Response To The Hypertonic Environment Of Osmoregulation Mechanism Of Anguilla Japonica

The Japanese eel, Anguilla japonica, is a catadromous fish. It spawns in the tropical Pacific Ocean west of the Mariana Island. Due to the complex life cycle with migrating between freshwater and seawater, Japanese eel becomes the ideal model for the research of osmoregulation. Japenase eel is an important aquaculture species in the Asian countries, such as Taiwan, Mainland China, Korea and Japan. Large numbers of elvers have been removed from estuaries for aquaculture purposes, preventing them from recruiting into the rivers. Due to over-exploitation and habitat degradation, the Japanese eel population has dramatically decreased to a very low level and many formerly important river fisheries are now closed. Research of the influence of salinity on physiological function is specially crucial for artificial propagation of eel for aquaculture, and for natural resource conservation. With respest to previous study on osmoregulation of Japanese eel, the expression of ion transport proteins, morphological change of chloride cells and the transformation of tissues or organs have been mainly focused on, but the regularity of instantaneous change of physiological characters of silver eel was little studied. The present research aims to study the salinity stress on physiological osmoregulation of Japanese eel, in the aspects of serology, enzymology and endocrine modulation.1. Change of rate of respiration and weight loss of Anguilla japonica in hypertonic environmentIn order to study the metabolic response of Japanese eel（505.1±35.7g） under salinity stress, the rate of respiration and weight loss were analyzed. The results showed that the respiratory rate of the eel increased with the elevated salinities. The respiratory rate of Japanese eel in salinity 33 group already reached 73±6 times·min^-1 at 1h, which increased by 78% compared with the freshwater group(41±2 times·min^-1)（P<0.05）. The respiratory rate in salinity 10 group reached 62 3 times·min^-1 at 1h, which increased by 51% compared with the freshwater group（P<0.05）. As the treatment time prolonged, the respiratory rate of each salinity group decreased, and achieved the level of freshwater group. The weight loss rate of Japanese eel increased as salinity elevated. Along with the time of treatment, weight loss rate in salinity 33 group and salinity 10 group is gradually increased, and reached the maximum 9.26±1.27% and 4.55±0.37%（P<0.05） at 24 h, separatedly. Then, the rate of weight loss in salinity groups decreased. The conclusion can be drawed that the respiratory rate of Japanese eel under hypertonic conditions increased, as well as the water loss rate. The eel can regulate water and salt metabolism through the excellent osmoregulation mechanism, and eventually reach a new balance.2. Osmoregulation mechanism of Anguilla japonica in hypertonic environmentIn order to study the ragularity of osmolality change of Japanese eel（505.1±35.7g） under salinity stress, the serum osmolality, serum ions concentration and activity of gill Na⁺/K⁺-ATPase under different salinities were analyzed. The result showed that the isosmotic point was estimated as 329.1 m Osm·kg^-1 and corresponded to salinity 10.48. With the duration of experiment, serum osmolality and ions（Na⁺, Cl-） concentration showed the similar fluctuating tendency that decreased after an initial increase. The serum K⁺ concentration was less effected by the salinity. The gill Na⁺/K⁺-ATPase activity in salinity 10 reached the minimum 5.40±0.72 μmol·mg^-1·h^-1 at 12 h, and then recovered to 7.17±0.85 μmol·mg^-1·h^-1 at 96 h, which was not significantly different compared with that in the control（P>0.05）. However, in salinity 33, the gill Na⁺/K⁺-ATPase activity descended acutely at the threshold of salinity treatment, then ascended rapidly and the maximum was observed at 24h(13.05±0.62 μmol·mg^-1·h^-1), which was 1.5 times higher than that in the control（P<0.05）. Therefore, there are three phases in the osmoregulation of Japanese eels response to salinity stress. The first was rapid increase period. Serum osmolality and ions（Na⁺, Cl-） concentration increased abruptly, whereas the gill Na⁺/K⁺-ATPase activity was inhibited. The second was slow increase period. Due to the eel took actions to compensate the loss of water to alleviate the rise of osmolality, serum osmolality and ions（Na⁺, Cl-） increased slowly, and the gill Na⁺/K⁺-ATPase activity was activated. The third was adaption period. The serum osmolality and ions（Na⁺, Cl-） concentration are gradually recovering, because the active transport ability was enhanced.3. Change of serum prolactin concentration, cortisol concentration and activity of kidney Na⁺/K⁺-ATPase of Anguilla japonica in hypertonic environmentIn order to study the ragularity of hormone regulation of Japanese eel（505.1±35.7g） under salinity stress, the serum osmolality, serum ions concentration and activity of gill Na⁺/K⁺-ATPase under different salinities were analyzed. The result showed that the serum prolactin concentration decreased with increasing salinity within 24 h. As the treat time prolonged, serum prolactin concentration in both of salinity groups showed firstly desended and then ascended trend. The salinity 10 group reached the minimum 15.76±0.70 ng·ml^-1（P<0.05） at 12 h, and returned to freshwater level at 96 h. The salinity 33 group reached the minimum 10.22±1.32 ng·ml^-1（P<0.05）, and returned to freshwater level at 14 d. The serum cortisol concentration raised with increasing salinity, The salinity 10 group achieved the maximum 5.64±0.27 ng·ml^-1 at 12 h, no significant difference with the freshwater. The salinity 33 group firstly declined at 1h, then achieved the maximum 7.52±0.21 ng·ml^-1 at 24h（P<0.05）, and fell to 7.52±0.21 ng·ml^-1（P<0.05）. As salinity increased, kidney Na⁺/K⁺-ATPase enzyme activity decreased. The salinity 10 group reached the minimum 10.63± 0.96 μmol·mg^-1·h^-1 at 12 h, which was significantly lower than the freshwater（P<0.05）, and returned to freshwater level at 96 h. The salinity 33 group reached the minimum 8.62±0.93 μmol·mg^-1·h^-1 at 24h（P<0.05）, and raised to 10.21±1.14 μmol·mg^-1·h^-1 at 96 h. The analysis indicated that salinity stress will quickly restrained the synthesis and release of prolactin of Japanese eel. Undergoing a short adaptation, prolactin would not affect the Japanese eel on osmoregulation in hypertonic environment. Na⁺/K⁺-ATPase enzyme activity was elevated by cortisol, enhancing the tolerance of salinity. Kidney Na⁺/K⁺-ATPase enzyme activity was inhibited in hypertonic conditions. Moreover, the interaction indeed exists among hormones, and prolactin can affect kidney Na⁺/K⁺-ATPase activity to regulate ion transport in freshwater.