Luteal LH receptors during the oestrous cycle and early pregnancy in the pig

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Abstract
The concentration, affinity constant and occupancy of the LH [luteinizing hormone, lutropin] receptor were measured in corpora lutea from 29 pigs at days 6-16 of the estrous cycle and from 25 pigs at days 12-30 of pregnancy using 125I-labeled porcine LH tracer. Investigation of the specificity of the receptor showed that cross-reactions of other pituitary hormones were accounted for by their contamination with LH. Luteal concentration of unoccupied receptors doubled between days 6-10 of the cycle and decreased between days 12-14; it increased 3-fold between days 20-30 of pregnancy but was lower on day 12 of pregnancy than at a comparable stage of the cycle. Concentrations of receptors occupied by LH increased early in the estrous cycle, in parallel with the total receptor concentraiton; in pregnancy the percentage occupancy dropped dramatically as total receptor concentration increased between days 20-30. Receptor affinity constants increased towards the end of the cycle and decreased between days 20-30 of pregnancy. The lower concentration of receptors in early pregnancy may reflect down regulation by circulating LH, concentrations of which are higher in early pregnancy than during the cycle. The increase in receptor concentration between days 20-30 of pregnancy may be due to a rise in circulating estrogen levels. The decrease in occupancy at this time may be caused by a decrease in affinity constant or by placental production of a chorionic gonadotropin-like compound.