Fructose administration stimulates glucose phosphorylation in the livers of anesthetized rats

A method allowing one to measure the rate of glucose phosphorylation in the livers of anesthetized rats is described. Upon injection of [2-3H]glucose into the portal vein, about 90% of the radioactivity remained in the liver for approximately 30 s. The proportion of radioactivity accounted for by tritiated water increased linearly at a rate of about 5%/min in control animals. Fructose injected into the penile vein stimulated this rate up to 2.2- and 2.7-fold in fed and overnight starved rats respectively, a maximal effect being observed at a dose of 50 mg/kg under both conditions. Fructose was also active when administered by intragastric infusion. The ketose caused increases in the concentration of fructose 1-phosphate, which reached values known to relieve the inhibition exerted on glucokinase by its regulatory protein.