Toxicology. 2003 Mar 14;185(1-2):23-33.
Placental transfer of mercury in pregnant rats which received dental amalgam restorations.
Takahashi Y, Tsuruta S, Arimoto M, Tanaka H, Yoshida M.
Mercury vapor released from one, two and four amalgam restorations in pregnant rats and mercury concentrations in maternal and fetal organs were studied. Dental treatment was given on day 2 of pregnancy. Mercury concentration in air samples drawn from each metabolism chamber with a rat were measured serially for 24 h on days 2, 8 and 15 of pregnancy. On each day of pregnancy, the amount of mercury in 24 h air samples was in proportion to the amalgam surface areas. Linear regression analysis showed relatively high correlation coefficients between the mercury content and amalgam surface areas, and the coefficients were statistically significant.
A highly significant correlation was also found between the number of amalgam fillings and their surface areas. Mercury concentrations in major maternal organs with one, two and four amalgam fillings tended to increase with the increasing amalgam surface areas.
Spearman's rank correlation test revealed significant correlations in the brain, liver, kidneys and placenta but not in the lung. Furthermore, significant correlations were also found between the mercury concentrations in all maternal organs and the amount of mercury in 24 h air samples on day 15 of pregnancy.
Mercury concentrations in fetal brain, liver and kidneys were much lower than those of the dams but liver and kidneys showed positive correlations between the mercury content and maternal amalgam surface areas. Similar correlations were observed between the mercury concentrations in fetal organs and the amount of mercury in 24 h air samples on day 15 of pregnancy. In fetal brain, no significant correlations were found between either maternal amalgam surface areas or the amount of mercury in 24 h samples on day 15 of pregnancy but significant uptake of mercury was found in the samples from the dams given four amalgam fillings.
The results of the present study demonstrated that mercury vapor released from the amalgam fillings in pregnant rats was distributed to maternal and fetal organs in dose-dependent amounts of the amalgam fillings.
Dental amalgam was introduced more than 150 years ago as a tooth filling restoration. Today it is still a popular restorative despite the introduction of new types of fillings because of its wide potential applications, ease of manipulation, adequate mechanical properties and relative low cost. It is a compound of mercury and silver-based alloys, but mercury is the principal component, usually accounting for about 50% by weight and this may have an adverse impact on human health. Dental amalgam is generally the major source of mercury vapor exposure (WHO, 1991). In human studies, several investigators have shown that mercury vapor is released from hardened dental amalgam (Gay et al., 1979; Svare et al., 1981; Abraham et al., 1984; Vimy and Lorscheider, 1985a; Berglund et a1., 1988; Langworth et a1., 1988; Aronsson et al., 1989; Berglund, 1990; Bjérkrnan and Lind, 1992; Skare and Engqvist, 1994; Fredin, 1994; Halbach, 1995; Berglund and Molin, 1996), and the release rate of mercury increased dramatically when the amalgam was stimulated by continuous chewing (Vimy and Lorscheider, 1985b; Aronsson et al., 1989). Further, mercury vapor levels were directly correlated with the number of amalgam fillings (Svare et al., 1981; Patterson et al., 1985; Abraham et al., 1984) or surface area of the amalgam fillings (Abraham et al., 1984). Most data are usually based on a single or a small series of samples of intra-oral or expired air, before and after mechanical stimulation of the amalgam surfaces. The estimate for the rate of release in people with amalgam filling is 3—17 pg Hg/day (WHO, 1991). The most recent estimate based on applying pharmacokinetic parameters to steady-state plasma levels in people with amalgam fillings suggests an average intake between 5 and 9 pg Hg/day (Sandborgh-Englund et al., 1998). The rate of release is dependent upon many factors including surface area, number, age, composition and structure of the amalgam fillings, as well as the quality of the surface oxide layer.
In human autopsy tissue samples, Nylander et al. (1987), Friberg and Nylander (1987) found that the mercury levels in tissues of subjects with amalgam fillings were higher than those of amalgam-free subjects, and a positive correlation was obtained between the number of amalgam fillings and mercury levels in brain and kidneys (Nylander et al., 1987; Eggleston and Nylander, 1987).
