Effect of graded doses of gonadotrophin-releasing hormone on serum LH concentrations in mares in various reproductive states: comparison with endogenously generated LH pulses.
Luteinizing hormone release induced by a range of small (3.3-33 micrograms) and large (300-500 micrograms) i.v. doses of gonadotrophin-releasing hormone (GnRH) was measured in acyclic (n = 4), luteal phase (n = 3) and follicular phase (n = 5) mares and compared with endogenously generated LH pulses in the same reproductive states. Extrapolation from log-linear dose-response curves showed that an LH pulse comparable to an endogenous one would be simulated by i.v. injection of 7.0 (n = 4) and 4.1 (n = 6) micrograms GnRH in luteal and follicular phase mares respectively; a much smaller dose than the 500 micrograms usually given clinically or experimentally. In acyclic mares (n = 4), LH pulses occurred too infrequently to be characterized. At small doses of GnRH the amount of LH released by the same dose was similar in all three reproductive states, although the steroid hormone milieu differed markedly. This implies that observed differences between states in mean (+/- S.E.M.) serum LH concentrations (0.7 +/- 0.01, 1.2 +/- 0.03 and 11.6 +/- 0.33 (microgram/l) in acyclic, luteal and follicular phase mares respectively) were produced by differences in GnRH pulse frequency and/or amplitude and not by steroid-mediated changes in pituitary response to GnRH. In acyclic, luteal and follicular phase mares, LH pulse frequency was: immeasurably low, 0.09 and 1.14 pulses/h respectively, which supports the important contribution of pulse frequency to determining mean LH concentration.(ABSTRACT TRUNCATED AT 250 WORDS)
D000818:Animals; D004971:Estrus; D005260:Female; D005498:Follicular Phase; D006736:Horses; D007986:Luteinizing Hormone / Q000097:blood / Q000378:metabolism*; D008597:Menstrual Cycle; D010902:Pituitary Gland / Q000187:drug effects / Q000378:metabolism*; D010906:Pituitary Hormone-Releasing Hormones / Q000494:pharmacology*; D013268:Stimulation, Chemical
D010906:Pituitary Hormone-Releasing Hormones; D007986:Luteinizing Hormone