Differential expressions of genes related to HCO_3~- secretion in the intestine of Gymnocypris przewalskiii during saline-alkaline water transfer

The high salinity and alkalinity of the saline-alkaline water is the primary factor restricting the survival and growth of aquatic organisms. Gymnocypris przewalskii is the most important economic fish in Qinghai Lake,a high saline-alkaline water body( p H 9. 3,salinity 15) in China. In recent years,the population of G.przewalskii has sharply declined due to the destruction of spawning habitat,long-term overfishing and the increasing salinity and alkalinity. Thus,researches on the molecular response of G. przewalskii under salinealkaline conditions are significant for the recovery of this endangered species. The paper is to clarify how the intestine participates in the osmotic and acid-base balance regulation of G. przewalskii upon transfer to saline and alkaline conditions by physiological and molecular methods. When freshwater-acclimated G. przewalskii were transferred to saline water[salinity 15. 02 ± 0. 02,carbonate alkalinity( 2. 12 ± 0. 05) mmol·L- 1],alkaline water [ salinity 1. 31 ± 0. 02,carbonate alkalinity( 30. 66 ± 0. 08) mmol · L- 1]or lake water[salinity 14. 84 ± 0. 03,carbonate alkalinity( 29. 57 ± 0. 11) mmol·L- 1],there was a significant increase of base( HCO-3) excretion into the rectal fluids. Real-time fluorescence quantitative PCR method was used to determine the relative mRNA expressions of slc4a1,slc4a2,slc4a4 and slc26a6 genes in the intestine after transfer. Among them,slc4a1,slc4a2 and slc26a6 were Cl--HCO-3exchangers,and slc4a4 was a Na+-HCO-3co-transporter. Results showed that the four genes were expressed in multiple tissues,and highly expressed in the intestine,while slc26a6 expression in the gill was the lowest,exhibiting tissue specificity. The significant changes in relative mRNA expressions of investigated genes in the mid-intestine were detected on the third day of alkaline water exposure except the slc4a1 gene. While transferred to saline or lake water,the relative mRNA expressions of all investigated genes in the mid-intestine were significantly elevated on the second day.Among them,those exposed to lake water showed the highest level of relative mRNA expressions. The increase of SLC gene family mRNA expressions were consistent with the increase in rectal base secretion following saline-alkaline water transfer. The mRNA expressions of all investigated genes in the mid-intestine almost recovered to the original level on the fourth day,suggesting that G. przewalskii was capable of tolerating high saline-alkaline stress. These results revealed that, when freshwater-acclimated G. przewalskii were exposed to saline water,they up-regulated the expression of SLC4 and SLC26 gene families in the intestine to increase the synthesis of relevant transporters,which could aid to excrete alkaline substances( HCO-3) into the rectal fluid and stimulate the intestinal water absorption. In addition,we also found an increase in rectal base secretion and up-regulation of these genes after transferring to alkaline water with low salinity and high alkalinity. These indicated that SLC4 and SLC26 gene families also played important roles in acid-base regulation. Hence,in G. przewalskii,the SLC4 and SLC26 gene families were important internal factors for high tolerance of salinity and alkalinity. At the same time, the coordination of acid-base regulation,osmoregulation and ion-regulation was the basis for aquatic organisms to survive in saline-alkaline environment. Multiple tissues,genes and proteins may participate in this complex physiological process.