Low molecular weight lipid-soluble luteotrophic factor(s) produced by conceptuses in cows

Peripheral plasma progesterone concentrations are higher (P less than 0.05) in pregnant than in inseminated non-pregnant or cyclic heifers between Days 10 and 18 after insemination. In several experiments, it has been demonstrated that bovine conceptuses produce a low molecular weight (Mr less than 10,000), lipid-soluble, heat-labile, dextran-coated charcoal-adsorbable luteotrophic substance(s) that has the ability to stimulate progesterone synthesis in dispersed bovine luteal cells. This substance does not appear to be PAF, since addition of PAF to dispersed luteal cells at several dose levels failed to affect either basal or LH-stimulated progesterone synthesis. However, results of in-vitro and in-vivo experiments suggest that platelet-derived products are luteotrophic. In in-vitro experiments it was shown that co-incubation of dispersed bovine luteal cells with bovine platelets augments basal progesterone synthesis. Serotonin and platelet-derived growth factor appear to be the major products of platelet activation responsible for the luteotrophic activity of platelets. Products of the arachidonic acid cascade do not appear to be important, since addition of the cyclo-oxygenase blocker indomethacin did not reduce the luteotrophic activity of platelets. In in-vivo experiments, it has been possible to demonstrate a significant thrombocytopenia in pregnant heifers between Days 7 and 16 after insemination. These results are compatible with the concept that release of platelet-derived products under the influence of factors produced by the early embryo play a role in stimulating progesterone synthesis by the corpus luteum during early pregnancy.