Studies in humans on mercury concentration in different tissues of fetuses and infants are scarce (Drasch et al., 1994; Lutz et al., 1996). Drasch et al. (1994) found a correlation between the number of maternal amalgam fillings and the concentration of mercury in fetal liver and kidneys, but unfortunately there is no data for fetal brain. In the study of Lutz et al. (1996), linear regression analysis of tissue mercury concentrations in the fetuses versus self-reported number of maternal amalgam fillings showed no significant association for the brain (P 0.23), but close to a significant association for the kidneys (P 0.06).
Investigators also demonstrated using various animals that when amalgam was placed in teeth, mercury levels rose in various organs and tissues (Fredin, 1987; Hahn et al., 1989, 1990; Vimy et a1., 1990; Danscher et a1., 1990; Hultman et al., 1998; Galic et 211., 1999; Takahashi et a1., 2001).
In our previous animal experiment, we found a mercury uptake in maternal and fetal tissues from the maternal amalgam fillings and that the amount of mercury vapor exhaled by the rat increased 20-fold after chewing (Takahashi et al., 2001). However, we could not find a relationship between the amount of mercury vapor released from amalgam placement in the mother and the mercury transfer to the fetus from a single amalgam restoration.
The purpose of this study was to examine the relationship between the amount of mercury vapor and amalgam occlusal areas and to determine whether or not the amount of mercury vapor or amalgam occlusal area would correlate with the mercury concentrations in maternal and fetal organs.
This study has demonstrated that mercury concentration increased significantly in 24 h air samples taken from pregnant rats given 1, 2 and four amalgam fillings. A correlation was found between the amount of mercury in the air samples and mercury concentration in the maternal brain, liver, kidneys, lung and placenta. Significant amounts of mercury from maternal organs were found to have been transferred to fetal organs. Mercury concentration in maternal organs tended to increase with the number of amalgam fillings and their surface areas.
The largest amounts of mercury concentration in the 24 h air samples were found on day 2 of pregnancy immediately after the placement of amalgam fillings (Table 1). During this first 24 h period, a significant amount of liquid mercury had probably reacted with the alloy to become chemically more firmly bound in the Ag2Hg3 phase that forms during the setting of the amalgam. The amalgamation with the alloy may progress gradually allowing mercury vapor emission to produce large amounts of mercury on day 2. However, there was still a surprisingly large amount of mercury concentration on days 8 and 15. This may be explained from the results of a previous study (Takahashi et al., 2001) in which we monitored rats given single amalgam restorations for 24 h using a video camera recorder with a video cassette recorder while the mercury in the air sample was being measured. The amount of mercury in 10 min air samples increased when the rats took food, amounts showing values from 7
amalgam restorations increased the level of mercury approximately 2—4-fold in the brain, 3-7-fold in the liver and 10-30-fold in the kidneys. Our study revealed that large amounts of mercury vapor were continually released from the amalgam fillings resulting in the accumulation of mercury in major maternal organs. In human autopsy studies, Nylander et al. (1987) found that mercury concentrations in subjects with amalgam fillings were approximately 2-3-fold higher in the brain and fold higher in the kidneys compared with amalgam-free subjects. In humans, Drasch et al. (1994) found a correlation between the number of maternal amalgam fillings and the concentration of mercury in the fetal liver and kidneys. A similar increase in mercury concentration in fetal kidneys with an increased number of amalgam fillings in the mother was reported by Lutz et al. (1996). Using BN rats and high copper silver amalgam fillings, Hultman et al. (1998) demonstrated a similar increase in mercury concentration in the kidneys to that shown in our present study. A recent study by Morgan et al. (2002) using pregnant rats, but a different dosage method from ours, revealed that mercury concentration in maternal organs increased with the increasing concentration of mercury vapor. Our study also revealed highly significant correlations between the number of amalgam fillings and their surface areas with the amounts of mercury concentration in maternal brain, liver and kidneys and between these and the placenta.
