Reproductive characteristics of pregnant mares passively immunized against equine chorionic gonadotropin Public Deposited

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  • Two experiments were conducted to examine the effects of passive immunization of pregnant mares with a monoclonal antibody [MAb, 518B7, antibovine βLH which crossreacts with equine chorionic gonadotropin (eCG)] on subsequent reproductive characteristics and changes in plasma concentrations of hormones. In Exp.1 control mares (C, n=3) were not treated. Another group (PGF, n=2) received prostaglandin F₂α (PGF₂α, 5 mg, i.m.) on d 45 and every 12 h until abortion. A third group (PGF/MAb, n=4) received PGF₂α as in group PGF plus MAb (35 mg, i.v.) on d 45 and 49. Because there were no significant differences in plasma concentrations of eCG and progesterone between groups PGF and PGF/MAb, data were combined. Treatment of mares with PGF or PGF and MAb caused a transient plateau or slight decrease in plasma eCG concentrations for 2 to 24 h compared with that of controls. However, on d 46, 47, 48 and 51 eCG concentrations were lower in treated mares (P < .05). Plasma progesterone in treated mares decreased to < 1 ng/ml within 24 h. Abortion occurred in all treated mares (mean ± SE, 5.8 ± 1.0 d) but only four of six mares exhibited post-treatment estrus (d 2, 4, 4 and 28) with a duration of estrus of 4.8 ± 1.0 d (mean ± SE); two mares exhibited estrus before abortion. The interval from treatment to first ovulation was 8.3 ± 1.0 d. There was a tendency for treated mares (aborted) to have fewer luteal structures (secondary corpora lutea or luteinized follicles) than in pregnant control mares (5.2 vs 8.0 structures, respectively; P = .13). Proportion of secondary corpora lutea (CL) (66%) to luteinized follicles (33%) was similar between groups. None of the treated mares that exhibited estrus with ovulation subsequent to abortion conceived at breeding; eCG remained elevated until end of the experiment on d 100. In Exp.2 repeated treatment with MAb (MAb, n=3) on d 35 (25 mg, i.v.), 36 (35 mg), 37 (45 mg) and 38 (55 mg) appeared to delay eCG production in treated vs control (C, n=3) mares during this time. However, mean concentrations of eCG were significantly lower (P < .05) on d 39 only in MAb treated mares (.05 IU/ml) but increased after d 40 and were similar to that of controls (13.7 IU/ml). Plasma progesterone was lower (P < .05) in treated mares on d 36 only (4.8 vs 7.8 ng/ml in controls). Thereafter, progesterone increased in mares of both groups and did not differ. Treated mares tended to have a higher proportion of luteinized follicles (70%), which occurred later in gestation (mean = d 71); control mares had more ovulatory follicles (78%, mean = d 58). In both experiments treatment with MAb, at the times and dosages used, had only a short-term, transient effect on eCG, whether mares had aborted (Exp.1) or remained pregnant (Exp.2). The significant reduction in plasma progesterone in pregnant MAb-treated mares (Exp.2) on d 36, when eCG is usually low or undetectable, was unexpected. This may indicate a necessity for continued tropic stimulation of the primary corpus luteum by luteinizing hormone before appearance of eCG.
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