Partial characterization of an equine conceptus prostaglandin inhibitory factor

Equine conceptuses are thought to produce antiluteolytic factors that inhibit endometrial PGF2alpha and, thus, prevent luteolysis in pregnant mares. The aim of the present study was to characterize partially the chemical nature of the prostaglandin inhibitory factor (PIF) produced by equine conceptuses in vitro. Embryos were collected from pregnant mares 13 +/- 0.5 days after ovulation and were cultured for 24 h. Harvested equine conceptus conditioned media (CCM) were assayed for antiluteolytic activity by determining the inhibition of endometrial PGF2alpha synthesis in vitro. Significant antiluteolytic activity was found in the CCM at 12, 18 and 24 h. After size fractionation, antiluteolytic activity was observed in the 3-10 kDa fraction and the activity was not destroyed by proteinase K digestion. In contrast, treatment with dextran-coated charcoal removed the antiluteolytic activity from the CCM at 12, 18 and 24 h of culture (P < 0.05). These results indicate that the prostaglandin inhibitory factor is a low molecular mass (3-10 kDa) proteinase K-resistant substance that may be adsorbed by dextran-coated charcoal. The chemical nature, antiluteolytic activity and time of synthesis of the PIF by early equine conceptuses indicate that the PIF identified in the present study may be involved in the maternal recognition of pregnancy factors in mares.