BOVINE CYCLIC ENDOMETRIUM CONTAINS HIGH-AFFINITY LUTEINIZING-HORMONE HUMAN CHORIONIC-GONADOTROPIN BINDING-SITES

High-affinity LH/hCG binding sites have been characterized in porcine, lepine, and murine uteri. In the present study, LH/hCG binding sites were characterized in bovine endometrium. Radioreceptor assays were performed with membrane homogenates of endometrial tissues and analyzed for binding site specificity and capacity. There was little competition for receptor occupancy between hCG and ovine FSH (5%) or ovine prolactin (<0.1%), but there was a 20% cross-reaction with eCG. There was no affinity for LH/hCG in crude membrane preparations of kidney, skeletal muscle, or vascular tissues. Concentrations of endometrial LH/hCG binding sites were determined during the bovine estrous cycle. LH/hCG receptors were found in cell preparations from Days 2-4 and 15-17 of the cycle, but not in preparations from the other stages of the cycle tested (Days 8-12, pre- and post-estrus, and ovulation). The concentration of uterine LH/hCG receptor varied during the estrous cycle, with higher values at Days 15-17 (3.1 fmol/mg protein) and lower values at Days 2-4 (1.2 fmol/mg protein). However, the binding capacity of hCG by luteal cells (9.7 fmol/mg protein) was 3-fold higher (p<0.01) than that by endometrial tissue on any day studied. No differences in affinity constant (K-a) were seen between endometrial LH/hCG receptors (either) from Days 2-4 or 15-17) and mid-cycle luteal cells (0.60 x 10(11) M(-1)). Using Western blot analysis, we determined the expression of cyclooxygenase (COX) during the estrous cycle of the cow. It was found that the signal for COX was strongest at 15-17 days. In the presence of LH, a 3-fold increase in COX expression was observed on Days 2-4 and 15-17. Incubation of endometrial cells with LH resulted in an increase in prostaglandin (PG) F-2 alpha production from 0.2 ng to 0.8 ng PGF(2 alpha)/3 x 10(5) cells/6 h. The data suggest that LH receptors are of physiological significance, since LH induces endometrial COX and increases PGF(2 alpha) release.