Role of gonadotropin-releasing hormone pulse frequency in differential regulation of gonadotropins in the gilt

Abstract

We tested the hypothesis that different GnRH pulse frequencies will affect serum LH and FSH differently. Ovariectomized gilts (n = 6), immunized against GnRH, were given 200-ng pulses of GnRH agonist (GnRH-A) every 180 min for 3 days (pretreatment), followed by GnRH-A pulses every 30, 60, or 180 min for 3 days (treatment) in a Latin rectangle design. Mean gonadotropin concentrations did not change over time when GnRH pulses were administered every 180 min. Initiation of high GnRH-A pulse frequency (30 min) caused a robust increase in serum LH to 265% of the pretreatment level (p ≤ 0.007) and a more moderate increase in serum FSH to 127% of pretreatment level (p ≤ 0.02). After 66 h of frequent pulsing, desensitization had occurred and serum LH concentrations were similar to pretreatment concentrations, but serum FSH had decreased to 53% of pretreatment levels (p ≤ 0.0008). After 72 h of treatment, 5 μg GnRH-A was infused to estimate residual releasable pools of LH and FSH, and the amounts of LH and FSH released were negatively correlated with GnRH-A pulse frequency. The results of this study imply that the LH surge is terminated because the pituitary gland becomes incapable of responding to an otherwise adequate stimulus, and not because of exhaustion of releasable LH pools. Our results confirm that in the pig the response to altered GnRH-A pulse frequency differs between LH and FSH. High GnRH pulse frequency is more effective in acutely releasing LH than FSH. Low pulse frequency of GnRH supports FSH synthesis and release, but is not as effective in increasing LH concentrations, while high GnRH pulse frequency inhibits FSH synthesis and release.