Endometrial prostaglandin E2-binding sites in the pig: characterization and changes during the estrous cycle and early pregnancy.

Because prostaglandins (PGs) are thought to mediate, in part, endometrial responses at implantation in a number of species and because pig conceptuses are capable of producing PGs, we have characterized the binding of [3H]PGE2 to endometrium obtained from pigs during the estrous cycle and early pregnancy. Binding of [3H]PGE2 to a membrane preparation obtained from endometrium was temperature- and pH-dependent, and reversible. Pretreatment of the membrane preparation with proteolytic enzymes, phospholipase A2, or heat reduced subsequent specific binding of [3H]PGE2. PGE1 and PGE2 competed equally for binding, whereas relative cross-reactivity for other prostanoids and compounds tested was less than 5%. Scatchard analysis of specific binding suggested that two classes of binding sites were present, a high-affinity binding site with an apparent dissociation constant (Kd) of approximately equal to 6 pM, and a lower-affinity site with an apparent Kd of approximately equal to 3 nM. For the high-affinity binding site, estimates of apparent Kd did not differ between days of the estrous cycle, but maximum binding capacity did change, being lowest on Day 10 and highest on Days 0-5 of the cycle. For the lower-affinity binding site, estimates of the apparent Kd differed, being lowest on Day 10 and highest on Day 15; numbers of binding sites were lowest on Day 5 and highest on Day 15. Estimates of these variables on Day 15 of pregnancy did not differ significantly from Day 15 of the cycle. These data are consistent with the presence of PGE receptors within the endometrium of the pig.