Aquaporin 1 Is Involved in Acid Secretion by Ionocytes of Zebrafish Embryos through Facilitating CO(2) Transport

Mammalian aquaporin 1 (AQP1) is well known to function as a membrane channel for H(2)O and CO(2) transport. Zebrafish AQP1a.1 (the homologue of mammalian AQP1) was recently identified in ionocytes of embryos; however its role in ionocytes is still unclear. In this study, we hypothesized that zebrafish AQP1a.1 is involved in the acid secretion by ionocytes through facilitating H(2)O and CO(2) diffusion. A real-time PCR showed that mRNA levels of AQP1a.1 in embryos were induced by exposure to 1% CO(2) hypercapnia for 3 days. In situ hybridization and immunohistochemistry showed that the AQP1a.1 transcript was highly expressed by acid-secreting ionocytes, i.e., H(+)-ATPase-rich (HR) cells. A scanning ion-selective electrode technique (SIET) was applied to analyze CO(2)-induced H(+) secretion by individual ionocytes in embryos. H(+) secretion by HR cells remarkably increased after a transient loading of CO(2) (1% for 10 min). AQP1a.1 knockdown with morpholino oligonucleotides decreased the H(+) secretion of HR cells by about half and limited the CO(2) stimulated increase. In addition, exposure to an AQP inhibitor (PCMB) for 10 min also suppressed CO(2)-induced H(+) secretion. Results from this study support our hypothesis and provide in vivo evidence of the physiological role of AQP1 in CO(2) transport.