Acute toxicity of arsenic and oxidative stress responses in the embryonic development of the common South American toad Rhinella arenarum

Abstract

Arsenic (As), a natural element of ecological relevance, is found in natural water sources throughout Argentina in concentrations between 0.01 mg/L and 15 mg/L. The autochthonous toad Rhinella arenarum was selected to study the acute toxicity of As and the biochemical responses elicited by the exposure to As in water during its embryonic development. The median lethal concentration (LC50) value averaged 24.3 mg/L As and remained constant along the embryonic development. However, As toxicity drastically decreased when embryos were exposed from heartbeat-stage on day 4 of development, suggesting the onset of detoxification mechanisms. Given the environmental concentrations of As in Argentina, there is a probability of exceeding lethal levels at 1% of sites. Arsenic at sublethal concentrations caused a significant decrease in the total antioxidant potential but generated an increase in endogenous glutathione (GSH) content and glutathione S-transferase (GST) activity. This protective response might prevent a deeper decline in the antioxidant system and further oxidative damage. Alternatively, it might be linked to As conjugation with GSH for its excretion. The authors conclude that toad embryos are more sensitive to As during early developmental stages and that relatively high concentrations of this toxic element are required to elicit mortality, but oxidative stress may be an adverse effect at sublethal concentrations.

Chronic toxicity of arsenic during Rhinella arenarum embryonic and larval development: Potential biomarkers of oxidative stress and antioxidant response

“…However, GSH levels are slightly affected at the beginning of the exposure and show an increasing response at the larval stage, suggesting a partial recovery, with no significant effect on GSH at As concentrations up to 10 mg/L after 23 d. The negative effect on total reactive antioxidant potential would thus indicate that other nonenzymatic antioxidants are negatively affected by As. A similar situation was previously reported for acute exposure to high concentrations of As during the early development in toads [13]. We observed that the As effect on total reactive antioxidant potential is manifested at very early stages (from day 3) and at concentrations at least 10 times lower than the no-observed-effect concentration (NOEC) value for long-term exposures.…”

“…This decrease was sustained during the whole assay. While in the acute exposure a decrease was observed at 10 mg/L , in the present study we observed an underlying effect at lower concentrations that was manifested after 16 d of exposure. Ferrari et al studied changes in antioxidant metabolism during R. arenarum embryonic development.…”

“…This decrease was sustained during the whole assay. While in the acute exposure a decrease was observed at 10 mg/L [13], in the present study we Figure 6. Effects of arsenic (As) on glutathione reductase activity measured in larvae of Rhinella arenarum chronically exposed to As.…”

“…However, this increase of GSH did not correlate with GR activity. The present results show a strong induction of GST activity in embryos and larvae chronically exposed to concentrations 10 times lower than the NOEC , 1 mg As/L and 10 mg As/L, from a very early embryonic stage (TB); and this answer was sustained during the whole assay. Ventura‐Lima et al reported that low levels of As affect the antioxidant response in zebrafish by increasing the activity of the enzyme glutamate‐cysteine‐ligase and GSH levels, even at a concentration of As considered safe for the environment.…”

“…Biomarkers of exposure and effect are needed to connect the presence of pollutants in the environment with their action in an organism . We previously reported that the main effect observed at acute exposure to sublethal concentrations of As in Rhinella arenarum embryos was oxidative stress , requiring high concentrations of this toxic element to elicit mortality.…”