In fetuses, mercury concentration in the liver increased in proportion to the surface area of maternal amalgam fillings with a strong positive correlation between them (P <0.01). A similar correlation was also found between the mercury content of the fetal liver and the amount of mercury in the 24 h air samples on day 15 of pregnancy (P <0.01). Fetal kidneys mercury uptake was not as high as that of fetal liver but significant correlations were observed between it and maternal filling surface areas or the amounts of mercury in 24 h air samples on day 15 of pregnancy (P 0.05, P 0.01, respectively). Mercury concentration in the fetal brain, however, did not increase in proportion to either the surface area of maternal fillings or amounts of mercury in
24 h air samples on day 15 of pregnancy, and neither were there any significant correlations. Only in the case of dams given four amalgam fillings was a significant amount of mercury uptake found in the fetal brain. We were not able to study what would happen from a greater number of fillings because the dental procedure used prevented us from placing fillings in all six molars. But it would appear from our data that more than four amalgam fillings would result in a larger deposit of mercury even in the fetal brain.
The blood-brain barrier is often likened to the placenta because of functional similarities (Villee, 1965). The blood-brain barrier prevents ionic mercury from entering brain tissues (Magos, 1968). In Magos et al., 1977, reported that catalase has a great capacity to oxidize mercury vapor into ionic mercury and that this enzyme is found abundantly in the liver. Blood returning to the placenta through the umbilical veins enters the fetal circulation through the ductus venous, mainly by passing through the liver (Guyton, 1976). In this study, Spearman's rank correlation test revealed a significant positive correlation between the mercury content in the placenta and in the fetal liver but not in the fetal brain
(Spearman's rank correlations, fetal brain: rs 0.529, P >0.05; fetal liver: rs=0.635, P <0.05). It is very likely, therefore, that oxidized mercury accumulated in the placenta and that elemental mercury crossing the placenta was oxidized into ionic mercury in the fetal liver and accumulated in that organ. This oxidization of the mercury vapor may prevent it from affecting the vulnerable fetal brain, because the blood—brain barrier prevents ionic mercury from entering the brain tissues.
Generally, fetal exposure to elemental mercury is expressed in the ratio of fetal (F) to maternal (M) mercury concentration in organs such as the brain and liver. In this study, uptake of mercury in the fetal brain and liver was lower than that of the mothers. For the brain, the ratio was 0.370.66, and for the liver, 0.30—0.47, results that were consistent with our previous study (Takahashi et a1., 2001) and with other animal studies of mercury vapor exposure (Danielsson et al., 1990; Yoshida et a1., 1986). The ratio decreased with increasing amounts of mercury in maternal brain and liver. When the placental transfer of mercury was demonstrated by the mercury concentration ratio of placenta to fetal liver, which showed the highest accumulation of mercury in the fetal organs, the ratios decreased significantly with increasing surface areas of the amalgam fillings (Spearman's rank correlation, rs -0.676, P 0.05). Even though the amount of mercury vapor generated from the amalgam surface increased, maternal-to-fetal transfer of mercury was clearly low. These findings indicate that mercury, which was oxidized to ionic mercury after exposure to mercury vapor from dental amalgam, increased and accumulated in the placenta. Therefore, it is assumed that the decreased mercury concentration in the fetal-to-maternal ratio is related to the increases of accumulation of mercury in maternal organs after the placement of amalgam fillings. However, except in the brain, mercury concentration in the liver and kidneys of the fetus correlated to the surface area of the amalgam fillings in spite of rapid oxidation of mercury vapor to ionic mercury in maternal organs. The results indicated that the amount of mercury vapor passing the placental transfer depended on the surface area of the amalgam fillings.
In conclusion, the present research has demonstrated that mercury vapor released from amalgam fillings in pregnant rats was distributed to maternal and fetal organs in significant amounts dependent on the number of fillings and their surface areas.
The study was limited to in utero exposure to dental amalgam placed in the oral cavity of dams. Further research needs to be pursued on the question of mercury exposure from breast milk after birth. Vimy et al. (1997) reported that the breast milk of mother sheep with 12 radioactive occlusal amalgam fillings contained more mercury than that of mothers with no fillings and that mercury concentrations in the organs of neonatal lambs was higher for those nursed by dams with amalgam fillings.