Uterine luteinizing hormone/human chorionic gonadotropin-binding sites in the early pregnant rat uterus: evidence for total occupancy in the periimplantation period.

We have investigated the presence of high affinity LH/hCG-binding sites (RLH) in crude membranes from early pregnant rats uteri. The uterine concentration of available RLH increased from day 1 to day 3 (1.3 +/- 0.2 vs. 2.8 +/- 0.4 fmol/mg protein) without a change in the affinity constant (approximately 5 x 10(10) M-1). However, unoccupied uterine RLH disappeared in the periimplantation period (days 4-6). To determine if the drop in available RLH was consecutive to their occupancy, uterine membranes were treated with acidified medium (25 mM Tris-HCl, and 5 mM MgCl2, pH 2.5) to remove endogenous ligand. The number and affinity of total (occupied plus available) RLH in acid-eluted membranes were estimated by Scatchard analysis of [125I]hCG binding and compared with those of available RLH in untreated membranes from the same uterine preparation. The uterine concentration of total RLH increased first between days 1 and 2 (2.2 +/- 0.5 vs. 4.2 +/- 0.8 fmol/mg protein), then between days 3 and 4 (4.2 +/- 0.6 vs. 6.5 +/- 0.8 fmol/mg protein), before plateauing until day 6. Thus, the reduction in the available uterine RLH in the periimplantation period is largely due to occupancy, rather than down-regulation, of RLH. The occupancy of uterine RLH 1) increased during early pregnancy (day 1, approximately 20%; days 2-3, approximately 40%; days 4-6, approximately 100%), 2) paralleled the increase in total RLH number, and 3) was probably due to pituitary LH only. However, the blastocyst itself seemed to influence uterine RLH occupancy, since available uterine RLH were detected on day 5 of pseudopregnancy. The increase in total uterine RLH as well as the perfect synchrony between their occupancy and the previously described pattern of uterine cAMP concentration during rat early pregnancy suggest that the response of uterine (and more precisely luminal epithelium) adenylate cyclase to LH (and/or related substance originating from embryo) may determine uterine receptivity for ovoimplantation and subsequent decidualization.