Respiratory control of acid-base status in lungfish

The acid-base status is a tightly regulated physiological process, resulting from a balance of ions in the organism relevant to acid-base. The efficiency of the regulatory systems importantly determines the compensatory pH changes for a given disturb. Vertebrates minimize (or compensate) an acid-base disturb by general processes, which include ion transfer and/or PCO₂ changes. Acid-base adjustment in fish is predominantly achieved by branchial exchange of acid-base relevant ions with correlated change in plasma HCO₃^- levels. Conversely, land vertebrates change blood PCO₂ through ventilatory process and hence respiratory control of acid-base regulation plays an important role as a compensatory mechanism. Lungfishes (Dipnoi) have central position on vertebrate's evolution being considered as the sister group to the tetrapods. With an aquatic life mode, lungfish share similarities of respiratory function with tetrapods. This article reviews evidence showing that lungfish's respiratory system regulates acid-base status, like terrestrial ectothermic vertebrates. In the South American lungfish, Lepidosiren paradoxa, the presence of central CO₂/pH chemoreceptors was unequivocally described. Also, the blood PCO₂ and acid-base status are typical of a terrestrial vertebrate. These aspects are discussed under different environmental conditions that require respiratory acid-base adjustments, such as, exposure to hypercarbia, hypoxia, high temperature and aestivation. Interesting questions regarding the location and cell phenotype of CO₂/pH central and peripheral chemoreceptors remain an open field to be explored in lungfish.