Activity of lactate dehydrogenase but not its concentration of messenger RNA increases with body size in barred sand bass, Paralabrax nebulifer (Teleostei).

In white skeletal muscle of conspecific pelagic fishes, the activity of enzymes associated with anaerobic glycolysis, e.g., lactate dehydrogenase (LDH), usually scale positively with increasing body size; this pattern is opposite to that found for enzymes of aerobic metabolism, which decrease in mass-specific activity with size (1-3). The higher mass-specific capacities for anaerobic ATP generation in larger conspecifics are thought to facilitate conservation of high-speed ("burst") swimming ability in fishes of different sizes (1). To investigate the mechanisms responsible for scaling of LDH activity, total RNA, and the specific mRNA for LDH-A (the skeletal muscle isoform of the enzyme) in white muscle of paralabrax nebulifer, the barred sand bass. We also measured total protein concentration and the concentration of actin, the major protein of thin filaments, and its specific mRNA. Although LDH activity scaled significantly with body size as predicted (1-4), no other biochemical trait measured showed a significant size-dependent concentration. We conclude that the regulation of LDH activity in white muscle of this species is not governed by LDH-A mRNA concentrations, but rather by one or more other mechanisms, for example rate of translation of LDH message or a reduced rate of degradation of LDH-A in larger fish.