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 viscera, the liver, and the hindquarters and to relate them to previously published variations in tissue energy expenditure. In vivo arterio-venous techniques were used, and animals were fed a conventional milk replacer. In the portal-drained viscera, net glucose absorption occurred 1 to 5 h postprandially with some lactate release. Arterial plasma non-esterified fatty acid (NEFA) concentrations decreased subsequently, as well as net NEFA uptake by the portal-drained viscera. Triglyceride absorption that occurred 1 as well as 7 or 8 h postprandially did not 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 by the liver, whereas urea was released. Only increases in NEFA uptake could partly explain the postprandial rise in hepatic energy expenditure. In the hindquarters, up to 48% of the glucose taken up could be converted into lactate; the remaining balance was possibly oxidized, thereby contributing as much as 33% to energy expenditure. No hourly patterns were noted in net nutrient fluxes through hindquarters. In conclusion, in preruminants, contribution of nutrients to oxidation differed from that noted in ruminants and seemed to vary with nutrient absorption and availability patterns, especially in the portal-drained viscera.