Progesterone and progesterone metabolite concentrations in implantation sites in the pregnant rat

Abstract

Endometrial uptake and metabolism of progesterone were studied in early pregnant rats to determine whether the presence of blastocysts alters endometrial progesterone dynamics during the periimplantation period. Animals were anesthetized and infused continously on Day 6 of pregnancy with [3H] progesterone. A control group of rats carried a surgically induced decidua in one cornu. At 90 min, rats were injected intravenously with Evans blue to stain implantation sites and decidua. At 120 min, blood was obtained from the inferior vena cava, the uterus removed, and placed on ice. Implantation and interimplantation sites, decidualized and undecidualized endometrium were identified, carefully scraped free of the myometrium and stored at −20°C until assayed. Radiolabeled progesterone and its metabolites were extracted from endometrial tissues with ethyl acetate, isolated by thin layer chromatography, counted, and calculated. Serum progesterone was measured by radioimmunoassay. The concentration of progesterone was significantly (p<0.03) greater in implantation (0.96±0.28 nmol/g) than interimplantation (0.53±0.17 nmol/g) sites. Progesterone metabolite concentration was significantly greater (p<0.02) in implantation than interimplantation sites (1.59 ± 0.40 and 1.21 ± 0.36 nmol/g, respectively). Tissue/serum ratios of both progesterone and progesterone metabolites were significantly (p<0.02) higher in implantation sites (1.7 and 0.63) than interimplantation sites (1.0 and 0.48, respectively). In control rats, progesterone concentrations in decidualized and undecidualized endometrium were 0.51 ± 0.18 and 0.63 ±0.10 nmol/g, while metabolite concentrations were 0.91 ± 0.33 and 0.86 ± 0.23 nmol/g, respectively. The results demonstrate that progesterone and metabolites of progesterone are higher in implantation than adjacent interimplantation sites. The data suggest that the presence of a blastocyst locally increases endometrial progesterone concentration and metabolism.