Metabolic sources of heat and power in tuna muscles. II. Enzyme and metabolite profiles.
Tuna appear able to maintain their muscles at 5-10 degrees C above ambient by balancing heat produced in situ and conserved by a counter-current heat exchanger with heat lost to the sea. Metabolite profiles under three different activity states (rest, burst swimming, and steady state swimming during feeding frenzies at sea) were used to identify which metabolic processes in white and red muscles could account for observed excess temperatures. During burst swimming, transient changes in metabolite levels indicate that the metabolism of both red and white muscle contributes to powering burst swimming; red muscle work is sustained mainly by oxidative metabolism while white muscle work depends upon an intense anaerobic glycolysis. The rate of metabolism in red muscle is easily high enough to account for the measured (10 degrees C) increase in temperature at this time. However, in white muscle, anaerobic glycolysis can account for only about a 2 degrees C maximum rise in temperature. The highest sustained swimming speeds and the highest muscle temperatures in skipjack are found during feeding frenzies at sea. As in burst swimming, during steady-state swimming red muscle temperatures can be accounted for by oxidative metabolism. In the case of white muscle, the lactate measurements indicate that anaerobic glycolysis could only lead to a 0.3 degree C temperature rise. However, if the fraction of utilized glycogen that is not fermented (about 60%) is assumed to be fully oxidized, enough heat is generated to raise white muscle temperatures by over 10 degrees C. The observed excess temperature at this time is about 8-10 degrees C, showing that anarerobic carbohydrate metabolism in white muscle is probably the major heat source during feeding frenzies. These interpretations are fully consistent with enzyme profiles of red and white muscles in tuna. They do not, however, explain why tuna have warm muscles. The latter problem is briefly discussed.