Cloning and expression of Na+/K+ -ATPase and osmotic stress transcription factor 1 mRNA in black porgy, Acanthopagrus schlegeli during osmotic stress.

We cloned complementary DNA (cDNA) encoding the Na(+)/K(+)-ATPase (NKA) and the osmotic stress transcription factor 1 (OSTF1) from the kidney and gill, respectively, of the black porgy, Acanthopagrus schlegeli. Black porgy NKA full-length cDNA consists of 3078 base pairs (bp) and encodes a protein of 1025 amino acids; OSTF1 partial cDNA consists of 201 bp. To investigate the osmoregulatory ability of black porgy when black porgy were transferred to freshwater (FW), we examined the expression of NKA and OSTF1 mRNA in osmoregulatory organs, i.e., gill, kidney and intestine, using quantitative polymerase chain reaction (QPCR). To determine the hypoosmotic stressor specificity of the induction of NKA and OSTF1, black porgy were exposed to 30 degrees C water temperature for 24 h. In the gill, NKA mRNA was 4.2 times higher in FW, its expression in the kidney was 5.7 times higher in 10 per thousand seawater (10 per thousand SW) than in SW. In contrast, OSTF1 mRNA in the gill was 3.7 times higher in FW than in SW. The expression of heat shock protein 90 (HSP90) mRNA occurred not only during transfer to FW, but also in high-temperature water in all tested tissues, although the mRNA levels were not significantly different. Plasma osmolality level was decreased and cortisol level was increased when the fish were transferred from SW to FW. These results suggest that NKA and OSTF1 genes play important roles in hormonal regulation in osmoregulatory organs and that these genes are specific to hypoosmotic stress, improving the hyperosmoregulatory ability of black porgy in hypoosmotic environments.