PULSATILE GONADOTROPIN-SECRETION DETERMINED BY FREQUENT SAMPLING FROMTHE INTERCAVERNOUS SINUS OF THE MARE - POSSIBLE MODULATORY ROLE OF PROGESTERONE DURING LUTEOLYSIS

Twelve horse mares were used in a repeated-measures design consisting of 3 replicates of 4 mares each, On Day 6 following ovulation, luteoly sis was initiated with an i.m. injection of prostaglandin F-2 alpha (P GF(2 alpha); Lutalyse, 10 mg). Either 12 (-12) or 36 (-36) h before PG F(2 alpha) (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 mu g/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 PGF(2 alpha) (POST). Progesterone reached a nadir 48 h after PGF(2 alpha), 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, respectiv ely. Three high-amplitude, concurrent ICS pulses of LH and FSH were ob served during this period, These were associated with elevated gonadot ropin levels in the corresponding JUG samples. In contrast, low-amplit ude ICS pulses at this time were predominately releases of FSH al one that were not detectable in the JUG. Following luteolysis, POST gonado tropin 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% o f the POST ICS pulses, none of which were measurable in corresponding JUG samples, LH and FSH responses to exogenous GnRH decreased (p < 0.0 5) dramatically from PRE to POST periods, Within the PRE period, LH-re sponses were significantly greater (p < 0.05) at -36 h compared to -12 h. From these data we conclude that 1) JUG samples alone are inadequa te to characterize gonadotropin secretion during the estrous cycle of the mare and 2) gonadotropin pulse frequency increases following induc ed luteal regression and prior to any increase in mean concentrations of estradiol, This is most likely due to loss of progesterone-negative feedback.