Changes in serum concentrations of luteinizing hormone and follicle-stimulating hormone following injection of gonadotropin-releasing hormone during pregnancy and after parturition in mares.
High concentrations of estrogens in the peripheral circulation during late gestation inhibit synthesis of LH and markedly reduce pituitary content of LH at the end of pregnancy in most domestic species. Because blood concentrations of estrogen peak shortly before mid-gestation in the mare and then gradually decrease until parturition, we hypothesized that pituitary content of LH may increase during late gestation. To test this hypothesis 10 horse mares were challenged with a maximally stimulatory dose (2 micrograms/kg) of GnRH on d 240 and 320 of gestation and d 3 after parturition. A separate group of four mares were treated with GnRH on d 2 or 3 estrus. Blood samples were collected at -2, -1, 0, .25, .5, .75, 1, 1.5, 2, 2.5, 3, 3.5, 4, 5, 6, 7 and 8 h relative to injection of GnRH and serum was analyzed for concentration of LH and FSH. Basal serum concentration and total quantity of LH released after GnRH stimulation (assessed by determining the area under the response curve) were not different on d 240 and 320 of gestation or on d 3 after parturition (12.5 +/- 3.5, 5.7 +/- 1.5 and 29.1 +/- 12.1 ng.min/ml, respectively) and were less (P less than .05) than on d 3 of estrus (311.0 +/- 54.0 ng.min/ml). There was little difference in the basal serum concentration of FSH at any of the time points examined. In contrast, GnRH-induced release of FSH continually decreased (P less than .05) from d 240 of gestation (559.8 +/- 88.9 ng.min/ml) to d 3 of estrus (51.8 +/- 6.2 ng.min/ml).(ABSTRACT TRUNCATED AT 250 WORDS)
Nett
T M
TM
Dept. of Physiol, Colorado State University, Fort Collins 80523.
Shoemaker
C F
CF
Squires
E L
EL
D000818:Animals; D004971:Estrus / Q000187:drug effects / Q000378:metabolism; D005260:Female; D005640:Follicle Stimulating Hormone / Q000097:blood*; D006736:Horses / Q000097:blood*; D007743:Labor, Obstetric / Q000097:blood* / Q000187:drug effects; D007986:Luteinizing Hormone / Q000097:blood*; D010906:Pituitary Hormone-Releasing Hormones / Q000494:pharmacology; D011247:Pregnancy; D011270:Pregnancy, Animal / Q000097:blood* / Q000187:drug effects
D010906:Pituitary Hormone-Releasing Hormones; D007986:Luteinizing Hormone; D005640:Follicle Stimulating Hormone