Effect of pulsatile gonadotrophin release on mean serum LH and FSH in peri-parturient mares

Changes in the pattern of LH and FSH in serum were studied in 6 mares foaling during the summer. Samples were collected frequently (every 15 min) for 24 h twice before foaling, -33 +/- 2 and -12 +/- 2 days, and for 12 h after foaling, on Day 0 and Day 4. Simultaneous pulses of FSH and LH were observed before foaling (r2 = 0.99). Before foaling, gonadotrophin pulses were infrequent (6 in 264 h of observation). On the day of foaling, LH and FSH pulse frequency increased (P less than 0.005) with 2-4 pulses per mare. The amplitudes of pulses of LH and FSH were higher before parturition than for those observed on Day 0 (P less than 0.001). The presence of FSH pulses was associated with an increased mean value of FSH for those days, including Day 0 (P less than 0.02). The presence of resolvable pulses of LH was not associated with an increase in mean LH values on those days (P greater than 0.6). The present results demonstrate that LH and FSH release is pulsatile in the periparturient mare. In addition, mean values of FSH in serum were elevated when pulses were detected, whereas mean concentrations of LH remained low. These results are consistent with the concept of one hypothalamic releasing hormone for LH and FSH in the mare.