LIVER BLOOD-FLOW AND METABOLIC-CLEARANCE RATE OF PROGESTERONE IN SHEEP

To study the effects of feeding on concentrations of peripheral plasma progesterone, ovariectomised ewes, given exogenous progesterone, were fed 750 g of chopped lucerne hav at either 09.00 (group A, n=5) or at 15.00 (group B, n=5) or were fed ad libitum regularly through the exp erimental period (group C, n=5). Peripheral blood samples were taken f rom each ewe at 09.00, 11.00, 15.00, 17.00 and 23.00. In ewes of group s A and B, mean plasma progesterone concentrations declined significan tly (P<0.05) after feeding. Mean progesterone concentrations of group C ewes remained low and were significantly different (P<0.05) from the pre-feeding values of group A and B ewes. These results showed that t he metabolic clearance rate of progesterone changed with the act of fe eding. In a second experiment, portal, hepatic and mesenteric vein can nulae were placed in ovariectomised ewes (n=17). They were then feda r ation calculated to maintain liveweight for seven days (M ration). The ewes were then infused with progesterone into a jugular vein and with p-amino hippuric acid into a mesenteric vein. Ewes were then allotted to receive either half M or twice M rations for seven days after whic h the infusions and blood sampling procedures were repeated. The mean rate of blood flow in the portal vein was directly related to the leve l of feed offered when ewes received either 1/2M, M or 2M rations. The liver and gut region removed a mean of 96 per cent of the progesteron e entering these tissues. It was concluded that an increased blood flo w through the gut and to the liver, coupled with the high efficiency o f these tissues to metabolise progesterone, explained the increased me tabolic clearance rate of progesterone associated with increased feed intake.