Long-term Mercury Exposure Accelerates the Development of Hypertension in Prehypertensive Spontaneously Hypertensive Rats Inducing Endothelial Dysfunction: the Role of Oxidative Stress and Cyclooxygenase-2

Mercury-induced toxicity: Mechanisms, molecular pathways, and gene regulation

Mercury is a well-known neurotoxicant for humans and wildlife. The epidemic of mercury poisoning in Japan has clearly demonstrated that chronic exposure to methylmercury (MeHg) results in serious neurological damage to the cerebral and cerebellar cortex, leading to the dysfunction of the central nervous system (CNS), especially in infants exposed to MeHg in utero. The occurrences of poisoning have caused a wide public concern regarding the health risk emanating from MeHg exposure; particularly those eating large amounts of fish may experience the low-level and long-term exposure. There is growing evidence that MeHg at environmentally relevant concentrations can affect the health of biota in the ecosystem. Although extensive in vivo and in vitro studies have demonstrated that the disruption of redox homeostasis and microtube assembly is mainly responsible for mercurial toxicity leading to adverse health outcomes, it is still unclear whether we could quantitively determine the occurrence of interaction between mercurial and thiols and/or selenols groups of proteins linked directly to outcomes, especially at very low levels of exposure. Furthermore, intracellular calcium homeostasis, cytoskeleton, mitochondrial function, oxidative stress, neurotransmitter release, and DNA methylation may be the targets of mercury compounds; however, the primary targets associated with the adverse outcomes remain to be elucidated. Considering these knowledge gaps, in this article, we conducted a comprehensive review of mercurial toxicity, focusing mainly on the mechanism, and genes/proteins expression. We speculated that comprehensive analyses of transcriptomics, proteomics, and metabolomics could enhance interpretation of "omics" profiles, which may reveal specific biomarkers obviously correlated with specific pathways that mediate selective neurotoxicity.

Does acute exposure to thimerosal, an organic mercury compound, affect the mitochondrial function of an infant model?

BACKGROUND

Thimerosal (TM) is a toxic, organometallic mercury compound (which releases ethyl-mercury-containing compounds in aqueous solutions) used as a preservative in vaccines. Mitochondria are organelle which are highly vulnerable to many chemical compounds, including mercury (Hg) and its derivatives.

METHOD

Wistar rats (at 21 days of age) were used to model a child's TM exposure following childhood vaccination, divided in two groups: TM exposed (20 μg/kg/day) and unexposed controls (saline solution), both for 24 h. Atomic Fluorescence Spectrometry was used to quantify the amounts of mercury in tissues. The electron transport chain (ETC) from isolated mitochondria was evaluated using an oxygen electrode. The mitochondrial membrane potential and H2O2 production were analyzed using selective fluorescence probes. The activity of some enzymes (SOD, CAT, GPx, and AChE) and secondary markers of oxidative stress (GSH, GSSG, total free thiol) were also examined in tissues.

RESULTS

Hg accumulation in the brain and liver was higher in exposed animals when compared to the control. Liver-isolated mitochondria showed that TM improved respiratory control by 23%; however, states 3 and 4 of the ETC presented a decrease of 16% and 37%, respectively. Furthermore, brain-isolated mitochondria presented an improvement of 61% in respiratory control. Brain enzyme activities were significantly impacted in TM-exposed rats compared to unexposed rats as follows: decreases in SOD (32%) and AChE (42%) and increases in GPx (79%) and CAT (100%). GPx enzyme activity in the liver was significantly increased (37%). Among secondary oxidative stress markers, the brain's total reduced thiol (SH) concentration was significantly increased (41%).

CONCLUSION

Acute TM treatment exposure in a Wistar rat model mimicking TM exposure in an infant following childhood vaccination significantly damaged brain bioenergetic pathways. This study supports the ability of TM exposure to preferentially damage the nervous system.

