Ion Regulation Under Acidic Conditions in Zebrafish (Danio rerio) Lacking Functional Corticosteroid Receptors

Abstract

Previous research suggests a role for cortisol in mediating ion balance in zebrafish (Danio rerio), an acid-tolerant freshwater fish that is used as a model species for understanding ionic, osmotic and acid-balance in teleost fishes. Exposure to acidic water (pH 4.0-4.5) increases diffusive ion loss and impairs ion uptake mechanisms. Cortisol, a corticosteroid hormone, exerts its effects through glucocorticoid (GR, encoded by the nr3c1 gene) and mineralocorticoid (MR, encoded by the nr3c2 gene) receptors. The objective of the present study was to generate nr3c1-/- and nr3c2-/- zebrafish lines to test the role of cortisol in eliciting compensatory responses to pH 4 water in adult zebrafish because most previous work has focused on larval stages. Whole-body Na+ and Ca2+ levels, plasma cortisol concentrations, ionocyte abundances and branchial expression of genes previously implicated in responses to pH 4 water were measured in nr3c1+/+nr3c2+/+ (i.e. wildtype, WT), nr3c1-/- (GR-KO), and nr3c2-/- (MR-KO) zebrafish exposed for seven days to control or pH 4 water. Despite the absence of cortisol signaling via GR, GR-KO fish tolerated pH 4 water similarly to WT and MR-KO fish, in contrast to the literature on larval fish that suggested that GR is essential for these responses. Subtle differences were observed; GR-KO adults exposed to pH 4 water had slightly but significantly lower whole-body Na+ content than WT fish, and higher abundance of H+-ATPase-rich (HR) cells under control conditions that decreased in response to pH 4 water. These fish also exhibited significantly lower transcript abundance of ncc, the Na+,Cl--cotransporter. Together, the results of the present study highlight the complexity of endocrine regulation of ionic balance in freshwater fishes.