Effect of methylmercury on some constituents of serum and urine

Modulating effects of dietary fats on methylmercury toxicity and distribution in rats

Fish consumption is the most important source of human exposure to methylmercury (MeHg). Since fish is also a rich source of n-3 polyunsaturated fatty acids, this study was conducted to examine the effects of dietary fats on MeHg-induced acute toxicity in rats. Weanling male Sprague Dawley rats were administered semi-purified casein-based isocaloric diet containing soy oil, seal oil, docosahexaenoic acid (DHA), fish oil, or lard for 28 days. Rats were then gavaged with 0, 1, or 3 mg MeHg/kg body weight (BW) per day and fed the same diet for 14 consecutive days. On 43rd day of the experiment, rats were sacrificed and blood samples were collected and analyzed for hematology. Liver and spleen were removed, fixed, and examined for pathological changes. Blood, feces, liver, and brain were analyzed for total mercury and/or MeHg contents. Serum samples were analyzed for clinical markers of hepatic injury and immunoglobulin. Total mercury contents in all tissues measured increased with dose. Mercury excretion in feces increased with dose and duration of MeHg treatment. Both diets and MeHg showed significant effects and interacted significantly on many of the toxicological endpoints measured. Many of the effects of MeHg were diet-dependent. For example, in the rats fed the lard diet, 3mg MeHg/kg BW significantly increased relative liver and spleen weight as compared with vehicle control; whereas in rats fed the fish oil, soy oil, seal oil, or DHA, this effect of MeHg was less obvious or absent, suggesting a protective effect of these diets. MeHg at 3mg/kg BW significantly decreased serum albumin level in all except DHA dietary groups, implying a protection by the DHA diet on this parameter. Only in the lard dietary group, did 3mg MeHg/kg BW significantly increase serum bilirubin level, indicating an enhancing effect of this diet on MeHg toxicity. MeHg suppressed the adaptive immune system and stimulated the innate immune system in rats in a diet-dependent fashion. The seal oil diet provided more resistance, while the fish oil diet rendered greater sensitivity to these effects of MeHg on the immune system. These results imply significant modulating effects of dietary fats on MeHg toxicity which may translate into more severe or protective clinical outcomes. Therefore, dietary fats are important factors to be considered in the risk assessment of MeHg exposure.

Influence of dietary protein levels on the acute toxicity of methylmercury in mice

The influence of dietary protein levels on the acute toxicity of methylmercury (MeHg) was investigated using C57BL/6N male mice fed either a 24.8% protein diet (normal protein diet, NPD) or a 7.5% protein diet (low protein diet, LPD). When MeHg was administered to each group of mice, all mice died at a medium or high dose (80 or 120 mumol/kg, respectively) within 16 or 7 days, respectively, but not at a low dose (40 mumol/kg) in both dietary groups. Although no difference was observed in the survival rate at a medium dose, NPD-fed mice died earlier despite lower brain Hg than LPD-fed mice at a high dose. Accordingly, death, in our observations, could not be due to neural damage by MeHg. When a high dose of MeHg was administered to mice, plasma aspartate aminotransferase and alanine aminotransferase activities increased in NPD-fed mice but not in LPD-fed mice in spite of similar hepatic Hg levels. Therefore, the higher susceptibility of the liver could be reason for the shorter survival period in NPD-fed mice. Since plasma creatinine increased within 24 h after MeHg administration at a medium or high dose, renal dysfunction could be a major factor in death. The present results suggest that susceptibility to acute MeHg toxicity was higher in NPD-fed mice than in LPD-fed mice, possibly due to the difference in hepatic susceptibility.