Increased gonadotrophin releasing hormone receptors on pituitary gonadotrophs: effect on subsequent LH secretion

GnRH, high potassium concentrations, and cAMP derivatives have been previously shown to increase GnRH receptor levels (GnRH-R) in cultured rat pituitary cells. However, the effect of these changes in receptor number on subsequent stimulated LH release has not been investigated. In this study pretreatment of pituitary cells with either 1 nM GnRH, 58 mM KCl, or 1 mM dibutyryl cAMP (dbcAMP) resulted in a 70-100% increase in GnRH-R 7-10 h later. Subsequent LH responses to GnRH in those cells pretreated with GnRH and KCl were markedly reduced and the dose-response characteristics altered such that the curves were non-sigmoidal. When corrected for depletion of cellular LH during the pretreatment period these GnRH response curves were similar to control, implying that hormone depletion was the explanation for apparent desensitisation. By contrast, dbcAMP and low-dose calcium ionophore (0.1 microM A23187) pretreatment, which did not deplete cellular LH, neither enhanced nor decreased subsequent sensitivity to GnRH. Thus, 4 agents which all, under these conditions, increased GnRH receptors did not sensitise gonadotrophs to GnRH. By contrast, pretreatment with 10(-9) and 10(-8) M GnRH for either 12 or 16 h rendered cells completely or partially refractory to further GnRH stimulation, despite an increase in GnRH receptors. This desensitisation could not be explained by cellular LH depletion, and was specific to the homologous ligand since dose-responses to the Ca2+ ionophore A23187 and KCl were normal when corrected for LH depletion. Non-receptor-mediated depletion of cellular LH during A23187 pretreatment (10 microM for 10 h) did not alter subsequent GnRH dose-responses, after correction for LH content. These data indicate that, under these in vitro conditions, the increased GnRH receptors are not functionally linked to the secretory apparatus of the gonadotroph. Furthermore, homologous ligand-induced desensitisation is both time- and concentration-dependent and is mediated largely by post-receptor cellular events independent of cellular LH content. Therefore, post-receptor cellular processes may be more important than changes in GnRH receptors in regulating gonadotrophin secretion. It is suggested that an increase in GnRH receptors may represent a cellular response to generalised gonadotroph activation by a variety of agents, and does not necessarily signify enhanced responsiveness to GnRH.