Role of fructose 2,6-bisphosphate in the control of glycolysis. Stimulation of glycogen synthesis by lactate in the isolated working rat heart

Abstract

Fructose 2,6-bisphosphate (Fru-2,6-P2) is the most potent stimulator of 6-phosphofructo-1-kinase (PFK-1), a key enzyme of glycolysis. We studied whether this regulator is involved in the changes of glycolysis that can be induced experimentally in the isolated working rat heart. The glycolytic flux was assessed by the rate of detritiation of [2-3H]- and [3-3H]glucose, by lactate output and by the changes in glycogen content. A 20-40% increase in Fru-2,6-P2 content was observed when glycolysis was stimulated by increasing either the workload (by increasing both preload and afterload) or the concentration of glucose (from 2 to 11 mM), or by adding 7 microM insulin. Anoxia decreased the external work developed by the heart, stimulated glycolysis by activating glycogenolysis, but did not increase Fru-2,6-P2. The increase of Fru-2,6-P2 content observed after insulin, high workload or glucose load might be related to a stimulation of glucose transport, and/or an activation of 6-phosphofructo-2-kinase (PFK-2), the enzyme responsible for the synthesis of Fru-2,6-P2. Addition to the perfusate of 0.5 to 10 mM lactate, which is a preferred substrate for the heart, with pyruvate in a 10:1 ratio, induced a dose-dependent inhibition of the glycolytic flux through PFK-1, with a maximal inhibition of 75% at 5 mM lactate. The accumulation of hexose 6-phosphates without change in fructose 1,6-bisphosphate and triose-phosphates concentrations confirmed that the inhibition of glycolysis was mainly exerted on PFK-1. This inhibition resulted from a doubling of the citrate concentration, an inhibitor, and from 75% decrease in Fru-2,6-P2. Despite the inhibition of glycolysis, glucose phosphorylation was barely affected by lactate, suggesting a change in glucose metabolism. Indeed, lactate induced a dose-dependent increase in glycogen content, which doubled at 5 mM lactate, reaching the level obtained after addition of 7 microM insulin. Increased glycogen synthesis was explained by the accumulation of UDP glucose, the substrate, and glucose 6-phosphate, a stimulator of glycogen synthase. We conclude that, during aerobiosis, Fru-2,6-P2 can be regarded as a glycolytic signal which is switched on by glucose availability, workload and insulin, and which is switched off by the availability of alternative oxidative substrates such as lactate. The latter also controls glucose metabolism by diverting glucose from glycolysis to glycogen synthesis.