Meeting Abstract
In Pacific bluefin tuna, vascular counter-current heat exchangers (retia) conserve metabolic heat, allowing aerobic (red) locomotor muscle temperature to be elevated above water temperature. We investigated molecular controls involved in the transition to regional endothermy in year 0 bluefin (20 cm fork length). Temperatures, activity of the mitochondrial enzymes citrate synthase (CS) and cytochrome c oxidase (COX), and gene expression for CS and COX1 and for peroxisome proliferator-activated receptor-γ coactivator (PGC-1α) were quantified in red and white muscle. 20-cm bluefin cannot elevate tissue temperatures significantly, but possess small red muscle retia and internalized red muscle with aerobic enzyme activities similar to those of larger, endothermic individuals. CS and COX activities per gram tissue were 6.2- and 10.4-fold higher in red compared with white muscle, indicating that mitochondria are more abundant in red muscle, as expected. We hypothesized that this would be due to increased transcription of the genes encoding those enzymes and the master regulator of mitochondrial biogenesis PGC-1α. CS, COX, and PGC-1α transcript abundances normalized to β-actin transcript abundance did not differ significantly between the two muscle types but, when normalized to the amount of RNA, CS, COX, and PGC-1α transcripts were 3.0-, 2.0- and 4.3-fold more abundant in red versus white muscle. Post-transcriptional regulation of CS and COX gene expression may also be involved because the fold-differences in enzyme activity were greater than those in gene expression. Results should inform aquaculture and conservation programs for bluefin tunas.
