CASTILHO, P.C.; LANDEIRA-FERNANDEZ, A.M.; MORRISSETTE, J.M.; SCHALLERT, R.; WENG, K.; BLOCK, B.A.; Hopkins Marine Station, Stanford University; Universidade Federal do Rio de Janeiro, UFRJ; Hopkins Marine Station, Stanford University; Hopkins Marine Station, Stanford University; Hopkins Marine Station, Stanford University; Hopkins Marine Station, Stanford University; ; : Salmon Shark (Lamna ditrops): an Endothermic Fish from Polar Area with Higher SERCA2 Activity

The endothermic sharks (family Lamnidae) and tunas (family Scombridae) display evolutionary convergence for a number of morphological and physiological properties. While both bluefin tuna and salmon sharks maintain elevated internal body temperatures, the heart operates at ambient temperature in both fishes. Electronic tracking data have shown that salmon sharks occupy waters ranging from 4-24^oC with high occupancy of cooler waters. In this study, we tested temperature effects on the salmon shark SERCA2 enzyme. This protein removes Ca²⁺ from the cytosol to lower the intracellular [Ca²⁺] calcium and allow the relaxation of the cardiac muscle. The results indicate that salmon shark ventricular SERCA2 activity exhibits a strong temperature dependence between 5 and 35^oC, with activity levels similar to those of rat. SERCA2 activity in salmon shark atrium was on average 2.5 times higher than the ventricle at all temperatures tested. Comparisons of temperature sensivity indicate that salmon shark cardiac SERCA2 activity is on average 5 times higher than that of the endothermic bluefin tuna (Thunnus thynnus) and 7 times higher than that of other sharks (mako Isurus oxyrinchu and thresher Alopias vulpinus). The higher salmon shark SERCA2 activity suggests that cardiac performance in cold may be strongly dependent on SR Ca²⁺ cycling and may be a key adaptation allowing these fishes to occupy niches in cold water. Funded by NSF.