Luteal Membrane Binding of Prostaglandin F2αand Sensitivity of Corpora Lutea to Prostaglandin F2α-Induced Luteolysis in Pseudopregnant Rats*

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Abstract
Newly formed corpora lutea of many mammalian species are resistant to the luteolytic effect of exogenous prostaglandin F2cc. The affinity and capacity of prostaglandin F2cc binding by membranes of rat corpora lutea were characterized on days 4, 6, 8, and 10 of pseudopregnancy to determine whether these factors affect the different response of the corpus luteum to prostaglandin F2cc at various stages of pseudopregnancy. The average affinity ranged from 9.4 ± 1.6 nm (mean ± SE) on day 4 to 10.7 ± 1.4 nm on day 8 of pseudopregnancy. The binding capacity ranged from 0.394 ± 0.082 pmol bound/mg protein on day 4 to 0.503 ± 0.086 pmol bound/mg protein on day 6. The affinity and binding capacity of luteal membranes for prostaglandin F2cc were similar at all stages of pseudopregnancy. The resistance of the rat on day 4 of pseudopregnancy to prostaglandin F2cc-induced luteolysis does not appear to be a consequence of low affinity or capacity of prostaglandin F2cc binding by the corpora lutea. Luteal cells isolated from corpora lutea of rats on days 4 and 8 of pseudopregnancy responded in vitro to LH with an increase in progesterone secretion. The addition of prostaglandin F2a had no effect on basal progesterone secretion but did inhibit LH stimulation of progesterone secretion by cells obtained on both day 4 and day 8. The ED50 for prostaglandin F2a inhibition of LH-stimulated progesterone secretion was about 50 nM and was similar on days 4 and 8 of pseudopregnancy. These results confirm the integrity of the prostaglandin F2cc receptor indicated by membrane binding analysis and suggest that the resistance to exogenous prostaglandin F2cc demonstrated in vivo by the rat corpora lutea on or before day 4 of pseudopregnancy is mediated by factors other than receptor-ligand interaction per se. (Endocrinology106: 1333, 1980)