Glycolytic flux in white muscle can be increased several-hundredfold by exercise. Phosphofructokinase (PFK; EC is a key regulatory enzyme of glycolysis, but how its activity in muscle is controlled is not fully understood. In order not to neglect integrative aspects of metabolic regulation, we have studied in frogs (Rana temporaria) a physiological form of muscle work (swimming) that can be triggered like a reflex. We analysed swimming to fatigue in well rested frogs, recovery from exercise, and repeated exercise after 2 h of recovery. At various times, gastrocnemius muscles were tested for glycolytic intermediates and effectors of PFK. All metabolites responded similarly to the two periods of exercise, with the notable exception of fructose 2,6-bisphosphate (F2,6P2), which we proved to be a most potent activator of frog muscle PFK. The first bout of exercise triggered a more than 10-fold increase in F2,6P2; PFK activity and the content of F2,6P2 in muscle were well correlated. F2,6P2 decreased to pre-exercise levels in fatigued frogs and it virtually disappeared during recovery. Varying by a factor of 70, F2,6P2 was the most dynamic of all metabolites in muscle. Even more surprisingly, F2,6P2 did not respond at all to a second bout of exercise. Other activators of PFK, such as Pi, AMP and ADP, are increased as a consequence of increased ATP turnover in contracting muscle cells. This does not apply to F2,6P2, which is likely to respond to extracellular signals and could be involved in mechanisms by which muscle metabolism is integrated into the metabolism of the whole body. Whether this phenomenon exists in vertebrates other than the frog, and maybe even in humans, and how the content of F2,6P2 in muscle is controlled are intriguing open questions.

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