Augmentation of pituitary responsiveness to luteinizing hormone/follicle stimulating hormone-releasing factor (LH-RF) as a result of acute ovariectomy in the four-day cyclic rat.
Within 6 h of ovariectomy in adult rats with 4-day estrous cycles, plasma FSH titers were significantly elevated above those of sham-operated controls at all stages of the estrous cycle, whereas plasma LH concentrations were raised by ovariectomy only during diestrus day 2, proestrus or estrus. A single intravenous injection of a partially purified extract of ovine stalk-median eminence tissue (LH-RF) into sham-operated control rats or incubation of pituitaries from similar rats with the same releasing factor extract elevated LH and FSH output at all stages of the cycle. This response to LH-RF was related to the stage of the estrous cycle, maximal pituitary responsiveness occurring on proestrus. Ovariectomy 6 h before administration of LH-RF significantly augmented pituitary LH and FSH responsiveness both in vivo and in vitro. This elevation in hypophyseal responsiveness observed in vitro followed a cyclic pattern which could be superimposed on the normal cyclic changes in pituitary responsiveness, whereas the cyclic increase in pituitary LH responsiveness to LH-RF in vivo following acute ovariectomy was first detectable 16 to 18 h earlier than in intact controls, that is, on late diestrus day 2 rather than on the morning of proestrus. These results indicate that acute ovariectomy can augment pituitary responsiveness to LH-RF and can also advance the onset of the cyclic augmentation of pituitary responsiveness to LH-RF during the 24 h immediately preceding the ovulatory gonadotropin surge. Possible explanations for these effects are discussed.
D000818:Animals; D002369:Castration; D004031:Diestrus; D004971:Estrus; D005260:Female; D005640:Follicle Stimulating Hormone / Q000378:metabolism; D007987:Gonadotropin-Releasing Hormone / Q000494:pharmacology*; D066298:In Vitro Techniques; D007986:Luteinizing Hormone / Q000378:metabolism; D010053:Ovary / Q000502:physiology*; D010902:Pituitary Gland / Q000187:drug effects*; D011247:Pregnancy; D011359:Proestrus; D051381:Rats; D013997:Time Factors
D007987:Gonadotropin-Releasing Hormone; D007986:Luteinizing Hormone; D005640:Follicle Stimulating Hormone