Normóxia, hipóxia e reoxigenação em jundiás frente à exposição ao manganês

Abstract

In recent decades, the global warming has increased the incidence of the hypoxia episodes in the world, changing the freshwater and oceanographic conditions. Thus, hypoxia periods, before seasonal, have occurred more frequently, mainly by human interference in the environment, raising concerns about the aquatic biota. As a result, it can be also observed by a decrease in oxygen levels and, in addition, increased water pollution generated by extractive activities like prospection of both minerals and fossil fuels. In this process, manganese (Mn) may suffer extravasation into the environment contaminating tributaries and compromising biodiversity. Based on this, this study evaluated the impact of changes in water oxygen levels (normoxia, hypoxia and reoxygenation) on the impairments generated by exposure to Mn in different silver catfish (Rhamdia quelen) tissues, a hypoxia-tolerant teleostean species of economic relevance. The acclimation of silver catfish to hypoxia (~3.09 mg L-1) was able to reduce Mn accumulation in different tissues when fish were exposed to Mn 9.8 mg L -1 and 8.1 mg L-1 (in the first and other studies, respectively). Additionally, hypoxia acclimation was able to minimize the oxidative damages to the membrane lipids and reduce protein carbonylation, preventing Mn-induced changes on both catalase (CAT) activity and Na +/K +-ATPase, as well as on prolactin (PRL) and somatolactin (SL) gene expression, pituitary hormones related to reproductive cycle, reinforcing the hormesis development hypothesis. Finally, when reoxygenation was established (restoration of the oxygen levels to normoxia: ~ 7.0 mg L-1), silver catfish acclimated to hypoxia and subsequently reoxygenated, presented adjustments in both hematological profile and antioxidant responses, which contributed to the reduction of the oxidative damages generated by exposure to the Mn under normoxia, observed by increased RS generation, increased protein carbonylation and increased serum transaminases. Hypoxia acclimation also produced morphological changes in the gill surface, important to the adaptive process to low oxygen levels, which, mostly, remained even after the establishment of reoxygenation. When the maze of social interaction memory or anxiety was accessed in the behavioral tests, no changes were observed on the memory of silver catfish acclimated to hypoxia or normoxia, exposed or not to Mn. However, fish accessed by the third time the maze trial, after the establishment of reoxygenation, showing memory related to social interaction. Additionally, silver catfish acclimated to hypoxia exhibited an increase per se in ventilatory rate, which was poorly reactive towards conspecifics, while fish under normoxia replied aggressively against conspecifics throughout the trial period. However, exposure to Mn under normoxia or hypoxia, and after reoxygenation, reduced the answer against conspecifics. Regarding stress coordinated by endocrine axis, silver catfish acclimated to hypoxia showed increased proopiomelanocortin-A (POMC-A) expression, while the expression of this pituitary hormone decreased during the exposure to Mn under hypoxia. The proopiomelanocortin-B (POMC-B), PRL and SL expression remained constant throughout the experimental period, regardless of the oxygen levels or Mn exposure. Silver catfish acclimated to hypoxia for 20 days, as well as in those animals exposed to Mn under hypoxia showed skin darkening, which persisted after reoxygenation. Taken together, these parameters allow to conclude that hypoxia-tolerant species as silver catfish develop over a short stress period by oxygen deprivation, both physiological and morphological adjustments mechanisms that increase their antioxidant defenses and reduce their energy consumption in order to face more hostile situations, such as excessive Mn concentration or a subsequent reoxygenation. Until now, the long-term hypoxia effects on silver catfish in its natural environment are unknown. However, it is possible that sustained hypoxia episodes cause behavioral impairments in the submission and dominance relationships among fish, changing the hierarchical relationships and reducing the aggression, injury risks, which may prolong survival but in contrast cause mischaracterization of this species, which naturally presents a carnivorous dominance behavior.