RESUMO
The time course of induction of activity of carbonic anhydrase (CA) and Na/K ATPase, two enzymes that are central to osmotic and ionic regulation in the eyryhaline green crab, Carcinus maenas, was measured in response to a transfer from 32 to 10 ppt salinity. CA activity was low in all gills in crabs acclimated to high salinity. Activity was induced in the posterior three gills (G6-G9) starting at 96 hr following transfer to low salinity, with activity peaking at seven post-transfer. Na/K ATPase activity in posterior gills was already high in crabs acclimated to 32 ppt salinity, and it did not increase as a result of transfer to 10 ppt. Acclimation of crabs to hypersaline (40 ppt) conditions resulted in uniformly low levels of Na/K ATPase activity, and transfer from 40 ppt to 10 ppt stimulated a four-fold induction of activity in the posterior gills that was evident by seven days of low salinity exposure. Low salinity stimulates the activity of both enzymes, but a different degree of salinity change appears to be necessary to cause the induction of each enzyme. The Na/K ATPase activity is already high at a salinity (32 ppt) at which the crab is still an osmotic and ionic conformer. CA activity, however, even when expressed in low levels, is still present in excess of what is needed to supply counterions at a rate adequate to match the rate of active ion transport. It is possible that two strategies exist for the regulation of these two enzymes that coincide with the crab's intertidal and estuarine lifestyle: short-term modulation of activity of highly expressed enzyme (Na/K ATPase) and long-term modulation of enzyme concentration by changes in gene expression (CA). For all ranges of low salinity exposure, crabs undergo hemodilution, cell swelling, and subsequent cell volume readjustment as evidenced by the increase in concentration of TNPS in the hemolymph. This response takes place before the induction of enzyme activity, and it could serve as the initial signal in the induction pathway.
