Carbonic anhydrase and Na/K-ATPase activities during the molt cycle of low salinity-reared white shrimp Litopenaeus vannamei

Changes in hemolymph osmolality, ion concentrations, and enzymatic activities of carbonic anhydrase (CA) in the gills and epidermal tissue, and Na/K-ATPase in the gills during the molt cycle were investigated in the white shrimp Litopenaeus vannamei. Hemolymph osmolality was high in the intermolt and early premolt stages, but started to decrease prior to ecdysis through to postmolt stages A and B. Changes in Na(+) and Cl(-) ion concentrations paralleled those in hemolymph osmolality. CA activity levels in the anterior and posterior gills were low at intermolt stage C(0) and premolt stage D(0), and maximum at premolt stage D(3). In the epidermal tissue, activity was relatively high at intermolt stage C(0) and premolt stage D(0), but fluctuated towards premolt stage D(3) and postmolt stage A. On the other hand, Na/K-ATPase activity in the gills decreased between intermolt stage C(0) and premolt stage D(2), but increased at premolt stage D(3) and postmolt stage A. The changes in patterns of CA activity during the molt cycle suggest that CA may be involved in supplying counter-ions for Na(+) and Cl(-) uptake during molting. Branchial Na/K-ATPase appears to be involved in producing local osmotic gradients in order to support water influx across the epithelium.