Environmentally mediated carbonic anhydrase induction in the gills of euryhaline crustaceans

The enzyme carbonic anhydrase appears to be a central molecular component i n the suite of physiological and biochemical adaptations to low salinity fo und in euryhaline crustaceans. It is present in high activities in the orga ns responsible for osmotic and ionic regulation, the gills, and more specif ically, the individual gills that are specialized for active ion uptake fro m dilute sea water. Within those gills carbonic anhydrase is distributed am ong different subcellular pools, the cytoplasm, mitochondria and microsomes . The cytoplasmic pool represents the largest subcellular fraction of carbo nic anhydrase activity, and it is this fraction that undergoes a tenfold in duction during acclimation to low salinity. Carbonic anhydrase activity is present in excess of that needed to support the general ion-transport proce sses, and so it is doubtful that carbonic anhydrase activity itself is a po int of short-term regulation in response to salinity changes. Rather, upreg ulation of carbonic anhydrase appears to be a result of selective gene expr ession, representing a permanent response to longterm adaptation to low sal inity, The exact signal that initiates the induction of carbonic anhydrase, and the pathway through which that signal is transduced to the activation of the carbonic anhydrase gene, are unknown, but two promising avenues of r esearch exist. First, induction of carbonic anhydrase is immediately preced ed by hemodilution and subsequent cell swelling, a potential initiating eve nt in the process. Second, recent work indicates that expression of carboni c anhydrase is under the control of a repressor substance, located in the e yestalk, whose effect is removed upon exposure to low salinity.