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Summary. Five new concepts concerning the control of corpus luteum function in the cow have been developed in recent years.

1. Prostacyclin (PGI-2) plays a luteotrophic role. Conversely, products of the lipoxygenase pathway of arachidonic acid metabolism, particularly 5 hydroxyeicosatetetraenoic acid (5-HETE), play luteolytic roles.

2. Luteal cells arise from two sources. The small luteal cells are all of theca cell origin; the large cells found early in the cycle (Days 2–6) are mainly of granulosa cell origin. However, a population of large cells found after Day 10 of the cycle are of theca cell origin.

3. Oxytocin of luteal cell origin plays a role in development of the corpus luteum and possibly in its regression.

4. The recently described Ca²⁺-polyphosphoinositol–protein kinase C second messenger system, as well as the LH–cAMP system, is involved in control of progesterone synthesis in the bovine corpus luteum.

5. Progesterone synthesis in the small theca-derived luteal cells is primarily controlled by the cAMP system. However, elevated intracellular calcium diminishes cAMP-mediated progesterone synthesis in these cells.

These findings modify our current concepts of the mechanisms of control of progesterone secretion by the corpus luteum and suggest several new lines of research.

Summary. Corpora lutea were collected from Holstein heifers on Days 10 and 12 of the oestrous cycle and the cells were dispersed with collagenase. The dispersed cells were separated into preparations of highly purified (90–99%) small (<20 μm) and large (> 25 μm) luteal cells by unit gravity sedimentation and fluorescence-activated cell sorting. Net progesterone accumulation by 1 × 105 small cells and 1 × 103 large cells during 2 and 4 h incubations, respectively, were measured after additions of LH, PGF-2α, and phorbol esters, alone and in combination. Progesterone synthesis was increased (P < 0·05) by phorbol dibutyrate (PBt₂) or PGF-2α (P < 0·05) in small, but not in large, luteal cells (10·1 ± 3·0 and 18·1 ± 5·0 ng/105 cells for 0 and 50 nm-PBt₂, and 19·9 ± 3·2 and 44·2 ± 9·3 ng/105 cells for 0 and 1 μg PGF-2α/ml). The previously reported stimulatory effects of PKC activation and PGF-2α addition to total dispersed cell preparations are therefore entirely attributable to the small, theca-derived cells. Small cells responded to low levels of LH (9·1 ± 1·1, 69·0 ± 5·4 and 154·7 ± 41·4 ng/105 cells for 0, 1 and 5 ng LH/ml, respectively, P < 0·05), while large cells responded only to high levels of LH (1635 ± 318, 2662 ± 459 and 3386 ± 335 pg/103 cells for 0, 100 and 1000 ng LH/ml, respectively, P < 0·05). PGF-2α inhibited LH-, 8-Br-cAMP-and forskolin-stimulated progesterone synthesis in the large cells (3052 ± 380, 3498 ± 418, 3202 ± 391 pg/103 cells for 1 μg LH/ml, and 0·5 mm-8-Br-cAMP, and 1 μm-forskolin respectively and 1750 ± 487, 2255 ± 468, 2165 ± 442 pg/103 cells for PGF-2α + LH, PGF-2α + 8-Br-cAMP and PGF-2α + forskolin, respectively), indicating that the inhibitory effect of PGF-2α on progesterone synthesis in large cells occurs at a site distal to cAMP generation. These results suggest that the large cells are the targets of the luteolytic effects of PGF-2α, while the small cells are responsible for the previously reported luteotrophic effect of PGF-2α in vitro.

Keywords: corpus luteum; cell sorter; prostaglandin-F2α; progesterone; cow