Maintenance of gonadotropin-releasing hormone (GnRH)-stimulated luteinizing hormone release despite desensitization of GnRH-stimulated cytosolic calcium responses.

Involvement of ionized cytosolic calcium ([Ca2+]i) and protein kinase-C (PKC) in GnRH-stimulated LH release was assessed by correlating measurable changes in [Ca2+]i and LH release in PKC-depleted and nondepleted gonadotropes. Primary cultures of anterior pituitary cells were loaded with the calcium-sensitive fluorescent dye fura-2 and placed in a perifusion chamber. GnRH pulses were delivered to the cells, and changes in fura-2 fluorescence and LH release were determined. The level of [Ca2+]i (assessed by fura-2) increased rapidly to a maximum within 20-40 sec, followed by a slower decline over the next minute (spike phase) to a sustained intermediate value (plateau phase). GnRH-stimulated LH release was unaffected by loading cells with fura-2. Both LH release and changes in [Ca2+]i were directly dependent on GnRH concentration. Pretreatment with the GnRH antagonist Antide (50 nM; [NAcD2Nal1-DpClPhe2-D3Pal3-Ser4-NicLys5-++ +DNicLys6-Leu7-ILys8-Pro9-DAla10]NH2 ) had no effect on basal [Ca2+]i or basal LH release, but did block both GnRH-stimulated calcium mobilization and GnRH-stimulated LH release. GnRH pretreatment (3.5 nM; 10 min) blocked the calcium spike phase, but not the plateau phase occurring in response to a GnRH pulse (10 nM; 5 min) delivered immediately after pretreatment. Inhibition of the calcium spike phase was transient (recovery within 15 min) and was dependent on pretreatment concentrations of GnRH. Calcium spike phase inhibition by GnRH pretreatment prevented increased LH release from PKC-depleted cells in response to a subsequent pulse of GnRH, but not from gonadotropes with normal levels of PKC. This suggests that initial LH release is dependent on changes in [Ca2+]i, but enhancement of LH release after periods of elevated GnRH concentrations may be dependent on PKC.