Net portal-drained visceral and hepatic metabolism of glucose, L-lactate, and nitrogenous compounds in lactating Holstein cows

Net portal-drained visceral and hepatic flux of glucose, L-lactate, alpha-amino N, NH3N, urea N, glutamate, and glutamine were measured in four Holstein cows. Cows were fed a 60:40 corn silage: concentrate diet ad libitum and milked at 12-h intervals. Six to 16 d postpartum chronic catheters were established in hepatic portal, hepatic, and mesenteric veins and a carotid artery was elevated. Twelve Measurements of net flux, the mathematical product of blood flow (measured by p-aminohippurate dilution) and venous-arterial concentration difference, were obtained for each cow at hourly intervals during 1 d of wk 4 and 8 postpartum. Dry matter, N, and energy digestion trials began 1 to 2 d after blood sampling. Dry matter intake and milk yield averaged 15.6 and 32.2 kg/d. Portal-drained visceral blood flow averaged 80% of hepatic blood flow (2041 L/h). Net flux of NH3N, urea N, and alpha-amino N across portal-drained viscera represented 68, 54, and 51% of N apparently digested. There was net use of glucose by portal-drained viscera. Hepatic glucose production (3.1 kg/d) exceeded calculated mammary glucose requirements. Net hepatic removal of L-lactate, alpha-amino N, and NH3N represented 115, 43, and 101%, respectively, of their net absorption by portal-drained viscera. Net hepatic L-lactate and alpha-amino N removal could account maximally for 17.4 and 16.5% of glucose produced.