Low level and sub-chronic exposure to methylmercury induces hypertension in rats: Nitric oxide depletion and oxidative damage as possible mechanisms
Departamento de Análises Clínicas, Toxicológicas e Bromatológicas, Faculdade de Ciências Farmacêuticas de Ribeirão Preto, Universidade de São Paulo, Ribeirão Preto, SP 14040-903, Brazil.Archives of Toxicology (Impact Factor: 5.98). 06/2009; 83(7):653-62. DOI: 10.1007/s00204-009-0437-8
Increased risk of hypertension after methylmercury (MeHg) exposure has been suggested. However, the underlying mechanisms are not well explored. In this paper, we have analyzed whether sub-chronic exposure to MeHg increases systolic blood pressure even at very low levels. In addition, we analyzed if the methylmercury-induced hypertension is associated with a decreased plasmatic nitric oxide levels and with a dysregulation of the activities of the antioxidant enzymes superoxide dismutase (SOD) and catalase (CAT), as well as the levels of MDA and glutathione. For this study, Wistar rats were treated with methylmercury chloride (100 microg/kg per day) or vehicle. Total treatment time was 100 days. Malondialdehyde (MDA) and circulating NOx levels and superoxide dismutase (SOD) and catalase (CAT) activities were determined in plasma, whereas glutathione levels were determined in erythrocytes. Our results show that long-term treatment at a low level of MeHg affected systolic blood pressure, increasing and reducing the levels of plasmatic MDA and NOx, respectively. However, the activity of SOD did not decrease in the MeHg exposed group when compared to the control. We found a negative correlation between plasmatic nitrite/nitrate (NOx) levels and systolic blood pressure (r = -0.67; P = 0.001), and a positive correlation between MDA and systolic blood pressure (r = 0.61; P = 0.03), thus suggesting increased inhibition of NO formation with the increase of hypertension. In conclusion, long-term exposure to a low dose of MeHg increases the systolic pressure and is associated, at least in part, with increased production of ROS as judged by increased production of malondialdehyde and depressed NO availability.
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[Show abstract] [Hide abstract] ABSTRACT: Aims: Methylmercury (MeHg) exposure results in increased risk of hypertension and cardiovascular diseases. In this study, we aimed to investigate whether the in vivo exposure of MeHg in mice affects blood pressure and basilar arterial responses to angiotensin II (Ang II) and acetylcholine (ACh), which are important modulators of cerebrovascular autoregulation. Main methods: Mice were exposed to MeHg (40ppm) in drinking water for 21days. Blood pressure was measured in conscious mice by an indirect tail-cuff method. Functional studies of the isolated arteries' response to vasoactive substances were performed using a micro-organ-bath system. Key findings: Systolic and mean blood pressures were significantly increased after 2 and 3weeks of treatment with MeHg, respectively. Ang II-induced contraction in an isolated basilar artery, which is mediated via Rho-kinase activation, was increased in MeHg-treated mice. ACh-induced relaxation, which is dependent on NO production from the endothelial cells, was decreased in MeHg-treated mice. However, alterations of vascular responses to Ang II and ACh were not observed in the isolated thoracic aorta. Significance: This study demonstrated that the cerebral vasculature appears to be particularly sensitive to in vivo exposure of MeHg. Our results suggest that in vivo MeHg increases blood pressure and causes alterations in the cerebrovascular reactivity in response to Ang II and ACh through enhancement of Rho-kinase activity and inhibition of NO bioavailability, respectively.
