Upregulation of heart PFK-2/FBPase-2 isozyme in skeletal muscle after persistent contraction

Fructose-2,6-bisphosphate (Fru-2,6-P2 )i s the most potent allosteric activator of liver 6-phosphofructo-1- kinase enzyme, which is crucial for glycolysis. It is present in skeletal muscle but its importance is controversial as a regulator of muscle glycolysis. This study aims to determine the role of Fru-2,6-P2 in the control of muscle glycolysis during contraction. Muscle contraction was produced by chronic low-frequency stimulation of rabbit tibialis anterior for 24 h, followed by a rest period of 48 h. To determine muscle glycolysis adaptation, we applied a short functional electrostimulation test using the same system of low- frequency stimulation for 1, 3, and 10 s. The variation in concentration of lactate and pyruvate was used to calculate the flux along the glycolysis pathway and the Fru-1,6-P2/ Fru-6-P ratio permitted to analyze the 6-phosphofructo-1- kinase activation. Fru-2,6-P2 levels increased over the 24 h of stimulation and remained elevated after the rest period, this being the only metabolite that kept the changes pro- duced by chronic low-frequency stimulation during the rest. During the short functional electrostimulation test, the gly- colytic pathway in stimulated and rested muscle was more active than in control muscle, which coincided with higher kinase activity of the 6-phosphofructo-2-kinase/fructose- 2,6-bisphosphatase (PFK-2/FBPase-2) enzyme. Further- more, we found a decrease in muscle, liver, and ubiquitous PFK-2/FBPase-2 isoform expression and an increase in heart isoform expression. For the first time, we demonstrate that a persistent increase in Fru-2,6-P2 produced by a change in PFK-2/FBPase-2 isoform expression may play an impor- tant role in the regulation of muscle glycolysis during the first moments of exercise.