CIRCADIAN CHANGES IN NET NUTRIENT FLUXES ACROSS THE PORTAL-DRAINED VISCERA, THE LIVER, AND THE HINDQUARTERS IN PRERUMINANT CALVES

The objective of this study was to describe in preruminant calves the circadian patterns in net nutrient fluxes across the portal-drained vi scera, the liver, and the hindquarters and to relate them to previousl y published variations in tissue energy expenditure. In vivo arterio-v enous techniques were used, and animals were fed a conventional milk r eplacer. In the portal-drained viscera, net glucose absorption occurre d 1 to 5 h postprandially with some lactate release. Arterial plasma n on-esterified fatty acid (NEFA) concentrations decreased subsequently, as well as net NEFA uptake by the portal-drained viscera. Triglycerid e absorption that occurred 1 as well as 7 or 8 h postprandially did no t take place via the portal vein. Changes in energy expenditure of the portal-drained viscera did not correlate with changes in net nutrient fluxes. In the liver, the maximum contribution of lactate to hepatic glucose production was 20%. The NEFA and triglycerides were taken up b y the liver, whereas urea was released. Only increases in NEFA uptake could partly explain the postprandial rise in hepatic energy expenditu re. In the hindquarters, up to 48% of the glucose taken up could be co nverted into lactate; the remaining balance was possibly oxidized, the reby contributing as much as 33% to energy expenditure. No hourly patt erns were noted in net nutrient fluxes through hindquarters. In conclu sion, in preruminants, contribution of nutrients to oxidation differed from that noted in ruminants and seemed to vary with nutrient absorpt ion and availability patterns, especially in the portal-drained viscer a.