Chronic exposure to mercury increases arrhythmia and mortality post-acute myocardial infarction in rats

Introduction: Mercury (Hg) is a heavy metal that causes a variety of toxic effects in eukaryotic cells. Previous studies have reported detrimental effects of mercury toxicity in the cardiovascular system. Given the importance of understanding the relationship between Hg and cardiovascular disease, we sought to investigate if the Hg could worsen the myocardial repercussions following ischemic injury. We demonstrated that once mercury toxicity is established, it can influence the outcome of myocardial infarction (MI). Methods: Male Wistar rats received intramuscular injections of either saline (NaCl 0.9%) or mercuric chloride (HgCl2, first dose of 4.6 μg/kg, and subsequent doses of 0.07 μg/kg/day) for 4 weeks. Three weeks post-exposure, we induced transmural infarction in the left ventricle free wall through coronary artery occlusion surgery. Results: ECG recordings obtained from MI groups demonstrated alterations in the rhythm of the heartbeat/heart electrical activity, as expected, including ventricular extrasystoles and ventricular tachycardia. However, the MI group exposed to Hg (MI-Hg) exhibited augmented ventricular extrasystoles and ventricular tachycardia compared to the MI group. Also, Basckó coefficient revealed that the arrhythmic events-after MI-were aggravated by Hg exposure. Discussion: Our results indicate that the significantly increased mortality in MI-Hg groups when compared to MI (21%, MI vs 32%, MI-Hg) is correlated with greater occurrence of arrhythmias. In conclusion, this study further supports the idea that exposure to mercury (Hg) should be recognized as a significant risk factor that exacerbates the impact of cardiac ischemic injury, potentially leading to an increased mortality rate among patients experiencing acute MI.

Cardiovascular Harmful Effects of Recommended Daily Doses (13 µg/kg/day), Tolerable Upper Intake Doses (0.14 mg/kg/day) and Twice the Tolerable Doses (0.28 mg/kg/day) of Copper

Copper is essential for homeostasis and regulation of body functions, but in excess, it is a cardiovascular risk factor since it increases oxidative stress. The objective of this study was to evaluate the effects of exposure to the recommended daily dose (13 µg/kg/day), upper tolerable dose (0.14 mg/kg/day) and twice the upper tolerable dose (0.28 mg/kg/day) via i.p. over 4 weeks on the vascular reactivity of aortic rings and the contraction of LV papillary muscles of male Wistar rats. It was also determined whether the antioxidant peptide from egg white hydrolysate (EWH) prevents these effects. Copper exposure at the doses evaluated did not change weight gain of male Wistar rats, the reactivity of the aortic rings or the cardiac mass. The dose of 0.13 µg/kg/day did not reduce the force of contraction, but it impaired the time derivatives of force. Doses of 0.14 and 0.28 mg/kg/day reduced the force of contraction, the inotropic response to calcium and isoproterenol, the postrest contraction and the peak and plateau of tetanized contractions. EWH treatment antagonized these effects. These results suggest that copper, even at the dose described as upper tolerable, can impair cardiac contraction without altering vascular reactivity. Antioxidative stress therapy with EWH reversed these harmful effects, suggesting a possible strategy for the amelioration of these effects.

Molecular Mechanisms of Cellular Injury and Role of Toxic Heavy Metals in Chronic Kidney Disease

Chronic kidney disease (CKD) is a progressive disease that affects millions of adults every year. Major risk factors include diabetes, hypertension, and obesity, which affect millions of adults worldwide. CKD is characterized by cellular injury followed by permanent loss of functional nephrons. As injured cells die and nephrons become sclerotic, remaining healthy nephrons attempt to compensate by undergoing various structural, molecular, and functional changes. While these changes are designed to maintain appropriate renal function, they may lead to additional cellular injury and progression of disease. As CKD progresses and filtration decreases, the ability to eliminate metabolic wastes and environmental toxicants declines. The inability to eliminate environmental toxicants such as arsenic, cadmium, and mercury may contribute to cellular injury and enhance the progression of CKD. The present review describes major molecular alterations that contribute to the pathogenesis of CKD and the effects of arsenic, cadmium, and mercury on the progression of CKD.

