Maylla Ronacher Simões

Toxic Effects of Mercury on the Cardiovascular and Central Nervous Systems

Environmental contamination has exposed humans to various metal agents, including mercury. This exposure is more common than expected, and the health consequences of such exposure remain unclear. For many years, mercury was used in a wide variety of human activities, and now, exposure to this metal from both natural and artificial sources is significantly increasing. Many studies show that high exposure to mercury induces changes in the central nervous system, potentially resulting in irritability, fatigue, behavioral changes, tremors, headaches, hearing and cognitive loss, dysarthria, incoordination, hallucinations, and death. In the cardiovascular system, mercury induces hypertension in humans and animals that has wide-ranging consequences, including alterations in endothelial function. The results described in this paper indicate that mercury exposure, even at low doses, affects endothelial and cardiovascular function. As a result, the reference values defining the limits for the absence of danger should be reduced.

Toxic effects of mercury, lead and gadolinium on vascular reactivity

Heavy metals have been used in a wide variety of human activities that have significantly increased both professional and environmental exposure. Unfortunately, disasters have highlighted the toxic effects of metals on different organs and systems. Over the last 50 years, the adverse effects of chronic lead, mercury and gadolinium exposure have been underscored. Mercury and lead induce hypertension in humans and animals, affecting endothelial function in addition to their other effects. Increased cardiovascular risk after exposure to metals has been reported, but the underlying mechanisms, mainly for short periods of time and at low concentrations, have not been well explored. The presence of other metals such as gadolinium has raised concerns about contrast-induced nephropathy and, interestingly, despite this negative action, gadolinium has not been defined as a toxic agent. The main actions of these metals, demonstrated in animal and human studies, are an increase of free radical production and oxidative stress and stimulation of angiotensin I-converting enzyme activity, among others. Increased vascular reactivity, highlighted in the present review, resulting from these actions might be an important mechanism underlying increased cardiovascular risk. Finally, the results described in this review suggest that mercury, lead and gadolinium, even at low doses or concentrations, affect vascular reactivity. Acting via the endothelium, by continuous exposure followed by their absorption, they can increase the production of free radicals and of angiotensin II, representing a hazard for cardiovascular function. In addition, the actual reference values, considered to pose no risk, need to be reduced.

Acute Lead Exposure Increases Arterial Pressure: Role of the Renin-Angiotensin System

Background Chronic lead exposure causes hypertension and cardiovascular disease. Our purpose was to evaluate the effects of acute exposure to lead on arterial pressure and elucidate the early mechanisms involved in the development of lead-induced hypertension. Methodology/Principal Findings Wistar rats were treated with lead acetate (i.v. bolus dose of 320 µg/Kg), and systolic arterial pressure, diastolic arterial pressure and heart rate were measured during 120 min. An increase in arterial pressure was found, and potential roles of the renin-angiotensin system, Na+,K+-ATPase and the autonomic reflexes in this change in the increase of arterial pressure found were evaluated. In anesthetized rats, lead exposure: 1) produced blood lead levels of 37±1.7 µg/dL, which is below the reference blood concentration (60 µg/dL); 2) increased systolic arterial pressure (Ct: 109±3 mmHg vs Pb: 120±4 mmHg); 3) increased ACE activity (27% compared to Ct) and Na+,K+-ATPase activity (125% compared to Ct); and 4) did not change the protein expression of the α1-subunit of Na+,K+-ATPase, AT1 and AT2. Pre-treatment with an AT1 receptor blocker (losartan, 10 mg/Kg) or an ACE inhibitor (enalapril, 5 mg/Kg) blocked the lead-induced increase of arterial pressure. However, a ganglionic blockade (hexamethonium, 20 mg/Kg) did not prevent leads hypertensive effect. Conclusion Acute exposure to lead below the reference blood concentration increases systolic arterial pressure by increasing angiotensin II levels due to ACE activation. These findings offer further evidence that acute exposure to lead can trigger early mechanisms of hypertension development and might be an environmental risk factor for cardiovascular disease.

