DUONG, C.A.*; DICKSON, K.A.; California State University Fullerton: The contribution of mitochondrial proton leak as a heat source in lamnid sharks

Endothermic fishes can elevate the temperature of certain tissues above ambient temperature because the tissues are served by counter-current heat exchangers that conserve metabolic heat. In lamnid sharks, warming of the locomotor musculature, visceral organs, and eye/brain is associated with lateral retia, the suprahepatic rete, and orbital retia, respectively. Whether these tissues are also specifically modified for thermogenesis remains unknown. This study examines the contribution of mitochondrial proton leak as a potential heat source in two endothermic tissues in the shortfin mako shark. Proton leak is an intrinsic, non-enzymatic property of the inner mitochondrial membrane whereby protons bypass ATP synthase and diffuse back into the matrix to dissipate energy. We compared mitochondrial proton leak rates in the red muscle and liver of three shark species and hypothesized that the endothermic shortfin mako would have higher proton leak rates than the ectothermic blue and leopard sharks. Respiration rate and membrane potential in isolated mitochondria were measured simultaneously using a Clark-type oxygen electrode and a lipophilic probe (TPMP+), respectively. Succinate-stimulated respiration rate was titrated with inhibitors of the electron transport chain to demonstrate a non-linear relationship between respiration rate and membrane potential. Although the proton leak rate in red muscle was higher than in liver for all three species, the rate did not differ significantly between endothermic and ectothermic sharks for either tissue. The results suggest that red muscle and visceral endothermy in lamnids is achieved principally through the presence of heat exchangers and not through an increase in energetically wasteful thermogenic pathways such as proton leak. (Supported by NSF and NIH.)