K + secretion by ionocytes in the skin of medaka larve

Epithelial ionocytes in fish gills and skin play a critical role in ionic and acid‐base regulation. Molecular mechanisms of sodium, chloride and acid‐base regulation in fish ionocytes have been well investigated. However, the regulatory mechanism of potassium, which is a major monovalent cation in fish, is still unclear. In a recent study, a sub‐group of ionocyte, which expressed potassium channels (ROMK) at the apical membrane was identified in zebrafish and Mozambique tilapia. In this study, we further investigated the mechanism of K+ secretion by ionocyte in medaka (Oryzias latipes). With a scanning ion‐selective electrode technique (SIET), K+ secretion was detected at the apical surface of a group of ionocyte in medaka larvae. Quantitative PCR showed that mRNA level of ROMK was induced in fish exposed to high K+ water. Loss‐of‐function studies showed that the K+ secretion was via ROMK. Taken together, this study suggests that the ROMK‐expressed ionocyte is responsible for K+ secretion in medaka.