The effects of cortisol on epithelial permeability and sodium (Na+) handling during acid exposure were investigated in larval zebrafish (Danio rerio). The results demonstrated that the whole-body absorption of the paracellular permeability marker polyethylene glycol-4000 (PEG-4000) decreased with increasing levels of exogenous cortisol. Western blot analysis revealed that the abundance of the epithelial tight junction proteins occludin-a and claudin-b was increased after cortisol treatment. Furthermore, translational gene knockdown of claudin-b using an antisense morpholino oligonucleotide caused an increase in the permeability to PEG-4000, which was mitigated by cortisol treatment, further suggesting a role for cortisol in reducing paracellular permeability. Exposure to acidic water (pH 4.0 vs 7.6) caused an expected increase in the diffusive loss of Na+ and a decrease in whole-body Na+ levels. These disruptive effects of acute acid exposure on Na+ balance were reduced by treatment of larvae with exogenous cortisol. Translational knockdown of the glucocorticoid receptor (GR) abolished the effects of cortisol on epithelial PEG permeability, suggesting that activation of GR was probably the major signaling pathway for reducing epithelial permeability. During acid exposure, the epithelial PEG permeability in the GR morphants was significantly higher than in the control fish. Additionally, GR morphants exhibited a more pronounced diffusive loss of Na+ than the control fish during acid exposure. These findings suggest that cortisol may help to minimize the negative consequences of acid exposure on Na+ homoeostasis via GR-mediated reductions in epithelial permeability and paracellular Na+ loss.
Raymond W M Kwong and Steve F Perry
Yusuke Kumai, Nicholas J Bernier and Steve F Perry
The contribution of the renin–angiotensin system (RAS) to Na+ uptake was investigated in larval zebrafish (Danio rerio). At 4 days post fertilization (dpf), the level of whole-body angiotensin-II (ANG-II) was significantly increased after 1- or 3-h exposure to acidic (pH=4.0) or ion-poor water (20-fold dilution of Ottawa tapwater), suggesting rapid activation of the RAS. Long-term (24 h) treatment of 3 dpf larvae with ANG-I or ANG-II significantly increased Na+ uptake which was accompanied by an increase in mRNA expression of the Na+-Cl− cotransporter (zslc12a10.2). Induction of Na+ uptake by exposure to ANG-I was blocked by simultaneously treating larvae with lisinopril (an angiotensin-converting enzyme inhibitor). Acute (2 h) exposure to acidic water or ion-poor water led to significant increase in Na+ uptake which was partially blocked by the ANG-II receptor antagonist, telmisartan. Consistent with these data, translational knockdown of renin prevented the stimulation of Na+ uptake following exposure to acidic or ion-poor water. The lack of any effects of pharmacological inhibition (using RU486), or knockdown of glucocorticoid receptors on the stimulation of Na+ uptake during acute exposure to acidic or ion-poor environments, indicates that the acute effects of RAS occur independently of cortisol signaling. The results of this study demonstrate that the RAS is involved in Na+ homeostasis in larval zebrafish.