Could hypoxia acclimation cause morphological changes and protect against Mn-induced oxidative injuries in silver catfish (Rhamdia quelen) even after reoxygenation?

Exposure to hypoxia has shown beneficial adjustments in different species, including silver catfish (Rhamdia quelen), especially in situations of aquatic contamination with pollutants such as manganese (Mn). Considering that hypoxia is seasonal in the natural aquatic environment, we decided to assess whether these adaptive mechanisms could be maintained when reoxygenation is established. Silver catfish acclimated to moderate hypoxia (∼3 mg L-1, 41% O2 saturation) for 10 days and subsequently exposed to Mn (∼8.1 mg L-1) for additional 10 days displayed lower (47%) Mn accumulation in the gills, and it was maintained (62.6%) after reoxygenation, in comparison to normoxia. Oxidative status in the gills allowed us to observe increased reactive species (RS) generation and protein carbonyl (PC) level together with decreased mitochondrial viability induced by Mn under normoxia. Inversely, while hypoxia per se was beneficial on RS generation and PC level, this acclimation was able to minimize Mn toxicity, as observed by the minor increase of RS generation and the minor reduction of mitochondrial viability, together with decreased PC level. Interestingly, after reoxygenation, part of the protective influences observed during hypoxia against Mn toxicity were maintained, as observed through a lower level of PC and higher mitochondrial viability in relation to the group exposed to Mn under normoxia. Only groups exposed to Mn under hypoxia showed increased activity of both catalase (CAT) and Na+/K+-ATPase in the gills, but, while CAT activity remained increased after reoxygenation, Na+/K+-ATPase activity was decreased by Mn, regardless of the oxygen level. Based on these outcomes, it is possible to propose that environment events of moderate hypoxia are able to generate rearrangements in the gills of silver catfish exposed to Mn, whose influence persists after water reoxygenation. These responses may be related to the adaptive development, reducing Mn toxicity to silver catfish. Moderate hypoxia generates rearrangements in the gills of Silver catfish, exerting beneficial and persistent protection against Mn toxicity.

Hypoxia acclimation and subsequent reoxygenation partially prevent Mn-induced damage in silver catfish.

This study investigated if hypoxia acclimation modifies the hematological and oxidative profiles in tissues of Mn-exposed silver catfish (Rhamdia quelen), and if such modifications persist upon subsequent reoxygenation. Silver catfish acclimated to hypoxia (~3mgL-1) for 10days and subsequently exposed to Mn (~8.1mgL-1) for additional 10days exhibited lower Mn accumulation in plasma, liver and kidney, even after reoxygenation, as compared to normoxia-acclimated fish. Hypoxia acclimation increased per se red blood cells count and hematocrit, suggesting adaptations under hypoxia, while the reoxygenation process was also related to increased hematocrit and hemoglobin per se. Fish exposed to Mn under normoxia for 20days showed decreased red blood cells count and hematocrit, while reoxygenation subsequent to hypoxia increased red blood cells count. Hypoxia acclimation also prevented Mn-induced oxidative damage, observed by increased reactive species generation and higher protein carbonyl levels in both liver and kidney under normoxia. Mn-exposed fish under hypoxia and after reoxygenation showed decreased plasma transaminases in relation to the normoxia group. Moreover, acclimation to hypoxia increased reduced glutathione levels, catalase activity and Na+/K+-ATPase activity in liver and kidney during Mn exposure, remaining increased even after reoxygenation. These findings show that previous acclimation to hypoxia generates physiological adjustments, which drive coordinated responses that ameliorate the antioxidant status even after reoxygenation. Such responses represent a physiological regulation of this teleost fish against oxygen restriction and/or Mn toxicity in order to preserve the stability of a particular tissue or system.