Does the mineralocorticoid receptor have a functional role in the teleost gill?

Cortisol has been shown to have a glucocorticoid as well as a mineralocorticoid role in teleosts. The dual role of cortisol, however, was thought to be through a single class of receptor, the glucocorticoid receptor (GR) in teleosts until the recent discovery of the gene for a mineralocorticoid receptor (MR) in this group of fishes. My research objective was to determine if MR has a functional role in ionoregulation and if the changes in MR expression were related to biochemical responses when fish move among waters that vary in salinity. I addressed this research question in two ways. In my first experiment, I transferred rainbow trout from fresh- to ion-poor or salt water. An increase in gill Na⁺, K⁺-ATPase (NKA) activity in salt water was associated with higher NKA α1b subunit mRNA. In contrast, there was little change in gill NKA activity following the transfer to ion-poor water, but mRNA for the α1a subunit was significantly elevated. Gill GR and MR mRNA showed little change when fish were transferred to either treatment, but prolactin receptor (Pr1R) and growth hormone 1 receptor (GH1R) mRNA showed significant changes. In my second experiment, I examined adult sockeye salmon migrating from the ocean to spawning grounds. Gill NKA activity declined consistently during migration, accompanied by significant increases in the gill NKA α1a isoform. Generally, mRNA for gill MR and GR declined during migration to fresh water, but gill Pr1R and GH1R increased - particularly after the fish were in fresh water. These findings suggest that cortisol may have a more limited role in ionoregulation than previously described.