LI, T.*; ROER, R.D.: Gill Na⁺, K⁺-ATPase activity as a function of size and salinity in the blue crab, Callinectes sapidus

Juvenile blue crabs (< 25mm carapace width) are found in large numbers in the oligohaline regions of the Cape Fear estuary. Previous work has shown that the juveniles lose Na to the dilute medium at rates significantly greater than adults. Juvenile crabs reduce gill Na permeability upon acclimation to dilute media, but not to any greater extent than do adults. The juveniles must, therefore, possess an enhanced ability to take up Na from dilute media relative to adult crabs. We previously demonstrated that crabs with weights < 5g had higher specific activities of gill Na⁺, K⁺-ATPase in both anterior and posterior gills relative to that of adults when acclimated to sea water or 150 mOsm dilute sea water. To further characterize the functional differences in gill Na⁺, K⁺-ATPase among different size crabs, Callinectes sapidus were collected from the Cape Fear River and acclimated in the laboratory for two weeks to dilute sea water (150 mOsm). Gills were removed from crabs, homogenized, and assayed biochemically for Na⁺, K⁺-ATPase activity. Enzymes kinetics were determined by performing assays over a range of Na concentrations in the reaction medium. Adult crabs (> 100 g) displayed very low levels of Na⁺, K⁺-ATPase activity in anterior gills. Posterior gills had a V_max of 73.6 units (�mol P/mg protein*hr). Sub-adult crabs (5 – 25 g) had a V_max of 72.5 and 76.9 units in anterior and posterior gills, respectively. Juvenile crabs (< 5 g) had a V_max of 500.0 and 1111.1 in anterior and posterior gills, respectively. The K_m was similar across sizes and averaged 14.5 mM Na in anterior gills compared to 7.8 mM in posterior gills.