Histological survey was made to determine nitrate toxicity on the Medaka fish,
Oryzias latipes. In order to investigate the effects of short-term exposure to nitrate, one-month-old Medaka fish was exposed to NaNO
3 at concentrations of 100 and 125 mg NO
3-N l
-1 for 96 hours. At the end of the exposure period, survival rate was found to be 30% and 10%, for the 100 and 125 mg NO
3-N l
-1 exposure concentrations, respectively. Histological examination of the organs showed that disruption of cell alignment was a common feature in the gills, intestinal ampulla, liver and kidney. A long-term exposure experiment was also carried out, whereby Medaka fish was exposed to NaNO
3 (100 and 125 mg NO
3-N l
-1) for three months from its egg stage. Eggs treated with NaNO
3 hatched within 10 days after fertilization. At the end of the exposure period, survival rate in the 100 and 125 mg NO
3-N l
-1 treatments were 40% and 30%, respectively. Fibrosis of the hepatic cells and curved spinal column were observed in the juveniles subjected to long-term nitrate exposure. The results of our experiments suggest that the high mortality resulting from short-term acute exposure to nitrate is caused by general dysfunction throughout the whole body. The chronic toxic effects attributed to nitrate, following long-term exposure, were likely to have resulted from nutrient deficiency caused by hepatic dysfunction.
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