Myocardial Contractile Dysfunction Induced by Ovariectomy Requires AT1Receptor Activation in Female Rats

Background/aim:Estrogen deficiency induces myocardial contractile dysfunction and increases cardiovascular disease risk. However, the mechanism underlying this response is unclear. Our aim was to investigate whether AT1 receptor blockade would prevent ovariectomy-induced myocardial contractile dysfunction. Methods: Female rats (8 weeks old, 280 g) that underwent bilateral ovariectomy were randomly assigned to receive daily treatment with losartan (OVX + LOS, 15 mg/kg, s.c., in 0.9 % NaCl), placebo (OVX), estrogen replacement (OVX + E2, 1 mg/ kg, once a week, i.m.) and SHAM for 58 days. Results: Losartan and estrogen treatment 1) prevented ovariectomy-induced weight gain and slight hypertrophy, 2) restored the positive inotropic responses to Ca2+ and isoproterenol in the isolated papillary muscle in the OVX group, 3) prevented the reduction in SERCA2a levels and the increase in phospholamban (PLB) expression in the OVX group, 4) abolished the increase in superoxide anion that was increased in the OVX group, and 5) normalized the increase in p22phox expression after ovariectomy. Estrogen treatment but not losartan restored the increase in serum angiotensin converting enzyme activity in the OVX group. Conclusion: This study demonstrated that myocardial contractile dysfunction induced by ovariectomy and expression of key Ca2+-handling proteins were prevented by losartan treatment and that AT1 receptor activation is involved in this response.

Chronic Lead Exposure Increases Blood Pressure and Myocardial Contractility in Rats

We investigated the cardiovascular effects of lead exposure, emphasising its direct action on myocardial contractility. Male Wistar rats were sorted randomly into two groups: control (Ct) and treatment with 100 ppm of lead (Pb) in the drinking water. Blood pressure (BP) was measured weekly. At the end of the treatment period, the animals were anaesthetised and haemodynamic parameters and contractility of the left ventricular papillary muscles were recorded. Blood and tissue samples were properly stored for further biochemical investigations. Statistical analyses were considered to be significant at p<0.05. The lead concentrations in the blood reached approximately 13 µg/dL, while the bone was the site of the highest deposition of this metal. BP in the Pb-treated group was higher from the first week of lead exposure and remained at the same level over the next four weeks. Haemodynamic evaluations revealed increases in systolic (Ct: 96±3.79 vs. Pb: 116±1.37 mmHg) and diastolic blood pressure (Ct: 60±2.93 vs. Pb: 70±3.38 mmHg), left ventricular systolic pressure (Ct: 104±5.85 vs. Pb: 120±2.51 mmHg) and heart rate (Ct: 307±10 vs. Pb: 348±16 bpm). Lead treatment did not alter the force and time derivatives of the force of left ventricular papillary muscles that were contracting isometrically. However, our results are suggestive of changes in the kinetics of calcium (Ca++) in cardiomyocytes increased transarcolemmal Ca++ influx, low Ca++ uptake by the sarcoplasmic reticulum and high extrusion by the sarcolemma. Altogether, these results show that despite the increased Ca++ influx that was induced by lead exposure, the myocytes had regulatory mechanisms that prevented increases in force, as evidenced in vivo by the increased systolic ventricular pressure.

MAPK pathway activation by chronic lead-exposure increases vascular reactivity through oxidative stress/cyclooxygenase-2-dependent pathways

