Corpus luteum maintenance during early pregnancy in the cow.

Holstein heifers (18 pregnant, 12 inseminated but non-pregnant, and 17 cyclic) were studied during the first 18 days after oestrus. Jugular vein plasma progesterone concentrations were significantly (P < 0.05) higher in pregnant than in non-pregnant and cyclic animals between Days 10 and 18. Jugular oestradiol concentrations were also higher in pregnant than in cyclic animals between Days 6 and 16 (P < 0.05). No differences were found among the three groups in jugular LH levels during Days 1-18. Ovarian and uterine vein progesterone and oestradiol 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 oestradiol 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). However, LH-stimulated synthesis (incubated with LH minus incubated alone) was nearly identical for the three groups of tissues. Therefore, loss of responsiveness to LH does not appear to be the initial event in luteolysis. Concentrations of PGF 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 than in pregnant animals. The data suggest that the embryo may produce one or more luteotrophic substances that stimulate increased progesterone secretion by the corpus luteum beginning as early as Day 10 of pregnancy.