Evidence for the Presence of Luteinizing Hormone/ Human Chorionic Gonadotropin-Binding Sites in the Porcine Uterus

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Abstract
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 (Ka = 7.8 .times. 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 [125I]hCG by LH receptors in female uterine tissue.