Allometric scaling of RNA, DNA, and enzyme levels: an intraspecific study

The activities of oxidative and glycolytic enzymes show body size-dependent relationships across a wide variety of taxa; however, the mechanistic basi s remains unknown. We sampled white epaxial muscle from rainbow trout (Onco rhynchus mykiss) spanning a 100-fold range in body mass. We measured activi ties of enzymes from aerobic and anaerobic metabolic pathways, RNA [total R NA and mRNA, pyruvate kinase (PK), citrate synthase (CS), and MyoD mRNA], a nd total DNA. Total RNA and DNA showed a biphasic relationship with body si ze, with a break point occurring after fish reached 1 yr of age. In contras t, total RNA/total DNA was constant across the entire size range. Neither C S activity nor CS mRNA levels scaled with body mass. PK activity and PK mRN A levels increased in parallel in yearling fish only (r(2) = 0.91, P < 0.01 ). This suggests that although PK expression is transcriptionally regulated in yearlings, the molecular mechanisms regulating expression change with g rowth and age. This was supported by a positive correlation between MyoD an d PK mRNA levels (r(2) = 0.17, P < 0.05).