Prostaglandin F2αInhibits Luteinizing Hormone (LH)-Induced Increase in LH Receptor Binding to Isolated Rat Luteal Cells*

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Abstract
The effect of the luteolytic hormone prostaglandin F2.alpha. (PGF2.alpha.) on parameters of LH [luteinizing hormone] receptor binding to isolated rat luteal cells was exmained. The quilibrium binding constant (0.55 .times. 1010 m), the association rate constant (0.89 .times. 108 M min), and the dissociation rate constant (1.70 .times. 10-2 M min) were not significantly altered by PGF2.alpha.. However, as [125I]iodo-hCG [human chorionic gonadotropin] binding approached equilibrium (< 2 h) and at equilibrium (> 3 h), PGF2.alpha., but not PGE2, consistently reduced LH binding by 10-20%. The LH receptor-binding capacity determined Scatchard analysis of [125I]iodo-hCG or [125I]iodo-hLH binding, was reduced from 7.5 .+-. 0.1 to 6.4 .+-. 0.1 .times. 104 receptors/cell in the presence of PGF2.alpha.. This reduction of total cell-bound radioactivity by PGF2.alpha. was not due to a change in the rate of hormone internalization and degradation. However, when cells were briefly treated with a pulse of LH (5 ng/ml; 5-10 min), [125I] iodo-hCG (LH) binding increased 20-30% within 2 h, and PGF2.alpha. prevented this LH-induced increase. A similar pulse of cAMP analogs did not alter LH binding. While the initial binding of LH to its receptor is not altered by PGF2.alpha., it does reduce the final equilibrium level of LH binding by a mechanism that involves a block in the appearance of LH-induced cryptic receptors.