Restoration of the LH secretory response in desensitized gonadotropes

Abstract

The time-dependent recovery of gonadotropin-releasing hormone (GnRH) responsiveness in desensitized gonadotropes was examined under conditions of altered membrane fluidity and GnRH exposure. Cultured pituitary cells were treated for 3 h with GnRH (10 −9 M; to provoke homologous desensitization) or vehicle alone (controls). When cells were washed and immediately rechallenged for 3 h with GnRH, gonadotrope responsiveness (assessed by luteinizing hormone (LH) release) was significantly lower in GnRH-pretreated cells than controls. If gonadotropes were allowed to recover in medium alone, membrane fluidity agents 2-(2-methoxyethoxy)-ethyl-8-( cis-2- n-octylcyclopropyl)-octanoate (A 2C; 10 −4 M) or cis-vaccenic acid (CVA; 0.5 mM) or a low dose of GnRH (10 −10 M) for up to 48 h prior to rechallenging with GnRH, responsiveness in all cases was significantly lower in GnRH-pretreated cells than controls. However, if cells were treated with either A 2C or CVA in the presence of GnRH (10 −10 M) during the recovery period, gonadotrope responsiveness to a subsequent challenge with GnRH was partially restored by 24 h; by 48 h no differences in the LH secretory response to GnRH was detected between GnRH-pretreated cells and controls. The possibility that restoration of the GnRH receptor-linked Ca 2+ channel is associated with recovery of the desensitized gonadotrope was also examined. Identical protocols to those described above were used except that the functional integrity of the Ca 2+ channel was assessed by measuring LH release in response to increasing doses of maitotoxin (MTX; a specific Ca 2+ channel activator). Again, GnRH-pretreated cells were significantly less responsive to MTX than controls when allowed to recover for 48 h in medium alone, A 2C (10 −4 M) or GnRH (10 -10 M). However, allowing cells to recover for 48 h under a condition of increased membrane fluidity and basal GnRH levels completely restored the MTX-stimulated LH secretory response in GnRH-pretreated gonadotropes. Taken together, these studies suggest that the physical state of the gonadotrope plasma membrane together with the appropriate hormonal milieu provide an important environment for the gonadotrope to recover from desensitization. Additionally, our results suggest that functional recovery of the GnRH-linked Ca 2+ channel may play a requisite role in restoring GnRH responsiveness to the desensitized gonadotrope.