Flying insects: model systems in exercise physiology.

Insect flight is the most energy-demanding exercise known. It requires very effective coupling of adenosine triphosphate (ATP) hydrolysis and regeneration in the working flight muscles. 31P nuclear magnetic resonance (NMR) spectroscopy of locust flight muscle in vivo has shown that flight causes only a small decrease in the content of ATP, whereas the free concentrations of inorganic phosphate (Pi), adenosine diphosphate (ADP) and adenosine monophosphate (AMP) were estimated to increase by about 3-, 5- and 27-fold, respectively. These metabolites are potent activators of glycogen phosphorylase and phosphofructokinase (PFK). Activation of glycolysis by AMP and Pi is reinforced synergistically by fructose 2,6-biphosphate (F2,6P2), a very potent activator of PFK. During prolonged flight locusts gradually change from using carbohydrate to lipids as their main fuel. This requires a decrease in glycolytic flux which is brought about, at least in part, by a marked decrease in the content of F2,6P2 in flight muscle (by 80% within 15 min of flight). The synthesis of F2,6P2 in flight muscle can be stimulated by the nervous system via the biogenic amine octopamine. Octopamine and F2,6P2 seem to be part of a mechanism to control the rate of carbohydrate oxidation in flight muscle and thus function in the metabolic integration of insect flight.