Summary. Fifty-one cyclic beef cows were mated with fertile bulls. At 36 h after the start of oestrus, cows were assigned to receive sesame oil (controls) or progesterone (100 mg) on Days 1, 2, 3 and 4 of pregnancy. Peripheral plasma concentration of progesterone was measured until slaughter on Days 5 or 14. Cows were randomly assigned to be slaughtered on Days 5 or 14 or remain intact and palpated per rectum on Day 40 to verify pregnancy. Uteri on Days 5 and 14 were flushed for recovery of luminal protein and conceptus tissue. Conceptus and endometrial tissues were cultured with [3H]leucine and submitted to two-dimensional-PAGE and fluorography.
Administration of progesterone increased peripheral plasma progesterone concentration on Day 2–5. Conceptuses recovered from progesterone-treated cows on Day 14 were advanced in development compared to conceptuses from control cows. Conceptuses recovered from progesterone-treated cows were viable as polypeptides associated with maintenance of pregnancy in cattle were synthesized and released at an earlier time and pregnancy was maintained beyond Day 40. Early progesterone stimulation altered the synthesis and release of polypeptides from endometrial explant cultures on Day 5. Results indicate a role of progesterone in the maternal regulation of conceptus growth and development in early pregnancy of cattle.