This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Irvine, C. H. G. Articles by Alexander, S. L. Search for Related Content PubMed PubMed Citation Articles by Irvine, C. H. G. Articles by Alexander, S. L.

Patterns of secretion of GnRH, LH and FSH during the postovulatory period in mares: mechanisms prolonging the LH surge

To study the mechanisms responsible for the unusually slow decline of the ovulatory LH surge in mares, secretion patterns of GnRH, LH and FSH were monitored in pituitary venous blood collected every 2 or 5 min for 10.5–18.0 h from five mares on the third (n = 4) or fifth day after ovulation (first sampling period). To determine the effectiveness of progesterone negative feedback, mares were then given a luteolytic dose of a prostaglandin analogue (PGF₂) and pituitary venous sampling (every 2 or 5 min for 16 h) recommenced 20–22 h later (second sampling period). During the declining arm of the LH surge, large peaks (detected by the Cluster algorithm) of concurrent LH and FSH secretion occurred infrequently, with four peaks being detected in a combined sampling period of 75 h. Outside the peaks, LH or FSH secretion continued (as assessed by a pituitary to jugular–venous concentration ratio 1.25) during 46% ± 13 or 40% ± 10, respectively, of the sampling period. GnRH immunoactivity was detected during each spontaneous gonadotrophin peak, but at other times was generally at assay sensitivity. After PGF₂, plasma progesterone fell (ng ml^–1, mean ± SEM; first sampling period: 8.6 ± 0.8; second: 2.0 ± 0.3; P = 0.001) and the frequency of LH (P < 0.05) and FSH (P < 0.02) peaks rose, with 28 peaks detected for each hormone in a total of 80 h sampling. Peaks in LH were smaller during the second period, with decreases observed in maximum (P= 0.027) and mean (P= 0.025) secretion rates. Maximum GnRH secretion rate during peaks also declined (P= 0.010); however, the decrement (–30 ± 6%) was less than that in maximum LH secretion rate (–82 ± 5%; P = 0.040), suggesting that other factors contribute to the reduced LH peak amplitude. In summary, gonadotrophin peak frequency during the downswing of the surge in mares is slow, as in the midluteal phase, and the slow rate of decline in peripheral gonadotrophin concentrations is due, at least in part, to continued secretion between pulses. Moreover, progesterone negative feedback is highly effective in early dioestrus, in that lessening it without complete removal markedly accelerates gonadotrophin pulse frequency.