Some fish sprint better than others, but even the Carl Lewises of the fish world have to keep going for long periods at more sedate speeds. Fish power slow swimming with red muscle, which is packed with mitochondria that generate energy rich ATP. Mitochondria can oxidise both lipids and carbohydrates to drive ATP production. Out of the two fuels, lipids produce more energy than carbohydrates, but carbohydrates release their energy more quickly. Chris Wood wondered which fuel the red muscle consumes as the fish change pace. Working with a team of students, they tested how the fish metabolise lipid and carbohydrate at different stages of an endurance swim. To their surprise, the fish kicked off by consuming carbohydrate, but switched to lipid metabolism once they'd established a steady pace(p. 2067).
Jeff Richards, a graduate student in Wood's group, was already familiar with the way that white muscle metabolises fuel for high-speed performance. Having designed a battery of tests to unravel the mysteries of anaerobic metabolism in white muscle, he turned his attentions to oxygen driven metabolism in the red muscle's mitochondria.
Working with a summer student, Ashley Mercado, they collected trout and put them into a swim tunnel where they could control the fish's speed. They set them swimming at 30% and 60% of their top speeds, to see how their muscles performed during a slow endurance swim. Then they tested the fish by turning the swim tunnel up to an unsustainable 90% of their top speed. At fixed times during each swim, they anesthetised the fish, and quickly sampled the muscle to catch a snap shot of the muscle's metabolic status.
Richards and Cheryl Clayton searched for enzymes and metabolic products that would tell them which fuel the red muscle had turned to at different stages of the three swimming tests. Pyruvate dehydrogenase is a key mitochondrial enzyme that only functions when the cells are consuming carbohydrate. By measuring the amount of an intermediate compound that is produced by metabolism of both fuels Richards and Clayton could tell whether the fish was using lipid or carbohydrate to drive their performance.
After months of intense biochemical analysis, the scientists were surprised to see that within the first few minutes of a slow endurance swim, the fish's red muscle opted for carbohydrate fuel, rather than the energy rich lipid. But after 15 minutes, the fish switched from fast release carbohydrate to energy rich lipid to power their progress. At unsustainably high speeds, the fish supplemented their red muscle with power from the white muscle, and switched back to carbohydrate metabolism in the red muscle, to give them an extra boost.
Richards admits that he is surprised by the way the fish switch back to carbohydrate fuel when the need arises. He says `to my knowledge, no study has ever focused on how the red muscle of fish chooses its fuel.' Having pinned down which fuels are used when, Wood and other scientists are keen to find out how the fuels are delivered and how muscle cells regulate key enzymes in both metabolic pathways.