Studies On Immune Responses To Heavy Metal Ions-Cd²⁺、Pb²⁺ In Ruditapes Philippinarum

In recent years, heavy metal contaminantion along costal areas has seriouslyincreased as a consequence of the rapid development of costal economy andanthropogenic activities. It could do serious harm to the marine environment andorganisms and has aroused the worldwide attention.Heavy metals have the characters of non-degradation and bioaccumulation andare recognized as the most potential deleterious contaminants presented in theenvironment. Cadmium (Cd²⁺) and lead (Pb²⁺) are important ingredients ofenvironmental endocrine disruptors and have been proved to induce a broad range ofphysiological, biochemical, and behavioral dysfunctions. Studying the physiologicaland biochemical effects of heavy metals on marine organisms would providescientific data for toxicological research on marine organisms and monitoring theearly heavy metal contamination of marine environments.The present study is conducted to evaluate the potentially toxic effects of heavymetals-Cd²⁺and Pb²⁺on R. philippinarum under controlled laboratory conditions. Themain research contents include:（1） The acute toxic effects on R. philippinarum whenexposed to sigle and combined stress of Cd²⁺and Pb²⁺;（2） The immune responses ofR. philippinarum to Cd²⁺or Pb²⁺stress. The activities of two kinds of antioxidantenzymes: superoxide dismuse （SOD） and catalase （CAT） in different tissue, twoenzymes relating to immune responses: acid phosphatase （ACP） and alkalinephosphatase （AKP） were analyzed. The index relating to lipidperoxidation–malondialdehyde （MDA） was also analyzed to study the possible toxicmechanism. Results showd that:1. Single toxic effect of Cd²⁺and Pb²⁺on R. philippinarumThe acute toxicity of Cd²⁺and Pb²⁺to R. philippinarum was evaluated and themedian lethal concentration （LC50） and safety concentrations （SC） of Cd²⁺and Pb²⁺toR. philippinarum were obtained. Results showed that: The24h,48h and96h LC50for Cd²⁺was11.146mg/L、6.546mg/L、4.740mg/L, respectively, while that for Pb²⁺was70.001mg/L、31.116mg/L、13.803mg/L, respectively. The safe concentration was0.677mg/L for Cd²⁺and1.844mg/L for Pb²⁺. Cd²⁺presented higher toxicitycompared to Pb²⁺in the present study.2. Combined toxic effect of Cd²⁺and Pb²⁺on R. philippinarumOn the basis of single factor acute toxic experiment, combined acute toxicexperiment of Cd²⁺-Pb²⁺were carried out in equitoxicity ratio. Results showed: Theaddition index（AI） of24h,48h and96h were0.142、0.244、0.541, respectively. Thecombined toxic effect of Cd²⁺and Pb²⁺was synergitic according to the changes of96h LC50. Synergistic action was strengthening with toxipollution time prolonging.3. Effects of Cd²⁺、Pb²⁺stress on immune and antioxidant enzyme activities in thegill and digestive gland of R. philippinarumIn gill and digestive gland of R. philippinarum, SOD and CAT activities wereincreased compared with the control group during the initial experimental time whenexposed to different concentrations of Cd²⁺、Pb²⁺. MDA presented little difference ascompared to the control （p>0.05） during the first2d after exposure, but increasedsignificantly with time increasing （p<0.05）, inferring the heavy metal exposure causea serious oxidative stress in organism. The activities of SOD and CAT in gill anddigestive gland decreased slowly when exposed to lower concentrations of heavymetals but decreased obviously in higher concentration groups （p<0.05）. SOD activityreached a peak on the second day after exposure while the peak of CAT activity wasobserved on the4thday. CAT activity in digestive gland was higher than that in thegill, but SOD activity in the gill revealed a much higher sensitivity to heavy metalsthan in digestive gland. This might be due to the different detoxification mechanisms.ACP and AKP activities presented different changing trend whenexposed to theheavy metals. ACP activity changes showed different patterns with Cd²⁺and Pb²⁺treatments: ACP activity was restrained and then increased in both gill and digestivegland when exposed to Cd²⁺while showed just the opposite trend in Pb²⁺groups.However, no regularity was observed in the activity of AKP when exposed to eitherCd²⁺or Pb²⁺. In summary, the research suggested that Cd²⁺or Pb²⁺exposure caused negativeeffects on immune systems of R. philippinarum, and the immune indicators indigestive gland were more sensitive than that in gill. Digestive gland might be thetarget tissue of heavy metal stress. SOD、CAT、ACP in the digestive gland of R.philippinarum could be used as biomarkers to Cd²⁺or Pb²⁺stress.