(full paper is archived in the Miller Library)
Title: Maintaining a high metabolic rate: evidence for sarcoplasmic reticulum calcium cycling in tuna hearts
Student Author(s): Massarotti, Haane
Faculty Advisor(s): Block, Barbara
Location: Final Papers Biology 176H
Date: June 2001
Abstract: The evolution of high metabolism has occurred in numerous lineages. Mammals, birds, tuna and squid all maintain high metabolic rates. Elevated metabolic rates demand high cardiac output. How the heart evolves the cellular machinery to deliver a high cardiac output remains unexplored. Tunas are exceptional amongst bony fishes in being endothermic. They maintain a high metabolic rate over a wide range of ambient temperatures. Bluefin tuna have been recorded to swim in ambient temperatures as low of 2.8° C. In cold waters, tuna maintain high temperatures in their muscles through a highly vascularized countercurrent heat exchange system. However, their heart temperatures are exposed to ambient temperatures. Tuna hearts deliver high cardiac outputs, high frequency contractions, and high power outputs in a wide range of temperatures. This project examined the mechanisms that allow tuna to produce their high cardiac outputs at the cellular level. In this project I investigated the importance of increased dependence on calcium cycling in the sarcoplasmic reticulum in tuna hearts. The presence of calcium ATPase and the SR calcium release channel were examined using biochemical purification of the SR membrane. The proteins are required to release and take up calcium from the cytoplasm into the SR during contraction and relaxation of the heart. SR protein was purified in wild yellowfin and bluefin tuna hearts. The SR preparation was separated on 7.5% and 3-12% gradient SDS gels. The Serca 2 pump was visualized using a Comassie blue stain of the gels and an antibody specific to Serca 2 on Western blots. Both gels and blots indicate that yellowfin and bluefin atria and ventricles have the calcium ATPase and ryanodine receptors. Densitometry using NIH Image suggests higher levels of Serca 2 in the atria than ventricles in both tuna. A functional assay of calcium uptake was performed on SR vesicles using a calcium-sensitive dye, Fura 2, in a spectroflourophotometer. Preliminary results indicate similar rates of SR calcium uptake between yellowfin and bluefin tuna and between tuna atria and ventricles. The detection of SR in tuna hearts provides insight into the mechanism by which tuna maintain high cardiac outputs in a wide range of ambient temperatures.