The Role of CO₂ for Gas Secretion in a Physoclist Swimbladder

In physoclist fish, that is, in fish with a closed swimbladder, oxygen has been shown to be the main gas in the swimbladder. Gas gland cells, crucial for the filling of the swimbladder, produce and secrete lactic acid. The resulting acidification of the blood releases oxygen from the hemoglobin via the Root effect, generating high oxygen partial pressures required for the diffusion of oxygen into the swimbladder. CO₂ in swimbladder gas was believed mainly to result from a wash out of HCO₃ ^- from the plasma due to acidification, but this reaction would diminish blood acidification. This study therefore attempted to revisit our current information about CO₂ production and also about CO₂ and HCO₃ ^- movements in swimbladder tissue to elucidate the role of CO₂ for swimbladder function.

Targeted literature search has been conducted to search for relevant publications.

The data reveal that European eel Anguilla anguilla gas gland cells produce most of the CO₂ in the pentose phosphate shunt. CO₂ diffuses into the swimbladder lumen and into the blood, facilitated by aquaporin 1. Membrane-bound carbonic anhydrase together with HCO₃ ^- transporters allow for CO₂ and HCO₃ ^- cycling across the basolateral gas gland cell membrane, supporting proton secretion and blood acidification. Countercurrent concentration of CO₂, HCO₃ ^- and protons in the rete mirabile results in a significant acidification of arterial blood in the rete.

CO₂ production together with CO₂ and HCO₃ ^- movements in swimbladder tissue significantly support blood acidification and the generation of high oxygen as well as CO₂ partial pressures.