Effect of Exogenous Glucose on Glucose Metabolism in dairy Cows

Abstract

Ruminants receive little alimentary glucose because of rumen fermentation, and hence they depend largely on gluconeogenesis for their metabolically essential glucose. Because of these circumstances, it seemed worthwhile to determine how glucose metabolism would be affected if large amounts of exogenous glucose were supplied to cows. Cows were given glucose-U-¹⁴C intravenously (jugular or ruminal vein) or into their duodenal lumen and the transfer of ¹⁴C from plasma glucose to respired CO₂ and milk components was assessed. The effect of various treatments on metabolism of glucose-U-¹⁴C was evaluated from changes in the specific activity of respired CO₂, plasma glucose and milk components. Treatments included infusion of unlabeled glucose into blood or into the duodenum, in short-term studies, and infusion of glucose into the duodenum at the rate of 1.5 kg/day for periods of 21 days. Long-term duodenal infusion of glucose had no apparent effect on rate of glucose absorption from the intestine, but exogenous glucose delivered into blood or the duodenum increased the transfer of ¹⁴C from glucose to respiratory CO₂ and to milk components. Under normal conditions lactating cows derive about 10 to 13% of their metabolic CO₂ from glucose oxidation but, with an exogenous supply, glucose oxidation increased to a level providing 20 to 23% of the metabolic CO₂. When glucose was supplied exogenously to cows, the glucose transfer rate increased from 1.2 g/minute to 1.7 g/minute and there was a decrease in endogenous glucose production. The mammary gland appears to have first-priority on blood glucose and may be responsible for the relatively low oxidation of glucose in lactating cows. The percentage of respiratory CO₂ derived from glucose increased in this order: hypoglycemic ketotic cows (5%), lactating cows (10–13%) and non-lactating cow (20%) or lactating cow with exogenous glucose (20–23%).