Estimation of the fructose 1,6-diphosphatase-phosphofructokinase substrate cycle and its relationship to gluconeogenesis in rat liver in vivo.

Substrate cycling of fructose 6-phosphate through reactions catalyzed by phosphofructokinase and fructose 1,6diphosphatase was estimated in rat liver in vivo. The rate of phosphorylation of fructose 6-phosphate was determined by measuring the release of tritium to intrahepatic water following the metabolism of intraperitoneally injected [5-3H, U-W]glucose. For net flux of hepatic glucose metabolism in the direction of glucose formation the rate of fructose 6-phosphate phosphorylation is effectively equal to the rate of substrate cycling. In contrast, when the net flux of glucose metabolism is in the direction of glycolysis, then substrate cycling is a fraction of the rate of fructose 6-phosphate phosphorylation. This fraction is related to the decrease in the 3H:14C ratio in hexose 6-phosphate. For either the fed or the 25-day-old rat that had been fasted for 24 hours or for the Z-day-old suckling rat a net synthesis of glucose from [U-Wllactate is always observed. At physiological levels of lactate (2 pmoles per g body weight) the net rate of gluconeogenesis is 0.22 pmole X min-1 X g-l (wet weight of liver) for either the suckling or fasted rat. In the fed rat, the net rate of gluconeogenesis is only 0.06 pmole X min-’ X g-l. These rates represent only a fraction of the maximal observed gluconeogenic capacity of liver since gluconeogenesis is proportional to the amount of lactate injected. Maximal rates of gluconeogenesis are achieved at a lactate concentration of 20 pmoles per g body weight. In contrast to these observations, a number of nonphysiological conditions (injection of hypoglycemic agents, anoxia, low temperature) can be used to produce a net rate of glycolysis in liver. The data from a variety of situations indicate that for the liver in vivo the rate of phosphorylation of fructose 6-phos-