Feed intake affects metabolic clearance rate of progesterone in sheep

To study the inverse relationship between feed level and concentrations of peripheral plasma progesterone, 36 ovariectomised ewes were fed rations at levels calculated to maintain liveweight (M). On the seventh day, the ewes were given an intravenous infusion of progesterone and the metabolic clearance rate (MCR) was calculated. The ewes were then randomly allotted to receive either 1/2M, M or 2M rations for seven days at which time the infusion and blood sampling schedules were repeated. The mean (SE) MCR of infused progesterone in ovariectomised ewes fed either 1/2M, M or 2M rations for seven days was 7.1 (0.59), 9.9 (1.64 and 13.0 (1.19) litre h-1 kg-1 of liveweight, respectively. Differences in MCR of progesterone between ewes fed 1/2M and 2M rations were significant (P < 0.05). The inverse relationship between level of feed intake and plasma progesterone concentration was attributed to differences in clearance rate of progesterone rather than to changes in the entry rate of the hormone into the blood.