Effect of pentobarbital on fructose 2,6-biphosphate metabolism in isolated rat hepatocytes

Addition of the commonly used anesthetic pentobarbital to hepatocytes from fed rats resulted in a dose-dependent decrease in the level of fructose 2,6-bisphosphate. At a concentration of pentobarbital (0.4 mM) that lowered fructose 2,6-bisphosphate by 60%, there was no significant change in the level of fructose 6-phosphate, ATP, or L-glycerol 3-phosphate. Higher concentrations of pentobarbital (2 mM) enhanced both glycolysis and glycogenolysis and fructose 2,6-bisphosphate levels were reduced to less than 10% of the control. Concomitant with these changes there was a decrease in ATP, glucose 6-phosphate, and fructose 6-phosphate and a two- and fivefold increase in ADP and AMP, respectively. In hepatocytes from starved rats pentobarbital also lowered ATP levels and inhibited gluconeogenesis but had no effect on either lactate production or the already low level of sugar diphosphate. However, in the fasted case pentobarbital completely prevented the 10-fold elevation of fructose 2,6-bisphosphate brought about by 30 mM glucose. The anesthetic had no effect on cAMP-dependent protein kinase activity or on pyruvate kinase activity in hepatocytes from fed or starved rats but caused reciprocal changes in the activities of the bifunctional enzyme 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase. Kinase activity was decreased and bisphosphatase activity was increased. These results suggest that the effects of pentobarbital on gluconeogenesis and glycolysis are due to inhibition of energy metabolism with elevated AMP levels causing activation of 6-phosphofructo-1-kinase and inhibition of fructose 1,6-bisphosphatase. The effect of pentobarbital on fructose 2,6-bisphosphate metabolism in hepatocytes from fed and starved rats results from both changes in the phosphorylation state of 6-phosphofructo-2-kinase/fructose 2,6-bisphosphate and from alterations in the level of substrates and effectors of the enzyme brought about by the inhibition of energy metabolism.