New insights into the many functions of carbonic anhydrase in fish gills

Carbonic anhydrase (CA) is a zinc metalloenzyme that catalyzes the reversible reactions of carbon dioxide and water: CO2 + H2O ↔ H+ + HCO3−. It has long been recognized that CA is abundant in the fish gill, with attention focused on the role of CA in catalyzing the hydration of CO2 to provide H+ and HCO3− for the branchial ion transport processes that underlie systemic ionic and acid–base regulation. Recent work has explored the diversity of CA isoforms in the fish gill. By linking these isoforms to different cell types in the gill, and by exploiting the diversity of fish species available for study, this work is increasing our understanding of the many roles that CA plays in the fish gill. In particular, recent work has revealed that fish utilize more than one model of CO2 excretion, that to understand the role of CA and the gill in ionic regulation and acid–base balance means characterizing the transporter and CA complement of individual cell types, and that CA plays roles in branchial sensory mechanisms. The goal of this brief review is to summarize these new developments, while at the same time highlighting key areas in which further research is needed.

► New findings concerning carbonic anhydrase (CA) in the fish gill are reviewed.
► In dogfish and hagfish, type IV CA dehydrates plasma HCO3− for CO2 excretion.
► Ionocytes vary in CA isoform distribution and role in ionic/acid-base regulation.
► CA contributes to CO2 sensing in neuroepithelial cells.
► CA contributes to acid-base sensing via soluble adenylyl cyclase.