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

In this study, the scaling of the oxidative metabolic enzymes citrate
synthase (CS) and cytochrome c oxidase (COX) and the glycolytic enzyme
pyruvate kinase (PK) in the red (‘slow-twitch’, oxidative) and the white
(‘fast-twitch’, glycolytic) 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 mass-specific metabolic heat production
potential either decreased or remained constant in the red muscle during
the ontogeny of red muscle endothermy. In contrast, total red muscle mass,
total red muscle CS activity and total red muscle COX activity 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, for CS and COX, transcript abundance was a
poor predictor of enzyme activity. Thus, transcriptional regulation played
a limited role in determining the differences between the two muscle types
and the scaling relationships for these enzymes.