Fueling the brain during exhaustive exercise

Brain cells called astrocytes store glycogen as an energy source. The breakdown of glycogen gives rise to lactate, which is transported by monocarboxylate transporters (MCT) into neurons. Physical exercise increases the brain’s energy needs, but the role of astrocytic glycogen in the exercising brain remains unclear. Takashi Matsui et al. (pp. 6358–6363) measured glycogen and MCT proteins in the brains and skeletal muscles of rats undergoing prolonged exhaustive exercise on a treadmill. Compared with sedentary rats, the muscles and brains of exhausted rats showed reduced glycogen. In addition, exercise led to an increase in the level of brain MCT2 protein, which transports lactate into neurons, as well as an increase in the level of muscle MCT proteins. Using metabolomics, the authors found that exhausted rats maintained preexercise levels of the energy source ATP in the brain, but not in muscles, through the use of lactate and other glucose-derived and glycogenderived energy sources. Injecting a glycogen breakdown inhibitor decreased both hippocampal lactate production and hippocampal ATP levels in exhausted animals and lowered endurance capacity. An inhibitor of brain MCT also decreased hippocampal ATP levels and accelerated exhaustion. According to the authors, lactate derived from astrocytic glycogen might play a protective role in the brain during prolonged exhaustive exercise and contribute to endurance capacity. - Read at PNAS