Aquatic animals often display physiological adjustments to improve their biological performance and hydrosaline balance in saline environments. In addition to energetic costs associated with osmoregulation, oxidative stress, and the activation of the antioxidant system are common cellular responses to salt stress in many species, but the knowledge of osmoregulation-linked oxidative homeostasis in amphibians is scarce. Here we studied the biochemical responses and oxidative responses of Xenopus laevis females exposed for 40 days to two contrasting salinities: hypo-osmotic (150 mOsm·kg −1 ·H 2 O NaCl, HYPO group) and hyper-osmotic environments (340 mOsm·kg −1 ·H 2 O NaCl, HYPER group). We found an increase of plasma osmolality and plasma urea concentration in the animals incubated in the HYPER treatment.At the end of the acclimation, animals were euthanized by decapitation (Katz & Hanke, 1993), the organs were weighted and tissues (liver, heart, ventral skin, and leg muscle) were frozen at −80°C for further biochemical analysis. Blood was collected from heart using capillary tubes, then centrifuged at 12,000g for 5 min, and the plasma (8 µL) was used for osmolality measurement by vapor pressure osmometry (Wescor 5130B). The remained plasma was stored (−80°C) for further analyses. The urea concentration (mg/dl) was determined by the urease/Berthelot method using a commercial kit 334 | HIDALGO ET AL.

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