In vitro synthesis and secretion of ovine trophoblast protein-1 during the period of maternal recognition of pregnancy.

The trophoblast of the sheep conceptus secretes a low mol wt, acidic polypeptide called ovine trophoblast protein-1 (oTP-1) from days 13-21 of pregnancy. This protein has been proposed to have an antiluteolytic role during pregnancy, since infusion of oTP-1 into the uterine lumen of nonpregnant ewes lengthened their estrous cycles. In the present study, mRNA from day 16 conceptuses was translated using a cell-free wheat germ lysate, and oTP-1 was immunoprecipitated from the translation mixture by means of a specific rabbit antiserum. The predominant protein synthesized under these conditions was oTP-1, which had a mol wt (21,000) higher than that of oTP-1 secreted by intact conceptuses into culture medium (mol wt, 17,000). In a separate experiment, in vitro secretion of oTP-1 from conceptuses on days 13, 17, and 21 of pregnancy was examined. Conceptuses were cultured for 24 h in the presence of [3H]leucine, and oTP-1 was immunoprecipitated from the culture medium. Incorporation of [3H]leucine into secreted macromolecules (mol wt, greater than 3500) increased 12-fold from days 13 to 21, while release of immunoprecipitable 3H-labeled oTP-1 increased 27-fold from days 13 to 17 and declined by over 50% from days 17 to 21. When results were expressed per microgram DNA, secretion of total macromolecules declined from days 13 to 17 and then remained constant until day 21. The pattern of oTP-1 secretion held whether results were expressed per conceptus or per unit DNA. The interestrous intervals for the donor ewes were shorter for ewes which had their conceptuses removed on day 13 than for ewes flushed on day 17 or 21. Thus, the period of oTP-1 synthesis corresponds closely to the time at which the conceptus acts to extend luteal lifespan.