Respiratory responses to hypoxia or hypercapnia in goldfish (Carassius auratus) experiencing gill remodelling

The presence of an interlamellar cell mass (ILCM) on the gills of goldfish significantly decreases the functional lamellar surface area and increases the diffusion distance for gas transfer and thus may impose a serious challenge for the transfer of respiratory gases (O2 and CO2). Here we tested the hypothesis that the presence of the ILCM in goldfish acclimated to 7 °C impedes the uptake of O2 and excretion of CO2. While PaO2 remained unaltered, the baseline values of PaCO2 were significantly higher in goldfish at 7 °C with ILCM present (5.55 ± 0.54 mmHg; mean ± SEM) than in goldfish at 25 °C without the ILCM (3.98 ± 0.18 mmHg). Carbonic anhydrase (CA) injections relieved the apparent diffusion limitation imposed by the presence of the ILCM on CO2 excretion (PwCO2 levels dropped to 3.07 ± 0.32 mmHg). Interestingly, the exposure of fish to acute hypoxia evoked similar changes in PaO2 at the two acclimation temperatures. Ethanol (EtOH) exposure was also used as a tool to further investigate the potential effects of the ILCM on branchial solute transfer. The results showed that the ILCM does not impede EtOH uptake in 7 °C goldfish. Overall, the results of this study demonstrate that the remodelling of the goldfish gill associated with acclimation to 7 °C water, while increasing PaCO2, has minimal impact on branchial O2 transfer.