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Acetate and plasma cholesterol as progesterone precursors in the intact ovary of the Day-16 pregnant rat

Summary. Rats at Day 16 of pregnancy were infused, via the parametrial artery, with radioactively labelled acetate or cholesterol. All of the venous effluent was collected and examined for radioactive progesterone and related compounds. Labelled progesterone was found in the venous effluent within 15 and 30 min of the start of cholesterol and acetate infusions, respectively, which shows that the ovary can extract both precursors from blood and convert them to progesterone in the intact animal. However, the proportion of secreted progesterone derived from acetate (estimated from labelled progesterone in venous blood) was only 0·21 ± 0·10% (mean ± s.e.m.; n = 5). Furthermore, the quantity of labelled progesterone and immediate progesterone precursors remaining in the ovary indicated that less than 0·22 ± 0·10% of progesterone could have been derived from acetate. In contrast, in rats infused with labelled cholesterol 42 ± 9% of the progesterone was derived from this source and, from labelled progesterone and immediate precursors of progesterone remaining in the ovary, it was calculated that 53 ± 10% could have been derived from blood-borne cholesterol.