EFFECT OF GONADOTROPIN-RELEASING-HORMONE PULSE FREQUENCY ON GONADOTROPIN-SECRETION AND SUBUNIT MESSENGER RIBONUCLEIC-ACIDS IN PERIFUSED PITUITARY-CELLS

Slow frequency GnRH pulses have been proposed to preferentially increase circulating FSH levels by increasing FSH synthesis and pulsatile release. Examination of this proposal using various in vivo models, however, has produced conflicting results. To examine directly the effects of GnRH pulse frequency on the pituitary, we compared the effects of 2.5-nM GnRH pulses administered every 1 h or every 4 h vs. no GnRH, using perifused rat pituitary cells. FSH secretion (total area under the response curve) was 2-fold greater (P < 0.01) with every hour than with every 4 h GnRH pulses. This difference resulted from the increased number of GnRH pulses and increased (P < 0.05) interpulse FSH secretion, whereas FSH pulse amplitude was unchanged. FSH-beta mRNA levels at the completion of the 11-h perifusion were increased 4.5-fold by GnRH every h (P < 0.01) and 3.3-fold by GnRH every 4 h (P < 0.05) above levels in untreated cells. FSH-beta mRNA levels were greater (P < 0.05) at the faster GnRH pulse frequency. Because more frequent stimulation delivered more GnRH during the study, cells were next stimulated with 2.5 nM GnRH every 1 h for nine pulses, 7.5 nM GnRH every 4 h for three pulses to equalize the GnRH dose, or 2.5 nM GnRH every 4 h for three pulses. Interpulse FSH secretion and FSH-beta mRNA levels were again greater (P < 0.05) with every hour than every 4 h GnRH pulses. Interpulse LH secretion, FSH and LH pulse amplitude, and LH-beta and alpha-subunit mRNA levels were not different between the groups. GnRH doses of 0.1-10 nM every hour increased FSH and LH pulsatile secretion dose-dependently, but FSH-beta, LH-beta, and alpha-subunit mRNA levels were similar. In conclusion, our data reveal that reducing the frequency of GnRH pulses from every hour to every 4 h reduces both FSH-beta mRNA levels and FSH interpulse secretion, but does not change GnRH-stimulated FSH pulsatile release. We suggest that the finding by others that slow frequency GnRH pulses increase circulating FSH levels under certain experimental conditions in vivo may instead be explained by complex hormonal interactions or changes in FSH clearance.