Cortisol and glucocorticoid receptor 2 regulate acid secretion in medaka (Oryzias latipes) larvae

Freshwater fish live in environments where pH levels fluctuate more than those in seawater. During acidic stress, the acid-base balance in these fish is regulated by ionocytes in the gills, which directly contact water and function as an external kidney. In ionocytes, apical acid secretion is largely mediated by H⁺-ATPase and the sodium/hydrogen exchanger (NHE). Control of this system was previously proposed to depend on the hormone, cortisol, mostly based on studies of zebrafish, a stenohaline fish, which utilize H⁺-ATPase as the main route for apical acid secretion. However, the role of cortisol is poorly understood in euryhaline fish species that preferentially use NHE as the main transporter. In the present study, we explored the role of cortisol in NHE-mediated acid secretion in medaka larvae. mRNA expression levels of transporters related to acid secretion and cortisol-synthesis enzyme were enhanced by acidic FW treatment (pH 4.5, 2 days) in medaka larvae. Moreover, exogenous cortisol treatment (25 mg/L, 2 days) resulted in upregulation of nhe3 and rhcg1 expression, as well as acid secretion in 7 dpf medaka larvae. In loss-of-function experiments, microinjection of glucocorticoid receptor (GR)2 morpholino (MO) caused reductions in nhe3 and rhcg1 expression and diminished acid secretion, but microinjection of mineralocorticoid receptor (MR) and GR1 MOs did not. Together, these results suggest a conserved action of cortisol and GR2 on fish body fluid acid-base regulation.