Corpus luteum maintenance during early pregnancy in the cow

Abstract

Holstein heifers (18 pregnant, 12 inseminated but non-pregnant and 17 cyclic) were studied during the first 18 days after estrus. Jugular vein plasma progesterone concentrations were significantly (P < 0.05) higher in pregnant animals than in non-pregnant and cyclic ones between days 10-18. Jugular estradiol concentrations were higher in pregnant than in cyclic animals between days 6-16 (P < 0.05). No differences were found among the 3 groups in jugular LH [luteinizing hormone lutropin] levels during days 1-18. Ovarian and uterine vein progesterone and estradiol concentrations suggested that the elevated peripheral plasma concentrations of these steroids in pregnant animals were due mainly to increased ovarian production, but the embryo itself may have contributed to the estradiol levels found at day 17. Luteal tissues from pregnant animals contained more progesterone when incubated alone or with LH than tissues from cyclic animals (P < 0.06). LH-stimulated synthesis (incubated with LH minus incubated alone) was nearly identical for the 3 groups of tissues. Loss of responsiveness to LH does not appear to be the initial event in luteolysis. PGF [prostaglandin F] concentrations in ovarian arterial plasma in cyclic and pregnant animals were similar at day 18, although uterine vein concentrations were higher (P < 0.05) in the cyclic animals. Free arachidonic acid concentrations in corpora lutea at day 18 were higher (P < 0.05) in cyclic animals than in pregnant ones. Apparently the embryo may produce 1 or more luteotropic substances that stimulate increased progesterone secretion by the corpus luteum, beginning as early as day 10 of pregnancy.