Scaling of locomotor muscle oxidative and glycolytic metabolic enzymes during the ontogeny of regional endothermy in Pacific bluefin tuna (Thunnus orientalis)

This study aimed to clarify the mechanisms contributing to ontogenetic differences in red locomotor muscle metabolic heat production in regionally endothermic tunas. To address this, the scaling of citrate synthase (CS), cytochrome c oxidase (COX) and pyruvate kinase (PK), enzymes involved in cellular respiration, in the red and white locomotor muscle of young (~ 2 to ~ 16 months of age) Pacific bluefin tuna (Thunnus orientalis) during the ontogeny of red muscle endothermy was investigated. On a mass-specific basis (units g⁻¹ muscle tissue), CS activity scaled negatively with body mass with scaling coefficients of −0.12 for red muscle and −0.21 for white muscle, whereas COX activity did not scale in either muscle type and PK activity scaled positively in white muscle, with a scaling coefficient of 0.09, but did not scale in red muscle. Thus, proxies for mass-specific metabolic heat production potential from cellular respiration either decreased or remained constant in the red muscle during the ontogeny of red muscle endothermy. In contrast, total red muscle mass and total CS and COX activities all scaled positively with body mass, with scaling coefficients of 0.90, 0.78 and 0.92, respectively, and each of these correlated positively with the magnitude of the red muscle thermal excess. Thus, increasing total, but not mass-specific, metabolic heat production capacity contributed to the increasing red muscle thermal excess with increasing body size in juvenile T. orientalis. Additionally, transcript abundance was a poor predictor of enzyme activity. Thus, transcriptional regulation played a limited role in determining the enzymes’ scaling relationships.