Difference in glucose sensitivity of liver glycolysis and glycogen synthesis. Relationship between lactate production and fructose 2,6-bisphosphate concentration.

Incubation of isolated rat hepatocytes from fasted rats with 0-6 mM-glucose caused an increase in fructose 2,6-bisphosphate] (0.2 to about 5 nmol/g) without net lactate production. A release of 3H2O from 3-3H]glucose was, however, detectable, indicating that phosphofructokinase was active and that cycling occurred between fructose 6-phosphate and fructose 1,6-bisphosphate. A relationship between fructose 2,6-bisphosphate] and lactate production was observed when hepatocytes were incubated with glucose] greater than 6 mM. Incubation with glucose caused a dose-dependent increase in hexose 6-phosphates]. The maximal capacity of liver cytosolic proteins to bind fructose 2,6-bisphosphate was 15 nmol/g, with affinity constants of 5 X 10(6) and 0.5 X 10(6) M-1. One can calculate that, at 5 microM, more than 90% of fructose 2,6-bisphosphate is bound to cytosolic proteins. In livers of non-anaesthetized fasted mice, the activation of glycogen synthase was more sensitive to glucose injection than was the increase in fructose 2,6-bisphosphate], whereas the opposite situation was observed in livers of fed mice. Glucose injection caused no change in the activity of liver phosphofructokinase-2 and decreased the hexose 6-phosphates] in livers of fed mice.