- "A previous study  also reported an increased incidence of hypertension and cerebrovascular disease among aging patients with Minamata disease in which the patients were exposed to high concentration of MeHg. Our results showed experimental evidence that prolonged exposure to MeHg increases SBP and MBP in mice, which is consistent with the findings of a previous study , who showed that subchronic exposure to MeHg induces increase in SBP in rats. We studied the effects of a ROS scavenger, tempol, on MeHg-induced elevated blood pressure, because excessive oxidative stress including generation of ROS is known to mediate MeHginduced toxicity not only in neuron [28,29] but also in endothelial cells  , and the ROS is an important factor in modulating blood pressure and cerebrovascular reactivity in mice . "
[Show abstract] [Hide abstract] ABSTRACT: Cardiovascular diseases, such as heart attack and stroke, are the major cause of death worldwide. It is well known that a high number of environmental and physiological risk factors contribute to the development of cardiovascular diseases. Although risk factors are additive, increased blood pressure (hypertension) is the greatest risk factor. Over the last two decades, a growing number of epidemiological studies associate environmental exposure to lead or mercury species with hypertension. However, cardiovascular effects beyond blood pressure are rarely studied and thresholds for effect not yet clear. To explore effects of lead or mercury species on the cardiovascular system, normal male Wistar rats were exposed to a range of doses of lead, inorganic mercury or methylmercury through the drinking water for four weeks. High-resolution ultrasound was used to measure heart and vascular function (carotid artery blood flow) at baseline and at the end of the exposure, while blood pressure was measured directly in the femoral artery at the end of the 4-week exposure. After 4 weeks, blood pressure responses to lead were biphasic. Low lead levels decreased blood pressure, dilated the carotid artery and increased cardiac output. At higher lead doses, rats had increased blood pressure. In contrast, methylmercury-exposed rats had increased blood pressure at all doses despite dilated carotid arteries. Inorganic mercury did not show any significant cardiovascular effects. Based on the current study, the Benchmark Dose Level 10% (BMDL10s) for systolic blood pressure for lead, inorganic mercury and methylmercury are 1.1, 1.3 and 1.0 μg/kg-bw/d, respectively. However, similar total mercury blood levels attributed to inorganic mercury or methylmercury produced strikingly different results with inorganic mercury having no observable effect on the cardiovascular system but methylmercury increasing systolic and pulse pressures. Therefore, adverse cardiovascular effects cannot be predicted by total blood mercury level alone and the mercury species of exposure must be taken into account.
- "In contrast, methylmercury exposure in rats (0.5 mg MeHg/kg-bw oral gavage for 23–28 days or a 10-fold higher dose) led to latent increases in systolic blood pressure either at 42 days or immediately after exposure, respectively (Wakita, 1987). In another study, rats gavaged with 100 mg/kg-bw/d methylmercury for 100 days also had significantly increased systolic blood pressure after four weeks (Grotto et al., 2009). Therefore, animal studies are highly suggestive of a causative association between methylmercury exposure and adverse cardiovascular effects, while the relationship between inorganic mercury and cardiovascular effects are unclear. "
[Show abstract] [Hide abstract] ABSTRACT: The objective of this study was to quantify the relationship between number of dental amalgam surfaces and urinary mercury levels. This study uses participant data from a large philanthropic chronic disease prevention program in Calgary, Alberta, Canada. Urine samples were analysed for mercury levels (measured in mug/g-creatinine). T-tests were used to determine if differences in urine mercury were statistically significant between persons with no dental amalgam surfaces and one or more dental amalgam surfaces. Linear regression was used to estimate the change in urinary mercury per amalgam surface. Urinary mercury levels were statistically significantly higher in participants with amalgam surfaces, with an average difference of 0.55 mug/g-creatinine. Per amalgam surface, we estimated an expected increase of 0.04 mug/g-creatinine. Measured urinary mercury levels were also statistically significantly higher in participants with dental amalgam surfaces following the oral administration of 2,3-dimercaptopropane-l-sulfonate (DMPS) and meso-2,3-dimercaptosuccinic acid (DMSA) which are used to mobilize mercury from the blood and tissues. Our estimates indicate that an individual with seven or more dental amalgam surfaces has 30% to 50% higher urinary mercury levels than an individual without amalgams. This is consistent with past literature that has identified seven amalgam surfaces as an unsafe level of exposure to mercury vapor. Our analysis suggests that continued use of silver amalgam dental fillings for restorative dentistry is a non-negligible, unnecessary source of mercury exposure considering the availability of composite resin alternatives.
- "Mercury vapor doses from dental amalgam fillings by industrial standards are considered to be small at any given point in time, but given that exposure to amalgams is essentially lifelong it can be argued that the current thresholds for mercury exposure should be lowered . Recent research on rats has speculated on particular mechanisms through which low, constant exposure to mercury negatively affected body systems [23,24]. A review on the health of humans and low levels of mercury concluded that the evidence base was too small to quantify the risk of chronic exposure to mercury . "