GEHNRICH, S.G.*; HENRY, R.P.; WEIHRAUCH, D.; TOWLE, D.W.: Identification of Carbonic Anhydrase Isoforms in the Gills of the Shore Crab, and Changes in their Expression During Acclimation to Low Salinity.

At low salinity, the epithelium in the posterior gills of the shore crab, Carcinus maenas, is believed to take up chloride and sodium from the environment in exchange for intracellular bicarbonate and protons, which are generated within the cell by the enzyme carbonic anhydrase (CA). During acclimation to low salinity, CA activity in the posterior gills increases during the first 96 hours of exposure, whereas CA activity in the anterior gills does not change significantly. Most of the increased CA activity is found in the cytoplasm, with a smaller increase associated with the microsomal fraction. Using RT-PCR we have identified and partially sequenced two isoforms of carbonic anhydrase from the posterior gills of the shore crab. These isoforms share several highly conserved, CA-specific, amino acid residues with mammalian isoforms, but have only a 30% amino acid homology with each other. Isoform-specific PCR primers were designed, and the levels of mRNA for each isoform was determined by quantitative PCR. Although mRNA for both isoforms was detected, one of the forms is expressed in much greater amounts than the other. During acclimation to low salinity, levels of mRNA for both isoforms increased approximately 2.5-fold, reaching maximal levels after 48-72 hours. Future studies will be done to determine the specific sub-cellular location of each isoform, and their exact role in ionic homeostasis. Supported by NSF (IBN-9807539) and Mt. Desert Island Biological Lab.