The dynamics of progesterone uptake and metabolism in the mammary gland of the goat have been measured and related to the metabolic clearance rate and production rate of the hormone determined by tracer kinetic techniques.
The metabolic clearance rate of progesterone from blood was 3·13 ± 0·35 (s.e.m.) 1/min in ten experiments on six goats; values tended to be slightly higher in pregnant than in non-pregnant goats. The production rate of progesterone at oestrus, and at day 3 of the normal cycle, was less than 0·01 μg/min. During the luteal phase of the oestrous cycle the production rate was 8·5 and 14·6 μg/min in 2 animals, and in the second half of pregnancy, 15·3 ± 0·6 μg/min (5 animals).
Progesterone was extracted from the circulation by the mammary gland of conscious goats with an efficiency of 49·4 ± 11·3% in non-pregnant, and 51·7 ± 11·5% in pregnant animals. The mean clearance rate of progesterone by the udder was 0·2791/min, 8·8% of the metabolic clearance rate. Mammary uptake of progesterone in goats with an actively secreting corpus luteum was 0·64 ± 0·29μg/min, which gave an estimated value of 0·11– 1·88 ng/min/g mammary gland.
The mammary extraction of progesterone was investigated in a goat 3 days after oestrus when any high affinity receptor sites would presumably be unoccupied. During the infusion of progesterone into a mammary artery, tissue samples were taken from various organs, including the mammary gland, and the concentration of labelled compounds at steady state was determined. A high mammary extraction of progesterone was found to be attributable principally to progesterone metabolism. The metabolites of progesterone were removed from the gland in venous blood and were not stored to any appreciable extent in mammary tissue.
Experiments in vitro confirmed the findings in vivo that mammary tissue metabolized labelled progesterone and also pregnenolone and androstenedione; metabolism of dehydroepiandrosterone, oestradiol-17β, oestrone and cortisol was relatively small. Confirmation of our previous finding that the mammary gland of the goat can synthesize progesterone from labelled pregnenolone infused into the gland in vivo, further implicates this organ as an active site of metabolism of certain steroids. The physiological role of steroid metabolism in the mammary gland is discussed.
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