Effect of mercuric chloride on the kinetics of cationic and substrate activation of the rat brain microsomal ATPase system

Chronic HgCl2 treatment increases vasoconstriction induced by electrical field stimulation: role of adrenergic and nitrergic innervation

In the present study, we have investigated the possible changes in rat mesenteric artery vascular innervation function caused by chronic exposure to low doses of HgCl2 (mercuric chloride), as well as the mechanisms involved. Rats were divided into two groups: (i) control, and (ii) HgCl2-treated rats (30 days; first dose, 4.6 μg/kg of body weight; subsequent dose, 0.07 μg·kg−1 of body weight·day−1, intramuscularly). Vasomotor response to EFS (electrical field stimulation), NA (noradrenaline) and the NO donor DEA-NO (diethylamine NONOate) were studied, nNOS (neuronal NO synthase) and phospho-nNOS protein expression were analysed, and NO, O2− (superoxide anion) and NA release were also determined. EFS-induced contraction was higher in the HgCl2-treated group. Phentolamine (1 μmol/l) decreased the response to EFS to a greater extent in HgCl2-treated rats. HgCl2 treatment increased vasoconstrictor response to exogenous NA and NA release. L-NAME (NG-nitro-L-arginine methyl ester; 0.1 mmol/l) increased the response to EFS in both experimental groups, but the increase was greater in segments from control animals. HgCl2 treatment decreased NO release and increased O2− production. Vasodilator response to DEA-NO was lower in HgCl2-treated animals. Tempol increased DEA-NO-induced relaxation to a greater extent in HgCl2-treated animals. nNOS expression was similar in arteries from both experimental groups, whereas phospho-nNOS was decreased in segments from HgCl2-treated animals. HgCl2 treatment increased vasoconstrictor response to EFS as a result of, in part, reduced NO bioavailability and increased adrenergic function. These findings offer further evidence that mercury, even at low concentrations, is an environmental risk factor for cardiovascular disease.

Interaction of mercury(II) with the microtubule cytoskeleton in IMR-32 neuroblastoma cells

On the background of the neurotoxicity of mercury compounds, the interaction of mercury(II) with the cytoskeleton was investigated in vitro using IMR-32 neuroblastoma cells. Conditions for culture of these cells on microscopic slides and procedures for immunofluorescence staining of the microtubule network were optimised. Both morphology and viability of IMR-32 cells were affected by mercury(II) at concentrations higher than 15 microM. Pronounced disintegration of the microtubule cytoskeleton was detected at 30 microM mercury(II). Compared to previous studies with fibroblasts, the no-observed-effect concentration was markedly lower, pointing to a particular sensitivity of nerve cells to mercury. This could be due to disturbed information transfer processes depending on an intact microtubule system.

In vitro effect of lead on Na+, K+-ATPase activity in different regions of adult rat brain

Lead interferes with cellular energy metabolism by inhibiting ATP (Adenosine triphosphate) synthesis and hydrolysis. This study was conducted to determine in vitro effects of lead on Na+, K+-ATPase activity in four regions of adult rat brain: the cerebellum, the hippocampus, the frontal cortex and the brain stem. Male rats (Wistar strain) weighing 125-150 g were sacrificed, whole brain excised and the four regions were isolated. Each tissue was homogenized separately in sucrose (0.25 M) and imidazole (10 mM) buffer (pH 7.5) and P2 fraction was prepared by following established methods. The activity of ATPase was determined by measuring inorganic phosphate (Pi) liberated from ATP hydrolysis. The delineation of Na+, K+-activated component of ATPase was obtained by the difference between total ATPase and Mg2+-ATPase using 1 mM ouabain. The P2 fraction was incubated with 0, 5, 10, 25, 50 and 100 microM of lead at 37 degrees C for 10 min. The enzyme activity was expressed as micromoles of Pi liberated/mg protein/hr. The results indicated a concentration-dependent and region-specific response to lead. In vitro lead at 50 and 100 microM significantly inhibited ATPase activity in all regions of the brain. It was also observed that in the control rats, the enzyme activity was high in cerebellum and hippocampus regions of the brain. In vitro dithiothreitol (DTT) protected the enzyme activity from IC50 lead in four regions of brain. In cerebellum and hippocampus, a 5 microM DTT provided 100% protection against IC50 lead. These results suggest that lead interferes with the ion transport mechanism and cellular energy metabolism of the brain and this effect is region specific.

