Permeabilization status affects the relationship between basal metabolic rate and mitochondrial respiration in great tit blood cells

Abstract Although mitochondrial respiration is believed to explain a substantial part of the variation in whole-animal basal (BMR) or resting metabolic rate (RMR), few studies have addressed the relationship between organismal and cellular metabolism and how this may vary in environments where individual demands for energy differ. We investigated the relationship between whole-individual metabolic rate, measured in temperatures ranging thermoneutrality to far below thermoneutrality, and mitochondrial respiration of intact or permeabilized blood cells in two separate studies on wild great tits ( Parus major L.). Our results show that, in permeabilized cells, there are significant positive relationships between BMR or RMR and several mitochondrial traits, including phosphorylating respiration rate through both complexes I and II (i.e., OXPHOS respiration). However, surprisingly, the LEAK respiration (i.e., basal respiration that mainly counteract for proton leakage) was not related to BMR or RMR. When measurements were performed using intact blood cells, BMR was positively related to ROUTINE respiration (i.e., mitochondrial respiration on endogenous substrates) in one of the two studies, but no other mitochondrial traits could explain variation in BMR or RMR in any thermal environment. These studies seem to show that the level of activation of mitochondrial metabolism as well as the permeabilization status of blood cells play a primary role on the extent to which blood metabolism might explain variations in the whole-individual metabolic rate.