Low levels of lead exposure induce oxidative damage and DNA damage in the testes of the frog Rana nigromaculata

We have investigated the chronic effects of low concentrations of lead (Pb) on oxidative damage and DNA damage in testes of the frog Rana nigromaculata. Sixty adult male frogs were randomly divided into six groups of ten. Based on the levels of the Integrated Wastewater Discharge Standard (GB 8978-1996) of China, five groups (II-VI) were treated by epidermal absorption with a PbNO3 solution at concentrations of 0.1, 0.2, 0.4, 0.8, 1.6 mg/l, respectively. The first group (I), which served as a control, was treated with distilled water only. Thirty days after treatment, all frogs were sacrificed and the testis tissues removed for the measurement of malondialdehyde (MDA) and glutathione (GSH) levels. DNA damage, including indicators of damage rate, DNA tail length (TL), and DNA tail moment (TM), was also analyzed by comet assays. Our data suggest that MDA levels in all treatment groups and GSH levels in the 0.2-1.6 mg/l Pb groups increased significantly relative to the controls (P < 0.01). Treatment with Pb at concentrations >0.4 mg/l also increased DNA damage rate and TM, while TL increased when the Pb level was >0.2 mg/l (P < 0.01 for DNA damage rate and TM, P < 0.05 for TL). Positive correlations were also found between DNA damage levels in the testes and MDA levels (r = 0.796 for DNA damage rate, r = 0.811 for TL, r = 0.796 for TM; P < 0.01 for all) as well between MDA and GSH levels (r = 0.455, P < 0.05) in the testes. Results from MDA measurements indicated that Pb-induced DNA damage in the testes of R. nigromaculata was possibly due to oxidative damage. Taken together, we conclude that Pb can induce male reproductive toxicity in R. nigromaculata.