Role of cytosolic carbonic anhydrase Ca17a in cardiorespiratory responses to CO₂ in developing zebrafish (<i>Danio rerio)</i>.

The sensing of environmental fluctuations and initiation of appropriate physiological responses is crucial to homeostasis. Neuroepithelial cells (NECs) in fishes are putative chemoreceptors, resembling mammalian Type I (glomus) cells, that respond in vitro to changes in O₂, CO₂, NH₃, and pH. Cytosolic carbonic anhydrase (Ca17a) is thought to be involved in CO₂ sensing owing to its presence in NECs. Zebrafish (<i>Danio rerio</i>) lacking functional Ca17a were generated via CRISPR/Cas9 technology and used to assess the role of Ca17a in initiating the cardiorespiratory responses to elevated CO₂ (hypercapnia). Unfortunately, the homozygous knockout mutants (<i>ca17a^-/-</i>) did not survive more than ∼12-14 days postfertilization (dpf), restricting experiments to early developmental stages (4-8 dpf). Changes in ventilation (<i>f</i>_V) and cardiac (<i>f</i>_H) frequency in response to hypercapnia (1% CO₂) in wild-type (<i>ca17a^+/+</i>), heterozygous (<i>ca17a^+/-</i>) and <i>ca17a^-/-</i> fish were used to investigate Ca17a-dependent CO₂ sensing and downstream signaling. Wild-type fish exhibited hyperventilation during hypercapnia as indicated by an increase in <i>f</i>_V. In the <i>ca17a^-/-</i> fish, the hyperventilatory response was attenuated markedly but only at 8 dpf. Hypercapnic tachycardia was observed for all genotypes and did not appear to be influenced by the absence of Ca17a. Interestingly, <i>ca17a^-/-</i> fish exhibited a significantly lower resting <i>f</i>_H that became more pronounced as the fish aged. The decrease in resting <i>f</i>_H was prevented ("rescued") when <i>ca17a^-/-</i> embryos were injected with <i>ca17a</i> mRNA. Collectively, the results of this study support a role for Ca17a in promoting hyperventilation during hypercapnia in larval zebrafish and suggest a previously unrecognized role for Ca17a in determining resting heart rate.