Differential physiological and endocrine responses of rainbow trout, Oncorhynchus mykiss, transferred from fresh water to ion-poor or salt water.

To understand the physiological and molecular endocrine changes that occur in response to a salinity challenge, we transferred rainbow trout from fresh water to an ion-poor or 24‰ saltwater treatment for 14 days. An increase in gill Na(+), K(+)-ATPase (NKA) activity in salt water was associated with higher mRNA expression for the NKA α1b subunit. In contrast, there was little change in gill NKA activity following transfer to ion-poor water, but the mRNA expression of NKA α1a was significantly elevated. Endocrine signals were assessed by measuring plasma cortisol concentrations and by quantifying changes in mRNA extracted from the gill for glucocorticoid receptors 1 and 2 (GR1 and GR2), mineralocorticoid receptor (MR), growth hormone receptor (GHR1), and prolactin receptor (PrlR). Cortisol increased after transfer to ion-poor and salt water, but both GR and MR mRNA in the gill showed little change. PrlR mRNA was significantly higher when fish were transferred to the ion-poor water and GHR1 mRNA was elevated during the saltwater challenge. This study demonstrated an increase in gill PrlR mRNA that parallels the changes in gill NKA α1a when rainbow trout were transferred to a lower salinity level. Furthermore, the increase in gill GHR1 mRNA supports the importance of GH for seawater acclimation as there is a corresponding increase in the expression of gill NKA α1b, the saltwater isoform. GH and Prl, therefore, may differentially determine the function of cortisol in both fresh- and saltwater ionoregulation.