Efeito agudo da deltametrina em respostas enzimáticas e na descarga do órgão elétrico de Microsternarchus cf. bilineatus (Gymnotiformes, Hypopomidae)

Abstract

he insecticide deltamethrin (DT) is a pyrethroid widely used in pest control in agriculture. With the escalation in the usage of pesticides in Brazil, these chemicals can more frequently reach the aquatic ecosystems by leaching and runoff, provoking exposure risks to the aquatic biota. Many studies evaluate the effect of pesticides on fish species with the aid of biochemical biomarkers. Glutathione-S-transferase (GST) is a key enzyme in the phase II of xenobiotics biotransformation in fish and acetylcholinesterase (AChE) is an enzyme that plays a key role in the maintenance of nerve impulses, chosen as a biomarker in this work due to the neurotoxic effects of DT. The species used in the present study was the electric fish Microsternarchus cf. bilineatus, belonging to the order Gymnotiformes. These fish have as main characteristic the ability to generate and use electric fields to perceive the environment around them. Due to this characteristic, it is proposed in the literature the use of these fish as biomonitors of environment quality, in real time. The present study aimed to evaluate the toxicity of DT in Microsternarchus cf. bilineatus. Toxicity was assessed from the determination of the median lethal concentration (LC50-96h) of DT for this species. Estimates of the activities of the enzyme GST and AChE were evaluated in vitro and in vivo in muscle, liver and nervous tissues of individuals exposed to DT. In a parallel experiment the effect of different concentrations of DT on several parameters of the electric organ discharge (EOD) of these fish, was verified. The activities of the GST and AChE enzymes were also evaluated in the same experiment. In the LC50 test, the lethal concentration of DT estimated for the species was 2.1 μg L-1, reflecting a high toxicity of this insecticide for the species studied. An increase of the muscle GST enzyme in the in vivo assay may indicate the role of the muscle of these fish in the defense against xenobiotics. DT did not alter muscle and cerebral AChE in the in vivo assay. However, a significant increase of muscle AChE was observed in the in vitro experiment at 4 μg L-1 concentration. DT reduced the mean repetition rate (Hz) of fish EODs in the first hour of exposure for all concentrations. This effect was more prominent in the concentration of 3 than in that of 4 μg L-1. There were no significant alterations in the AChE and GST enzymes in the tissues evaluated for the exposure times associated to these experiments. The nonlinear pattern of the fish’s bioelectrical responses to the insecticide may be related to internal physiological and biochemical compensation mechanisms when these fish are exposed to high concentrations of pollutants. It is concluded that DT is highly toxic to Microsternarchus cf. bilineatus, whom presented the lowest value of LC50 registered in the literature for an Amazonian species; at concentrations above the LC50, it is likely that the induction of the muscle GST enzyme observed in the present study reflects the role of muscle in the defense against xenobiotics. Also, this species can be used as a quality biomonitor of aquatic environments by altering its discharge patterns to insecticide exposure in less than three hours of testing.