Thermal acclimation and regulation of metabolism in a reptile (Crocodylus porosus): the importance of transcriptional mechanisms and membrane composition.

Energy metabolism is fundamental for animal fitness because it fuels locomotion, growth, and reproduction. Mitochondrial capacities often acclimate to compensate for negative thermodynamic effects. Our aim was to determine the importance of transcriptional regulation and membrane fatty acid composition in modulating oxidative capacities at body temperatures selected in a cold and a warm environment by a reptile (Crocodylus porosus). In the cool environment (mean selected T(b) = 21 degrees C), mRNA concentrations of the transcription factor peroxisome proliferator-activated receptor gamma (PPARgamma) and its coactivator PPARgamma coactivator 1 alpha (PGC-1alpha), as well as of the cytochrome c oxidase (COX) subunits COX1 and COX5, were significantly higher in the liver but not in skeletal muscle compared with animals in the warm environment (mean selected T(b) = 29 degrees C). F(O)F(1)-ATPase subunit alpha mRNA concentrations were significantly higher in both muscle and the liver in the cool animals. A positive relationship between PGC-1alpha and PPARgamma mRNA concentrations, with an indicator of mitochondrial density (16S rRNA) in muscle and COX and F(O)F(1)-ATPase subunit alpha mRNA concentrations in liver, suggest that these proteins regulate quantity increases of mitochondria during acclimation. The percent saturated fatty acids in liver membranes of cool animals was significantly lower, and the n3 fatty acid content was significantly higher, compared with in warm animals. The n3 fatty acid content was positively related to COX enzyme activity in the liver, and there was a negative relationship between n7 fatty acid content and COX activity in muscle. Hence, metabolic acclimation is mediated by both transcriptional regulation and membrane fatty acid composition. The importance of PGC-1alpha and PPARgamma in a reptile indicate that the mechanisms that regulate metabolism are conserved among vertebrates.