Effect of Glucose Load on Synthesis of Plasma Glucose in Lactating Cows

Abstract

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.

Effects of intrajugular glucose infusion on feed intake, milk yield, and metabolic responses of early postpartum cows fed diets varying in protein and starch concentration

“…In the present study, if the elevated insulin concentration by GI only decreased hepatic uptake of glucose precursors other than propionate, there might be less effect on the pool of tricarboxylic acid intermediates than if it decreased gluconeogenesis from propionate during the timeframe of meals. However, those results are in contrast to a study with lactating cows (Thompson et al, 1975) in which short-term intrajugular glucose infusion at approximately twice the infusion rate of the current experiment increased plasma insulin concentration and decreased propionate conversion to glucose by 43%. Also, increased glucose demand with phlorizin treatment increased gluconeogenesis from propionate in steers (Veenhuizen et al, 1988).…”

“…Hyperinsulinemic-euglycemic clamps quadrupled plasma insulin concentration and decreased feed intake 33% in fresh cows (Leury et al, 2003). In ruminants, a reduction in hepatic gluconeogenesis might force propionate to pursue a different biochemical pathway than that for glucose synthesis; glucose infusion increased plasma insulin concentration and decreased propionate conversion to glucose in lactating cows (Thompson et al, 1975). Propionate can be converted to pyruvate and then to acetyl CoA by PDC; however, this route is likely reduced for cows in the early postpartum period that have high concentrations of acetyl CoA from β-oxidation of NEFA because acetyl CoA inhibits PDC (Roche et al, 2001).…”

“…We speculate that the suppression of DMI was caused by increased hepatic oxidation of acetyl CoA within the timeframe of meals. Increasing glucose availability by glucose infusion decreased gluconeogenesis from propionate and other glucose precursors in lactating cows (Thompson et al, 1975), and elevated liver acetyl CoA concentration from β-oxidation decreases conversion of pyruvate to acetyl CoA by inhibiting pyruvate dehydrogenase complex (PDC; Roche et al, 2001). Repletion of tricarboxylic acid intermediates during meals likely increases their pool size to a greater extent when gluconeogenesis is downregulated and PDC is inhibited, enhancing oxidation of acetyl CoA within meals, causing satiety sooner (Allen et al, 2009).…”

Hyperglycaemia in transition dairy cows: Effects of lactational stage and conjugated linoleic acid supplementation on glucose metabolism and turnover

“…() found a lower insulin response to glucose infusion in lactating cows compared with non‐lactating animals, which indicates the necessity of maintenance of nutrient direction towards the mammary gland during lactation (Mann et al., ). Furthermore, Thompson, Weiser, Seto, and Black () and Lomax, Baird, Symonds, Mallinson, and Shaw () reported a decreased hepatic glucose production and reduced uptake of gluconeogenic precursors in cows following glucose infusion. Metabolic clearance rate (MCR) of glucose was shown not to be independent of glucose concentration, but at high insulin concentrations as observed during the hyperglycaemic clamps the MCR can be assumed to be independent of the prevailing glucose concentration (Proietto et al., ).…”

Expression and activity of key hepatic gluconeogenesis enzymes in response to increasing intravenous infusions of glucose in dairy cows1

“…Although surplus supply of glucose has often been shown to downregulate hepatic glucose production (Bartley and Black, 1966;Thompson et al, 1975;Lomax et al, 1977;Rigout et al, 2002), it is a crucial question whether this also involves a downregulation of the involved enzymes. A downregulation of gluconeogenic enzymes could have consequences extending beyond the period of surplus glucose or glucogenic substrate supply.…”