High affinity luteinizing hormone/human chorionic gonadotropin (LH/hCG) receptors have been identified in porcine, rabbit and rat uteri and immunocytochemically demonstrated in the human uterus. We have now assessed the effect of estradiol and progesterone on the capacity and affinity of LH/hCG binding sites in crude membrane fractions of porcine myometria. Nineteen cross-bred gilts were ovariectomized at 6–7 months of age. Five weeks later, the experiment was conducted and gilts were given estradiol benzoate 2 mg (N= 5), progesterone 50mg (N=4) and 2 mg of estradiol benzoate plus 50 mg of progesterone in 2 ml of corn oil (N = 6), im for five consecutive days. Controls (N = 4) received 2 ml of the vehicle. Gilts were hysterectomized 24 h after the last injection. Blood samples for assays of LH, estradiol and progesterone were collected 1 h before hysterectomy. The numbers and affinities of unoccupied LH/hCG binding sites were characterized in all samples of myometrium. The results indicate that treatment with estradiol benzoate increases (p <0.01) the number of LH/hCG binding sites compared with gilts receiving corn oil. Progesterone treatment caused elevation in the number of LH/hCG receptors (p<0.05), when compared with estradiol alone (2.9±0.3 vs 1.2±0.1 fmol/mg protein, respectively). Combined administration of estradiol and progesterone increased receptor capacity to 2.7±0.4. Steroid treatment did not alter the affinity (Ka) of [125I]hCG binding to receptors and it varied from 1.8±0.8 to 2.9±0.2 ×1011 l/mol. Concentrations of LH (pmol/l) were maintained at high (64.3±14.3), medium (28.6±10.7), and low (10.7±3.6) levels in control, progesterone and both estradiol and estradiol plus progesterone treated groups, respectively. These results indicate that estradiol and progesterone independently stimulate the appearance of unoccupied LH/hCG receptors in the porcine myometrium.

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