Effect of pulsatile infusion of gonadotrophin-releasing hormone on plasma oestradiol-17β concentrations and follicular development during naturally and artificially maintained high levels of plasma progesterone in heifers

To stimulate a follicular-phase pattern of pulsatile LH release, gonadotrophin-releasing hormone (GnRH; 5 micrograms) was infused (i.v.) hourly into heifers for periods of 5-11 days during the luteal phase of the oestrous cycle, and also when plasma progesterone levels were increased artificially by means of a progesterone-releasing intravaginal device. Plasma oestradiol-17 beta concentrations increased from basal (less than or equal to 2.5 pmol/l) to preovulatory peak levels (20-30 pmol/l) during the first 3 days of GnRH treatment. They were maintained at these values before returning to basal levels within 24 h of cessation of infusion. This response occurred regardless of the source of progesterone (endogenous or administered). Follicular development was observed by ovarian palpation (per rectum) in some heifers at the time of maximum secretion of oestradiol-17 beta. There was no detectable cervical mucus secretion or oestrous behaviour during these periods of high oestradiol-17 beta levels and ovulation did not occur. Treatment with GnRH did not affect plasma progesterone concentrations or oestrous cycle length. The study shows that oestradiol-17 beta secretion and follicular development (and the accompanying oestrus and ovulation) are suppressed during the luteal phase of the cycle by high concentrations of plasma progesterone, and provides strong indirect evidence that such inhibition is associated with a reduction in the pulse frequency of LH release.