JOHNSON, LK; KINSEY, ST; University of North Carolina at Wilmington; University of North Carolina at Wilmington: Effects of muscle fiber size on post-contractile lactate recovery in the blue crab, Callinectes sapidus.

Cells typically fall within a size range of 10-100 &mum. Exceeding this size range leads to a decreased surface area to volume ratio (SA:V) and increased intracellular diffusion distances, both of which may compromise aerobic metabolism. However, diameters of some fast-twitch muscle fibers in the adult blue crab, Callinectes sapidus, exceed 500 &mum whereas juvenile fibers are <100 &mum. Burst contractile function, which does not rely on aerobic metabolism, is not affected by the ontogenetic hypertrophy of the white muscle fibers. However, it is hypothesized that the subsequent aerobically-dependent recovery process will be significantly impeded by large fiber size. In this study, recovery of accumulated lactate after anaerobic burst exercise was examined as a function of fiber size. Burst escape responses were elicited in crabs ranging from <1 g to >250 g and lactate recovery was monitored in hemolymph and white levator muscle. The amount of lactate accumulated immediately after exercise was not different between small and large crabs. However, in large crabs, post-contractile lactate accumulation was significantly greater than that seen in small crabs, and lactate continued to rise 180 min after exercise in large crabs, while lactate in small crabs had recovered to resting levels by this time. In addition, activities of cytochrome-c oxidase, an indicator of aerobic potential, were found to scale negatively with increasing body mass in white levator muscle, with a slope greater than would be expected from normal scaling effects. These data are consistent with the idea that aerobic metabolic processes are limited by SA:V and intracellular diffusion constraints in large cells.