Supplementation with the Symbiotic Formulation Prodefen® Increases Neuronal Nitric Oxide Synthase and Decreases Oxidative Stress in Superior Mesenteric Artery from Spontaneously Hypertensive Rats

In recent years, gut dysbiosis has been related to some peripheral vascular alterations linked to hypertension. In this work, we explore whether gut dysbiosis is related to vascular innervation dysfunction and altered nitric oxide (NO) production in the superior mesenteric artery, one of the main vascular beds involved in peripheral vascular resistance. For this purpose, we used spontaneously hypertensive rats, either treated or not with the commercial synbiotic formulation Prodefen^® (10⁸ colony forming units/day, 4 weeks). Prodefen^® diminished systolic blood pressure and serum endotoxin, as well as the vasoconstriction elicited by electrical field stimulation (EFS), and enhanced acetic and butyric acid in fecal samples, and the vasodilation induced by the exogenous NO donor DEA-NO. Unspecific nitric oxide synthase (NOS) inhibitor L-NAME increased EFS-induced vasoconstriction more markedly in rats supplemented with Prodefen^®. Both neuronal NO release and neuronal NOS activity were enhanced by Prodefen^®, through a hyperactivation of protein kinase (PK)A, PKC and phosphatidylinositol 3 kinase-AKT signaling pathways. The superoxide anion scavenger tempol increased both NO release and DEA-NO vasodilation only in control animals. Prodefen^® caused an increase in both nuclear erythroid related factor 2 and superoxide dismutase activities, consequently reducing both superoxide anion and peroxynitrite releases. In summary, Prodefen^® could be an interesting non-pharmacological approach to ameliorate hypertension.

Chronic mercury exposure induces oxidative stress in female rats by endothelial nitric oxide synthase uncoupling and cyclooxygenase-2 activation, without affecting oestrogen receptor function

Mercury has been shown to be a significant health risk factor and is positively associated with cardiovascular diseases. Evidence reveals that men are more likely to develop cardiovascular diseases than women during reproductive age. However, the effects of mercury in females remain poorly investigated, despite the finding that female hormones demonstrate a cardioprotective role. In the present study, we evaluated whether chronic mercury chloride exposure could alter blood pressure and vascular function of the female rat aorta. Ten-week-old female Wistar rats were divided into two groups: control (vehicle) and mercury treated (first dose of 4.6 μg/kg, subsequent daily doses of 0.07 μg/kg), im. Mercury treatment did not modify systolic blood pressure (SBP) but increased vascular reactivity due to the reduction of nitric oxide bioavailability associated with the increase in reactive oxygen species from endothelial nitric oxide synthase (eNOS) uncoupling. Furthermore, increased participation of the cyclooxygenase-2 pathway occurred through an imbalance in thromboxane 2 and prostacyclin 2. However, the oestrogen signalling pathway was not altered in either group. These results demonstrated that chronic exposure to mercury in females induced endothelial dysfunction and, consequently, increased aortic vascular reactivity, causing vascular damage to the female rat aorta and representing a risk of cardiovascular diseases.

Endothelial Dysfunction Induced by Cadmium and Mercury and its Relationship to Hypertension

Hypertension is an important public health concern that affects millions globally, leading to a large number of morbidities and fatalities. The etiology of hypertension is complex and multifactorial, and it involves environmental factors, including heavy metals. Cadmium and mercury are toxic elements commonly found in the environment, contributing to hypertension. We aimed to assess the role of cadmium and mercury-induced endothelial dysfunction in the development of hypertension. A narrative review was carried out through database searches. In this review, we discussed the critical roles of cadmium and mercury in the etiology of hypertension and provided new insights into potential mechanisms of their effect, focusing primarily on endothelial dysfunction. Although the mechanisms by which cadmium and mercury induce hypertension have yet to be completely elucidated, evidence for both implicates impaired nitric oxide signaling in their hypertensive etiology.