Chronic exposure to low lead concentration produces hypertension; however, the underlying mechanisms remain unclear. We analyzed the role of oxidative stress, cyclooxygenase-2-dependent pathways and MAPK in the vascular alterations induced by chronic lead exposure. Aortas from lead-treated Wistar rats (1st dose: 10 μg/100g; subsequent doses: 0.125μg/100g, intramuscular, 30days) and cultured aortic vascular smooth muscle cells (VSMCs) from Sprague Dawley rats stimulated with lead (20μg/dL) were used. Lead blood levels of treated rats attained 21.7±2.38μg/dL. Lead exposure increased systolic blood pressure and aortic ring contractile response to phenylephrine, reduced acetylcholine-induced relaxation and did not affect sodium nitroprusside relaxation. Endothelium removal and L-NAME left-shifted the response to phenylephrine more in untreated than in lead-treated rats. Apocynin and indomethacin decreased more the response to phenylephrine in treated than in untreated rats. Aortic protein expression of gp91(phox), Cu/Zn-SOD, Mn-SOD and COX-2 increased after lead exposure. In cultured VSMCs lead 1) increased superoxide anion production, NADPH oxidase activity and gene and/or protein levels of NOX-1, NOX-4, Mn-SOD, EC-SOD and COX-2 and 2) activated ERK1/2 and p38 MAPK. Both antioxidants and COX-2 inhibitors normalized superoxide anion production, NADPH oxidase activity and mRNA levels of NOX-1, NOX-4 and COX-2. Blockade of the ERK1/2 and p38 signaling pathways abolished lead-induced NOX-1, NOX-4 and COX-2 expression. Results show that lead activation of the MAPK signaling pathways activates inflammatory proteins such as NADPH oxidase and COX-2, suggesting a reciprocal interplay and contribution to vascular dysfunction as an underlying mechanisms for lead-induced hypertension.

Acute exposure to lead increases myocardial contractility independent of hypertension development.

We studied the effects of the acute administration of small doses of lead over time on hemodynamic parameters in anesthetized rats to determine if myocardial contractility changes are dependent or not on the development of hypertension. Male Wistar rats received 320 µg/kg lead acetate iv once, and their hemodynamic parameters were measured for 2 h. Cardiac contractility was evaluated in vitro using left ventricular papillary muscles as were Na+,K+-ATPase and myosin Ca2+-ATPase activities. Lead increased left- (control: 112 ± 3.7 vs lead: 129 ± 3.2 mmHg) and right-ventricular systolic pressures (control: 28 ± 1.2 vs lead: 34 ± 1.2 mmHg) significantly without modifying heart rate. Papillary muscles were exposed to 8 µM lead acetate and evaluated 60 min later. Isometric contractions increased (control: 0.546 ± 0.07 vs lead: 0.608 ± 0.06 g/mg) and time to peak tension decreased (control: 268 ± 13 vs lead: 227 ± 5.58 ms), but relaxation time was unchanged. Post-pause potentiation was similar between groups (n = 6 per group), suggesting no change in sarcoplasmic reticulum activity, evaluated indirectly by this protocol. After 1-h exposure to lead acetate, the papillary muscles became hyperactive in response to a β-adrenergic agonist (10 µM isoproterenol). In addition, post-rest contractions decreased, suggesting a reduction in sarcolemmal calcium influx. The heart samples treated with 8 µM lead acetate presented increased Na+,K+-ATPase (approximately 140%, P < 0.05 for control vs lead) and myosin ATPase (approximately 30%, P < 0.05 for control vs lead) activity. Our results indicated that acute exposure to low lead concentrations produces direct positive inotropic and lusitropic effects on myocardial contractility and increases the right and left ventricular systolic pressure, thus potentially contributing to the early development of hypertension.

Activation of K+ channels and Na+/K+ ATPase prevents aortic endothelial dysfunction in 7-day lead-treated rats.

