BLANK, J.M.*; MORRISSETTE, J.M.; MASSAROTI, H.; BLACKWELL, S.B.; BLOCK, B.A.: Physiology & Biochemical Characteristics of Pacific & Atlantic Bluefin Tuna Hearts

Electronic tagging data indicates that bluefin tunas encounter a wide range of environmental temperatures from 2 to 30°C while maintaining endothermic muscle and visceral temperatures of 23-30°C. The hearts of bluefin operate at ambient temperature due to close proximity to the gills and the absence of a heat exchanger. This creates a physiological paradox as the tuna heart must meet the oxygen demands of warm, aerobic tissues in both cold and warm waters. We investigated the effects of temperature on cardiac function in bluefin tuna using an in situ perfused heart preparation. Perfused bluefin hearts have a heart rate of ~23 beats/min at 5°C and ~110 beats/min at 30°C. The Q₁₀ for heart rate of bluefin preparations is ~1.9 between 5 and 30°C. At 10°C and 20°C, isolated bluefin hearts maintain higher heart rates than comparable preparations on yellowfin tuna, which exhibit a more restricted thermal niche in the wild. Bluefin tuna hearts maintain high cardiac outputs at both 5 and 20°C. To investigate the hypothesis that the high heart rates of bluefin tuna require increased use of internal Ca2+ stores, we have used SDS PAGE and western blotting to demonstrate the presence of the sarcoplasmic reticulum Ca2+ ATPase and Ca2+ release channel. High heart rates place large aerobic demands on the myocardium, and tuna hearts exhibit correspondingly high citrate synthase activities, which are a shared physiological character among the hearts of scombrid fishes.