LH/hCG-binding sites were measured in crude membrane fractions of porcine uteri. Specific high affinity and low capacity receptors for LH/hCG were found in all (n = 17) membrane preparations of myometrium but in only 5 of 17 crude membrane fractions of endometrium of porcine uteri. There was very little competition between hCG and porcine GH (pGH), bovine TSH, pFSH, and pPRL (0.5%, 0.3%, 0.2%, and<0.005%, respectively). Specificity of [¹²⁵I]hCG binding to other tissues was determined by incubating crude membrane preparations of heart, skeletal muscle, liver, and kidney. Numbers and affinities of available LH/hCG-binding sites were characterized in all samples of myometrium and 5 endometrium membrane preparations that were positive for LH/hCG receptors.

The results indicate that the number of uterine LH-binding sites in myometrium (0.66 ± 0.17 fmol/mg) is 10 times less than the receptor capacity in porcine corpora lutea (7.46 ± 0.54 fmol/ mg) when expressed per mg protein of crude membrane preparation. However, it is approximately 60 times less when expressed per mg DNA equivalent of initial homogenate (1.31 ± 0.28 vs. 81.18 ± 3.64 fmol/mg, respectively). Receptor affinities of uterine LH/hCG-binding sites remained comparable to those of corpora lutea receptors (Kfl = 7.8 X 1010 M⁻¹). Concentrations of LH/hCG-binding sites in myometrium taken from gilts in the late follicular phase of the estrous cycle (0.13 ± 0.06 fmol/mg protein; n = 5) were significantly less (P< 0.05 and P< 0.01) compared to those in myometrium from luteal phase (0.85 ± 0.22 fmol/mg protein; n = 6) or early pregnancy (1.03 ± 0.15 fmol/mg protein; n = 6), respectively. This is probably the first evidence demonstrating specific binding of [126I]hCG by LH receptors in female uterine tissue. (Endocrinology119: 1159–1163,1986)