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 activities of enzymes associated with anaerobic glycolysis, e.g., lactate dehydr ogenase (LDH), usually scale positively with increasing body size; thi s 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 conspe cifics are thought to facilitate conservation of high-speed (''burst'' ) swimming ability in fishes of different sizes (1). To investigate th e mechanisms responsible for scaling of LDH activity in white muscle, we quantified LDH activity, total RNA, and the specific mRNA for LDH-A (the skeletal muscle isoform of the enzyme) in white muslce of Parala brax nebulifer, the barred sand bass. We also measured total protein c oncentration and the concentration of actin, the major protein of thin filaments, and its specific mRNA. Although LDH activity scaled signif icantly with body size as predicted (1-4), no other biochemical trait measured showed a significant size-dependent concentration. We conclud e 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 mor e other mechanisms, for example, an elevated rate of translation of LD H message or a reduced rate of degradation of LDH-A in larger fish.