A comparison of the effects of environmental XXXX exposure on the Asian freshwater stingray Himantura signifer and the Amazonian freshwater stingray Potamotrygon motoro.

@entity12116 @entity1133 @entity3265 @entity1160 @entity7222 @entity3655 @entity369
The white-edge whip tail ray Himantura signifer inhabits a freshwater environment but has retained the capability to synthesize @entity1133 de novo through the @entity369 - @entity7222 - @entity1133 cycle (OUC). The present study aimed to elucidate whether the capacity of @entity1133 synthesis in H. signifer could be upregulated in response to environmental @entity3265 exposure. When H. signifer was exposed to environmental @entity3265 , fairly high concentrations of @entity3265 were accumulated in the plasma and other tissues. This would subsequently reduce the net influx of exogenous @entity3265 by reducing the @entity3655 partial pressure gradient across the branchial and body surfaces. There was also an increase in the OUC capacity in the liver. Since the @entity3265 produced endogenously could not be excreted effectively in the presence of environmental @entity3265 , it was detoxified into @entity1133 through the OUC. In comparison, the South American freshwater stingray Potamotrygon motoro, which has lost the capability to synthesize @entity1133 de novo, was unable to detoxify @entity3265 to @entity1133 during @entity3265 loading. No increase in @entity1160 was observed in the various tissues of H. signifer exposed to environmental @entity3265 despite a significant increase in the hepatic @entity1160 synthetase activity. These results indicate that the excess @entity1160 formed was channelled completely into @entity1133 formation through @entity12116 synthetase III. It has been reported elsewhere that both @entity1133 synthesis and @entity1133 retention were upregulated in H. signifer exposed to 20 per thousand water for osmoregulatory purposes. By contrast, for H. signifer exposed to environmental @entity3265 in freshwater, the excess @entity1133 formed was excreted to the external medium instead. This suggests that the effectiveness of @entity1133 synthesis de novo as a strategy to detoxify @entity3265 is determined not simply by an increase in the capacity of @entity1133 synthesis but, more importantly, by the ability of the animal to control the direction (i.e. absorption or excretion) and rate of @entity1133 transport. Our results suggest that such a strategy began to develop in those elasmobranchs, e.g. H. signifer, that migrate into a freshwater environment from the sea but not in those permanently adapted to a freshwater environment.
