The concentration of 6-keto-prostaglandin F1alpha is markedly elevated at the site of blastocyst implantation in the rat.

The initiation of blastocyst implantation in the rat is indicated by localized increases in endometrial vascular permeability at the sites where blastocysts are present. The concentrations of 6-keto-prostaglandin F1alpha (6-keto-PGF1alpha), a stable metabolite of prostaglandin I2 (PGI2), were measured by gas chromatography-mass spectrometry in the areas of increased endometrial vascular permeability (uterine dye sites), and compared with those in the remainder of the uterus (uterine non-dye sites). For rats killed either on the evening of Day 5 of pregnancy or on the morning of Day 6, measurable amounts of 6-keto-PGF1alpha were found in the dye sites of all animals, whereas 1 of 6 and 4 of 6 rats killed on Days 5 and 6, respectively, had undetectable amounts (less than 1 ng) in non-dye site tissue. It was estimated that, on average, the concentration of 6-keto-PGF1alpha in dye sites on the evening of Day 5 is at least 40-fold that in non-dye sites. The possible role of PGI2 in the initiation of blastocyst implantation is discussed.