Summary. Corpora lutea were collected from sheep on Days 6,10, and 15 of the oestrous cycle and Day 25 of pregnancy and dissociated into single cell suspensions. Purified preparations of large and small luteal cells were prepared by elutriation on all days except Day 6. Basal progesterone production by large cells was 6–8-fold higher than by small cells (36–65 vs 6–9 fg/cell/min). Oxytocin secretion was maximal on Day 6(1·0 fg/cell/min) and declined thereafter. The number of receptors for LH increased between Day 6 and Day 10 and the two cell types had an equal number of receptors on Days 10 and 15 (19 000–23 000). Large cells on Day 25 of pregnancy had fewer receptors (12 000) than did small cells (26 000). Progesterone secretion by small luteal cells from all days examined was stimulated by LH (0·01–1000ng/ml) in a dose-dependent manner; maximum sensitivity to LH occurred on Day 10. Despite the presence of receptors for LH on large cells, LH failed to stimulate progesterone production. Basal production of progesterone by large and small cells, and the response of small cells to LH, was not influenced by day examined. Re-combinations of large and small cells from Day 10 synergized to increase progesterone secretion. Prostaglandin E-2 (0·1–1000 ng/ml) did not stimulate progesterone secretion by large or small cells.
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