Scaling with body mass of mitochondrial respiration from the white muscle of three phylogenetically, morphologically and behaviorally disparate teleost fishes
White muscle (WM) fibers in many fishes often increase in size from <50 μm in juveniles to >250 μm in adults. This leads to increases in intracellular diffusion distances that may impact the scaling with body mass of muscle metabolism. We have previously found similar negative scaling of aerobic capacity (mitochondrial volume density, Vmt) and the rate of an aerobic process (post-contractile phosphocreatine recovery) in fish WM. In the present study, we examined the scaling with body mass of oxygen consumption rates of isolated mitochondria (VO2mt) from WM in three species from different families that vary in morphology and behavior: an active, pelagic species (bluefish, Pomatomus saltatrix), a relatively inactive demersal species (black sea bass, Centropristis striata), and a sedentary, benthic species (southern flounder, Paralichthys lethostigma). In contrast to our prior studies, the measurement of respiration in isolated mitochondria is not influenced by the diffusion of oxygen or metabolites. Vmt was measured in WM and in high-density isolates used for VO2mt measurements. WM Vmt was significantly higher in the bluefish than in the other two species and VO2mt was independent of body mass when expressed per milligram protein or per milliliter mitochondria. The size-independence of VO2mt indicates that differences in WM aerobic function result from variation in Vmt and not to changes in VO2mt. This is consistent with our prior work that indicated that while diffusion constraints influence mitochondrial distribution, the negative scaling of aerobic processes like post-contractile PCr recovery can largely be attributed to the body size dependence of Vmt.