Effect of glucose load on synthesis of plasma glucose in lactating cows.

We assess the effect of an intravenous glucose load on production of glucose in lactating dairy cows. Isotope dilution techniques were utilized to measure transfer rate of plasma glucose (with tritiated glucose) and gluconeogenesis (with carbon-14 labeled propionate or carbon dioxide). A glucose load equivalent to the cow's normal transfer rate decreased endogenous glucose output two-thirds and decreased gluconeogenesis by half. Glucose concentration and transfer rate were higher within 2 h after glucose load started while carbon-14 transfer to plasma glucose became increasingly depressed and persisted for at least 1 h after loading terminated. Reduced plasma glucose synthesis from propionate during loading was associated with increased transfer of propionate carbon to plasma phospholipids, triglycerides, and cholesterol esters, and with increased plasma insulin concentration. It appears that glucogenic precursors are utilized for greater synthesis of lipid (and presumably glycogen) and that loading leads to metabolic changes causing net reduction in gluconeogenic capacity. These changes could have important implications for use of intravenous glucose in treating hypoglycemic metabolic conditions.