The effects of feed intake and body fatness on progesterone metabolism in ovariectomized gilts

We studied the effects of feed intake and fatness on metabolic clearance ra te (MCR) and half-life of progesterone in 40 ovariectomized gilts (10 x 4 l ittermates). One gilt from each litter was randomly allocated to each of fo ur treatments in a randomized block design. Gilts were reared to be either lean (Ln), 113 kg BW and 10 mm backfat measured 65 mm from the midline at t he level of the last rib (P2), or fat (F), 124 kg BW and 20 mm P2 backfat. They were ovariectomized and fitted with bilateral jugular catheters. Fat a nd Ln gilts were then fed either low (1.15 x maintenance energy, L), or hig h (2.30 x maintenance energy, H) feed intakes. Gilts received an i.m. injec tion of 130 mg of progesterone on two consecutive days (d 1 and 2). From d 3, progesterone was infused at 5.4 mg/h (130 mg/d) for 60 h. Blood samples for progesterone analysis were taken during the last 24 h of infusion and f or a further 72 h. Gilts were then slaughtered, and livers were sampled for microsomal studies. Fatness did not affect any aspect of progesterone meta bolism measured. Postprandial MCR was greater in H than in L gilts, 103.0 v s 76.1 mL.min(-1).kg BW-1 (P < .01), respectively. Feed intake did not affe ct the disappearance rate constant of progesterone (mean -.019), and the es timated half-life of progesterone was 36.5 h. High-intake gilts had larger proportional liver size (P < .001) than L gilts. Microsomal metabolism of p rogesterone and P450 enzyme concentration were similar across treatments. W e conclude that increasing feed intake increases MCR of progesterone and co uld be used to manipulate progesterone concentration in sows.