Combined effects of cadmium, hypoxia and temperature on mitochondrial bioenergetics in rainbow trout (Oncorhynchus mykiss)

Aquatic organisms are exposed to diverse and dynamic combinations of stressors in their environment that may interact to alter mitochondrial function. It is important to understand mechanisms of joint effects of stressors to more accurately monitor and predict adverse biological outcomes. I studied the mechanisms of interactions of cadmium (Cd), hypoxia-reoxygenation (H-R) and temperature induced stress on mitochondrial bioenergetics. My overall hypothesis was that when present together Cd, hypoxia and temperature affect common target sites exacerbating single stressor-induced effects on mitochondrial function. In the first investigation I studied the effects of hypoxia-cadmium interactions on rainbow trout (Oncorhynchus mykiss) mitochondrial bioenergetics and showed that H-R enhances the sensitivity of mitochondria to Cd-induced stress. Interestingly, I observed that Cd at low dose attenuates H-R-induced proton leak. In the second study, I investigated how temperature modulates cadmium-induced mitochondrial dysfunction and volume changes. I showed that high temperature exacerbates Cd-induced mitochondrial dysfunction and volume changes in part by increasing metal uptake through the mitochondrial calcium uniporter. In the third study, I investigated the effect of H-R on the thermal sensitivity of complex 1 oxidative capacity. I showed that effects of H-R on mitochondrial function are exacerbated by thermal stress. My fourth study investigated the combined effects of cadmium, temperature and hypoxia-reoxygenation on mitochondrial function. I found that the ternary interactions of Cd, H-R and temperature exacerbate their binary effects on mitochondrial function. I linked the alterations in mitochondrial function to impaired volume homeostasis, complex I A-D transition, dissipation of mitochondrial membrane potential, increased ROS production and loss of structural integrity. Although the effects of Cd and/or H-R were greater at both high and low temperatures, this was not explained by increased Cd accumulation. Overall oxidative stress could explain to a large extent the effects of Cd, H-R and temperature on mitochondrial structure and function. In the fifth study I investigated the role of mitoKATP and the effects of pharmacological modulators on H-R-induced mitochondrial dysfunction. I found that in the presence of Mg-ATP both the opening of mitoKATP channels and bioenergetic effects of diazoxide were protective against the deleterious effects of H-R while in the absence of Mg-ATP only the bioenergetic effects of diazoxide was protective. Overall, my research unearthed previously unknown mechanisms of interactions of Cd, hypoxia and temperature on mitochondrial bioenergetics and increased our understanding of the impact of multiple stressors on cellular energy metabolism in aquatic organisms.