. This concentration of vinblastine had no effect on LH release. These results indicate that receptor stimulation by GnRH, K⁺ and dbcAMP is a calcium-dependent process requiring the integrity of the microtubule system and there is a different calcium requirement for the processes of GnRH receptor up-regulation and LH secretion. J. Endocr. (1985) 107, 49–56