Identification and immunocytochemical localization of two different carbonic anhydrase isoenzymes in teleostean fish erythrocytes and gill epithelia

Carbonic anhydrase was purified from the gills (CAB) of the rainbow trout Salmo gairdneri and from erythrocytes (CAE) of the fresh water carp Cyprinus carpio. The purification of the isozymes was confirmed by SDS acrylamide gel electrophoresis. Antibodies against the purified CAB and CAE were then raised in rabbits. Specificity was verified by immunoblotting. No cross-reaction was found between them, using the immunodot technique. CAB antiserum was used to specifically localize gill CA in the trout. Immunoperoxidase labelling revealed a concentration of enzyme on the apical region of the outer layer of the gill epithelial cells. The inner layer of the epithelium was only weakly positive. Results obtained using the immuno-gold technique confirmed the immunoperoxidase labelling: there was a concentration of label in the apical regions of chloride cells. In mucous cells, only the mucous granules were labelled. In the lamellae, the label was distributed in the apical part of the pavement cells. The villi and microplicae were strongly positive. CAE antiserum stained the red blood cells. The discrepancy between histochemical localization in the gill or in the opercular skin of killifish and our present immunolocalization was discussed.It was concluded that the most typical localization of CA is on the apical surface of the lamellar epithelium lying in contact with the environment. The result suggests that one of the main roles of gill CA may be to facilitate the diffusion of CO2 from blood to water.