The present study examined the effects of LH, prostaglandin E₂ (PGE₂), 8-bromo-cyclic AMP (cAMP) and forskolin on progesterone secretion by small and large pig luteal cells. Corpora lutea were isolated from gilts (n ≥ 3 per day) on days 9, 12 and 14 of the oestrous cycle and days 9, 12, 14 and 30 of pregnancy. After enzymatic dissociation of the corpora lutea, small and large luteal cells were obtained by elutriation. Culture plates (24-well) were then seeded with 150 000 small luteal cells or 30 000 large luteal cells per well in 1 ml M199 medium in the absence or presence of LH, PGE₂, LH plus PGE₂, 8-bromo-cAMP or forskolin. After 12 h of incubation, culture plates were centrifuged, and the supernatant collected and frozen for subsequent assay of progesterone. Differences within day were not detected between cyclic and pregnant gilts, and thus, results were combined for days 9, 12 and 14. Basal progesterone secretion by small luteal cells was less (P < 0.05) on days 14 and 30 than days 9 and 12. Treatment with LH, PGE₂, 8-bromo-cAMP or forskolin increased (P < 0.05) progesterone secretion by small luteal cells on days 9 and 12; however, treatments had no effect on days 14 and 30. Basal progesterone production by large luteal cells was less (P < 0.05) on day 30 compared with other days. PGE₂ stimulated (P < 0.001) progesterone production by large luteal cells at all days. In contrast, 8-bromo-cAMP and forskolin inhibited progesterone production by large luteal cells on day 12 (P < 0.05), and day 14 (P < 0.001). These data show that pregnancy status does not alter luteal cell response to the aforementioned secretagogues. However, regulation of progesterone secretion differs between small and large luteal cells, and the age of the corpora lutea. Also, it is unlikely that the stimulatory actions of PGE₂ involve increased cAMP production in pig large luteal cells.

This study examined the effects of LH and PGE₂ on progesterone secretion by small and large porcine luteal cells with or without low-density lipoproteins. Corpora lutea were isolated from gilts 13–14 days after administering gonadotrophins; enzymatically dissociated and small and large cells were isolated by elutriation. Culture plates, 24-well, were then seeded with 150 000 small or 30 000 large luteal cells suspended in 1 ml M199 medium supplemented with 5 μg insulin ml⁻¹, 40 ng hydrocortisone ml⁻¹ and with or without low-density lipoproteins (50 μg cholesterol ml⁻¹) or PGE₂. Cells were cultured for up to 24 h in a humidified incubator at 37°C under 5% CO₂ in air. The low-density lipoproteins stimulated (P < 0.05) progesterone secretion by large, but not small, luteal cells. Prostaglandin E2 stimulated (P < 0.05) progesterone production by large luteal cells in a dose-dependent manner, and the stimulatory effects of PGE₂ were greater (P < 0.05) in the presence than in the absence of low-density lipoproteins. Progesterone secretion by small luteal cells was not significantly affected by PGE₂. Progesterone production by small luteal cells was enhanced (P < 0.05) by LH, and the stimulatory effects of LH were greater (P < 0.05) in the presence than in the absence of low-density lipoproteins. In the absence of these lipoproteins, LH had no effect on progesterone secretion by large luteal cells; however, in the presence of low-density lipoproteins, LH increased (P < 0.05) progesterone secretion by large cells, though to a lesser (P < 0.05) extent than the effect of LH on small cells. These data demonstrate that progesterone secretion by porcine luteal cells is stimulated differentially by LH and PGE2 and that small luteal cells are more responsive to LH and PGE2 acts primarily on large luteal cells.