Changes in Plasma Estrogen, Luteinizing Hormone, Follicle-Stimulating Hormone and 13, 14-Dihydro-15-Keto-Prostaglandin F2α during Blockade of Luteolysis in Pigs after human Chorionic Gonadotropin Treatment1

An injection of human chorionic gonadotropin (HCG) or estrogen on d 12 of the estrous cycle delays luteolysis in the pig. In an experiment to determine if HCG stimulated estrogen secretion, 21 cyclic pigs received one of five different amounts of HCG–(A) 0, (B) 125, (C) 250, (D) 500 or (E) 1,000 IU–as a single, im injection in 2 ml of distilled water on d 12 of the estrous cycle. Blood was collected from the jugular vein immediately before HCG injection and once daily thereafter until d 20 of the estrous cycle. Plasma progesterone, estrogen (unconjugated) and 13,14-dihyro-15-keto-pros-taglandin F2α (PGFM) were quantified for pigs in all groups; luteinizing hormone (LH) and follicle-stimulating hormone (FSH) were quantified for pigs in groups A and E. The HCG injection exerted a dose-related increase on the mean interestrus interval (groups A, B, C, D and E were 20.5, 20.2, 22.5, 31.0 and 61.4 d, respectively) and on the delay of luteolysis as measured by mean plasma progesterone on d 16 (A, B and C vs D and E, respectively, 1.9, 1.2 and 10.4 vs 34.1 and 47.1 ng/ml; P<.05). The HCG injection caused a transitory increase in plasma estrogen from d 12 (5 to 10 pg/ml before treatment) to d 15 (35.5 pg/ml, group D) and to d 16 (90.2 pg/ml, group E) before it decreased to preinjection levels on d 17 (group D) and 18 (group E). No preovulatory surge of LH or FSH was detected for pigs in group E; plasma LH and FSH were depressed compared with group A (P<.05). In contrast, in groups A and B, plasma estrogen peaked on d 19 and culminated in preovulatory LH and FSH surges on d 20 in group A. The late luteal phase increases in plasma PGFM typical of cyclic pigs were present in pigs of groups A and B; these increases in plasma PGFM were absent in pigs of group E. Data are consistent with the hypothesis that injection of HCG stimulated follicular estrogen production and secretion.