Seven day exposure to a low concentration of lead acetate increases nitric oxide bioavailability suggesting a putative role of K+ channels affecting vascular reactivity. This could be an adaptive mechanism at the initial stages of toxicity from lead exposure due to oxidative stress. We evaluated whether lead alters the participation of K+ channels and Na+/K+)-ATPase (NKA) on vascular function. Wistar rats were treated with lead (1st dose 4 μg/100 g, subsequent doses 0.05 μg/100g, im, 7 days) or vehicle. Lead treatment reduced the contractile response of aortic rings to phenylephrine (PHE) without changing the vasodilator response to acetylcholine (ACh) or sodium nitroprusside (SNP). Furthermore, this treatment increased basal O₂⁻ production, and apocynin (0.3 μM), superoxide dismutase (150 U/mL) and catalase (1000 U/mL) reduced the response to PHE only in the treated group. Lead also increased aortic functional NKA activity evaluated by K+-induced relaxation curves. Ouabain (100 μM) plus L-NAME (100 μM), aminoguanidine (50 μM) or tetraethylammonium (TEA, 2 mM) reduced the K+-induced relaxation only in lead-treated rats. When aortic rings were precontracted with KCl (60 mM/L) or preincubated with TEA (2 mM), 4-aminopyridine (4-AP, 5 mM), iberiotoxin (IbTX, 30 nM), apamin (0.5 μM) or charybdotoxin (0.1 μM), the ACh-induced relaxation was more reduced in the lead-treated rats. Additionally, 4-AP and IbTX reduced the relaxation elicited by SNP more in the lead-treated rats. Results suggest that lead treatment promoted NKA and K+ channels activation and these effects might contribute to the preservation of aortic endothelial function against oxidative stress.

Body weight loss after myocardial infarction in rats as a marker of early heart failure development.

BACKGROUND AND AIMS We evaluated the use of body weight (BW) loss soon after acute myocardial infarction (MI) in rats as a marker of acute heart failure (HF). METHODS Female Wistar rats (200-240 g) were submitted either to sham operation or to coronary artery occlusion. In individual cages, daily BW and food and water intake were measured. Seven days later, cardiac function was evaluated by left ventricular catheterization. HF was defined by a left ventricular end-diastolic pressure greater than the upper limit of the 95% confidence interval. MI group was then divided into those that developed HF (n = 27; MI-HF) and those that did not (n = 47; MI). RESULTS The MI-HF group experienced increased BW loss (sham: 4.2 ± 0.6% MI: 0.4 ± 0.8%, MI-HF: -4.9 ± 1.2%; p <0.05) and reduced water and food intake compared with other groups. HF animals showed greater lung weight (sham: 1.460 ± 0.076 g, MI: 1.748 ± 0.086 g, MI-HF: 2.033 ± 0.13 g; p <0.05). Infarct area was significantly different between the groups (MI: 35.9 ± 0.9%, MI-HF: 39.7 ± 1.3%; p <0.05). ROC curve showed that BW loss over 7 days has 100% sensitivity and 72.3% specificity for identifying acute HF. Moreover, excluding the effect of infarct area on these results, a sample of animals with the same infarct area displayed similar morphometric and hemodynamic patterns as the entire sample. Multivariate linear regression analysis confirmed that BW loss is a HF marker independent of infarct area. CONCLUSIONS BW is an easy and reliable noninvasive method to detect HF early after MI in rats.

High salt intake does not produce additional impairment in the coronary artery relaxation of spontaneously hypertensive aged rats.

The effect of a salt-based diet on the coronary responsiveness in aged hypertensive rats (SHR) still is unclear. We investigated the effects of high salt intake on the relaxation properties of coronary arteries of aged SHRs. Male SHR (32 week-old) received drinking water (SHR) or 1% NaCl solution (SHR-Salt) for 8 weeks. Isolated coronary segments were subjected to concentration-response curves to acetylcholine (ACh) in the presence or absence of L-NAME (100 μM), enalaprilate (10 μM), losartan (10 μM), and spironolactone (100 μM). Salt intake did not increase blood pressure in old SHRs, but caused ventricular hypertrophy. The endothelium-dependent relaxation in SHRs was lower than in Wistar rats. However, salt intake did not add further impairment. Both enalaprilate and losartan reduced the vasodilator response in coronary arteries from Wistar, but did not affect SHR-salt rats. Conversely, losartan attenuated the impaired ACh relaxation observed in SHR. Spironolactone reduced the relaxation induced by ACh in coronary arteries from Wistar rats but not in SHR. The renin-angiotensin-aldosterone system participates in the impaired coronary relaxation in aged SHR, but does not partake in this deleterious effect under increased salt intake, indicating that age could differentiate the effects of high sodium intake in coronary arteries of SHR.