Cortisol regulation of ion transporter mRNA in Atlantic salmon gill and the effect of salinity on the signaling pathway

Based on real-time RT-PCR, analysis of transcripts of selected ion-regulatory proteins (Na+, K+-ATPase alpha 1a and alpha 1b subunit, Na+, K+, 2Cl(-) cotransporter, cystic fibrosis transmembrane conductance regulator (CFTR), and H+-ATPase B-subunit), the regulatory role of cortisol and the associated receptor signaling pathway (glucocorticoid (GR) versus mineralocorticoid (MR)) of cortisol was investigated in the salmon gill. Using a gill organ culture technique, the effect of cortisol with and without addition of specific hormone receptor antagonists (RU486 and spironolactone) was analyzed in gills from freshwater (FW) and seawater (SW) acclimated fish. The effect of cortisol was highly dependent on acclimation to salinity. In FW, cortisol stimulated the transcript levels of CFTR-I and Na+, K+-ATPase alpha 1a and alpha 1b. Addition of RU486 totally abolished the cortisol effects on CFTR-I and Na+, K+-ATPase alpha 1b, suggesting that signaling was mediated via GR. Interestingly, both spironolactone and RU486 were able to inhibit the cortisol effect on Na+, K+-ATPase alpha 1a indicating a role for both MR and GR in regulation of this target gene. In SW, cortisol increased the transcript levels of CFTR-I, CFTR-II, Na+, K+-ATPase alpha 1a and alpha 1b, and NKCC. Interestingly, the effect of cortisol on CFTR-I and Na+, K+-ATPase alpha 1a was mediated through GR and MR respectively, while both GR and MR signaling were required in the regulation of CFTR-II and Na+, K+-ATPase alpha 1b. In FW gills, GR1 and MR transcript levels were not significantly affected by cortisol. In SW gills, GR1 and MR transcripts were downregulated by cortisol; GR1 was regulated via the MR and MR regulation was mediated via GK.