PROSTAGLANDIN-F2-ALPHA RECEPTORS ON ENZYME-DISSOCIATED PIG LUTEAL CELLS THROUGHOUT THE ESTROUS-CYCLE

A deficiency of luteal cell prostaglandin F2α (PGF2α) receptors might help explain the well documented refractoriness of pig corpora lutea to the luteolytic effects of PGF2α administered in vivo before day 12 of the estrous cycle. Accordingly, experiments were conducted to measure the levels of [3H] PGF2α -binding sites/receptors on collagenase-dispersed pig luteal cells taken at different stages of the estrous cycle. Pig corpora lutea were obtained surgically at various stages of the estrous cycle and dissociated with collagenase in medium 199. Dissociated mixed luteal cells (∼5-15 X 104 large luteal cells/tube) were assayed for [3H]PGF2α-binding activity by saturation (Scatchard) analysis. In preliminary experiments it was determined that PGF2o binding was maximal after incubation for 45 min at 30 C in assay buffer of pH 5.75. Additionally, it was determined that [3H]PGF2α binding was displaceable by PGF2α = PGD2 > PGE2 > 13, 14-dihydro-15-keto-PGF2α. Other eicosanoids did not inhibit [3H]PGF2α binding. Two distinct classes of binding sites (high affinity Kd = 19-64 nM; low affinity Kd = 262-3103 nM) were observed at all stages of the estrous cycle. From studies using enriched (by elutriation) large (>30 µm) and small (10-20 µm) luteal cells it appeared that the high affinity binding site was largely confined to large luteal cells, whereas the low affinity binding site was found on both large and small luteal cells. The concentrations (number per large luteal cell) of high affinity PGF2a-binding sites of mixed (unelutriated) luteal cell preparations was low on days 6-8 (0.6 x 106 sites/cell) and increased gradually up to 1.4 x 106 sites/cell on day 12. The concentrations of binding sites were increased approximately 3- fold on day 13 (4.6 x 106 sites/cell; P < 0.05 vs days 6-12) and remained elevated on days 14 and 16-17 (∼ 3 X 106 sites/cell). In summary, these results indicate the existence of a high affinity PGF2α-binding site in pig (large) luteal cells, which is probably the luteal PGF2α receptor. The numbers of these putative PGF2α receptors are low during the early luteal phase (before day 12), but increase thereafter (days 13, 14, and 16-17). This may provide one explanation for the observed refractoriness in vivo of pig corpora lutea to PGF2α before, and increased sensitivity to PGF2α after, day 12 of the estrous cycle. © 1990 by The Endocrine Society.