Concentrations of progesterone in the backfat of pigs during the oestrous cycle and after ovariectomy

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Abstract
Small samples of backfat were taken daily during 1 estrous cycle and more frequently after ovariectomy from 12 gilts by means of a simple biopsy technique, and the levels of progesterone were determined. Compared to the levels of progesterone in peripheral plasma changes in backfat levels during the estrous cycle were delayed by 1-2 days. Maximal levels with 89.7 .+-. 9.2 (mean .+-. SEM [standard error of the mean]) ng progesterone/100 mg backfat were recorded on day 15 of the estrous cycle. It was estimated that, on this day, a total amount of .apprx. 36 mg progesterone is stored in the adipose tissue, which is .apprx. 200 times that present in total blood and corresponds to the daily production of the corpora lutea of the sow on Day 11. Initial half-life of progesterone in backfat after ovariectomy was estimated to be .apprx. 34 h compared to an initial half-life of plasma progesterone of .apprx. 120 min. The exact calculation of half-lives was, however, confounded by an obvious effect of anesthesia or surgery on progesterone levels. Changes in backfat or plasma progesterone concentrations were not affected by the fat-to-lean ratio of the gilts. Fat progesterone levels determined in 44 additional pregnant and non-pregnant sows 17 or 20 days after mating indicated that reliable diagnosis of non-pregnant sows was possible on day 20. The endocrinology of the estrous cycle in pigs is related to the enormous storage of progesterone in the fat.