The teleost fish are thought to lack the mineralocorticoid hormone aldosterone but possess mineralocorticoid receptor (MR) homologs. Here we describe the characterization of two rainbow trout ( Oncorhynchus mykiss) MRs, called rtMRa and rtMRb. The open reading frame of rtMRa cDNA encoded a protein of 1041 amino acids. The rtMRb predicted protein sequence is similar, differing in only 10 amino acids in the nonconserved A/B domain and lacking a three-amino acid insertion between the two zinc fingers of the C domain. Expression of rtMR mRNA ( sum of both forms), measured in juvenile trout by real-time RT-PCR, shows that the transcripts are ubiquitous. Expression was significantly higher in brain than the other tissues studied ( eye, trunk kidney, head kidney, gut, gills, liver, spleen, ovary, heart, white muscle, skin). Hormonal stimulation of receptor transactivation activity was studied in COS-7 cells transiently cotransfected with receptor cDNA and a mouse mammary tumor virus-luciferase reporter. The mineralocorticoids 11-deoxycorticosterone and aldosterone were more potent enhancers of rtMRa transcriptional activity (EC50 = 1.6 +/- 0.5 x 10(-10) and 1.1 +/- 0.4 x 10(-10) M, respectively) than the glucocorticoids cortisol and 11-deoxycortisol (EC50 = 1.1 +/- 0.3 x 10(-9) and 3.7 +/- 1.9 x 10(-9) M, respectively). A similar response was observed in transactivation assays with rtMRb. These results are discussed in the view of reported circulating levels of corticosteroids in trout.
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