Metabolic effects of oral fructose in the liver of fasted rats

Twenty-four-hour-fasted rats were given glucose (4 g/kg) by gavage. Glucose absorption and portal and peripheral plasma glucose, lactate, and insulin concentrations, as well as liver glucose, UDPglucose, glucose-6-P, lactate, ATP, and inorganic phosphate (Pi), and % glycogen synthase I and % phosphorylase a were measured at 10, 20, 30, 40, 60, and 120 min after the glucose was given. Liver and muscle glycogen also were measured. Ninety-one percent of the glucose load had disappeared from the gut in 2 h. Despite increased plasma glucose and insulin levels the liver continued to produce glucose. Lactate produced in the periphery was the major substrate for gluconeogenesis, and lactate utilization could account for the hepatic glycogen synthesized. Glucose ingestion did not affect lactate production by the splanchnic bed. In the liver glucose-6-P was transiently increased; UDP glucose decreased after glucose administration. ATP and Pi were unchanged. Glycogen synthase was activated by 20 min without a significant change in phosphorylase a. Hepatic glycogen increased linearly after 20 min. Total glucose storage as glycogen was similar in liver (20%) and muscle (19%). We could account for 41% of the glucose absorbed as glycogen, unmetabolized glucose, or glucose metabolites. Most of the remainder probably was oxidized.