Twelve horse mares were used in a repeated-measures design consisting of 3 replicates of 4 mares each. On Day 6 following ovulation, luteolysis was initiated with an i.m. injection of prostaglandin F2α (PGF2α; Lutalyse, 10 mg). Either 12 (−12) or 36 (−36) h before PGF2α (PRE), blood samples were collected simultaneously from the intercavernous sinus (ICS) and jugular (JUG) vein at 10-min intervals for an 8-h period. Pituitary capacity to exogenous GnRH (2 μg/kg BW, i.v.) was evaluated at the alternate time point within this period. Frequent sampling and GnRH challenge were repeated 36 (+36) or 60 (+60) h after PGF2α (POST). Progesterone reached a nadir 48 h after PGF2α, while the first significant elevation in estradiol (p < 0.05) was not detected until 156 h. PRE LH and FSH pulse frequencies in ICS samples were 0.6 ± 0.3 and 1.8 ± 0.8 pulses/8 h, respectively. Three high-amplitude, concurrent ICS pulses of LH and FSH were observed during this period. These were associated with elevated gonadotropin levels in the corresponding JUG samples. In contrast, low-amplitude ICS pulses at this time were predominately releases of FSH alone that were not detectable in the JUG. Following luteolysis, POST gonadotropin pulse frequencies in the ICS increased (p < 0.001) to 7.4 ± 0.4 and 6.8 ± 0.5 pulses/8 h for LH and FSH, respectively. Concurrent LH and FSH releases of relatively low amplitude characterized 87.7% of the POST ICS pulses, none of which were measurable in corresponding JUG samples. LH and FSH responses to exogenous GnRH decreased (p < 0.05) dramatically from PRE to POST periods. Within the PRE period, LH-responses were significantly greater (p < 0.05) at −36 h compared to −12 h. From these data we conclude that 1) JUG samples alone are inadequate to characterize gonadotropin secretion during the estrous cycle of the mare and 2) gonadotropin pulse frequency increases following induced luteal regression and prior to any increase in mean concentrations of estradiol. This is most likely due to loss of progesterone-negative feedback.
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Meyer et al. (1995) studied this question.