Effects of mercury on the arterial blood pressure of anesthetized rats

The available data suggests that hypotension caused by Hg2+ administration may be produced by a reduction of cardiac contractility or by cholinergic mechanisms. The hemodynamic effects of an intravenous injection of HgCl2 (5 mg/kg) were studied in anesthetized rats (N = 12) by monitoring left and right ventricular (LV and RV) systolic and diastolic pressures for 120 min. After HgCl2 administration the LV systolic pressure decreased only after 40 min (99 +/- 3.3 to 85 +/- 8.8 mmHg at 80 min). However, RV systolic pressure increased, initially slowly but faster after 30 min (25 +/- 1.8 to 42 +/- 1.6 mmHg at 80 min). Both right and left diastolic pressures increased after HgCl2 treatment, suggesting the development of diastolic ventricular dysfunction. Since HgCl2 could be increasing pulmonary vascular resistance, isolated lungs (N = 10) were perfused for 80 min with Krebs solution (continuous flow of 10 ml/min) containing or not 5 microM HgCl2. A continuous increase in pulmonary vascular resistance was observed, suggesting the direct effect of Hg2+ on the pulmonary vessels (12 +/- 0.4 to 29 +/- 3.2 mmHg at 30 min). To examine the interactions of Hg2+ and changes in cholinergic activity we analyzed the effects of acetylcholine (Ach) on mean arterial blood pressure (ABP) in anesthetized rats (N = 9) before and after Hg2+ treatment (5 mg/kg). Using the same amount and route used to study the hemodynamic effects we also examined the effects of Hg2+ administration on heart and plasma cholinesterase activity (N = 10). The in vivo hypotensive response to Ach (0.035 to 10.5 microg) was reduced after Hg2+ treatment. Cholinesterase activity (microM h-1 mg protein-1) increased in heart and plasma (32 and 65%, respectively) after Hg2+ treatment. In conclusion, the reduction in ABP produced by Hg2+ is not dependent on a putative increase in cholinergic activity. HgCl2 mainly affects cardiac function. The increased pulmonary vascular resistance and cardiac failure due to diastolic dysfunction of both ventricles are factors that might contribute to the reduction of cardiac output and the fall in arterial pressure.

Selective inhibition of the mouse brain Mn-SOD by methylmercury

Changes in mRNA levels, protein contents and enzyme activities for brain Cu,Zn- and Mn-SOD by methylmercury chloride (MMC) administration, were examined, over a period of 12 days in ICR male mice. After subcutaneous administration of MMC (10 mg/kg) to mice, brain mercury content reached a maximum at 2 days and remained at that level for at least 5 days. MMC exposure resulted in a time-dependent decrease in the Mn-SOD activity: the enzyme activity at 5 days after exposure to MMC was about 60% of control level whereas this exposure was without effect on the Cu,Zn-SOD activity, indicating differential sensitivity of SOD isozymes to the metal. However, levels of mRNA and protein synthesis for Mn-SOD were unaffected by MMC administration. The direct effect of MMC on the both SOD activities were further examined with purified enzyme preparations. After each SOD isozyme (10 U) was incubated with 0.2 mM MMC for 24 h at pH 7.8, the enzyme activities for Cu,Zn- and Mn-SOD were 90% and 37% of control, respectively. Incubations at a ratio of SOD to MMC (1 : 600) for 24 h resulted in a substantial decrease in the enzyme activity of the Mn form; this isozyme-selective inactivation was noted at alkaline pH. A combination of isoelectric focusing-agarose gel electrophoresis (IEF-AGE) and synchrotron radiation X-ray fluorescence (SR-XRF) analysis revealed that Mn-SOD rather than Cu,Zn-SOD underwent modification. Furthermore, a decrease in native form of Mn-SOD protein after MMC exposure was confirmed by gel filtration chromatography. These results indicate that Mn-SOD, but not Cu,Zn-SOD, is susceptible to modification by MMC and the resulting alteration in structure appears to cause a loss of enzyme activities.

Comparison of the inhibitory effects of mercury and cadmium on the creatine kinase from Electrophorus electricus (L)

We have determined the effects of mercury and cadmium on the creatine kinase activity of the electric organ of Electrophorus electricus (L.) which catalyses the transphosphorylation reaction between phosphocreatine and magnesium adenosine-5'-di-phosphate and has essential sulfhydryl groups. The kinetic effects of these heavy metals, which have high affinity for sulfhydryl groups, on the creatine kinase activity were analysed with the three reaction components: phosphocreatine, adenosine-5'-di-phosphate and magnesium. The kinetic data were analysed with a non-linear regression program (Sigmaplot for Windows). Both metals inhibit creatine kinase activity in the micromolar range, mercury being a more potent inhibitor than cadmium. With phosphocreatine as substrate, mercury behaved as a mixed partial hyperbolic inhibitor, non-competitive inhibitor with adenosine-5'-di-phosphate, and with magnesium mercury behaved as a competitive inhibitor. Cadmium inhibition was shown to be of a classical competitive nature with respect to both substrates, phosphocreatine or adenosine-5'-di-phosphate, and non-competitive when magnesium was the variable in the reaction mixture. The results suggest that the binding site of mercury is at or near the phosphocreatine site, but it is not the same as adenosine-5'-di-phosphate, whereas cadmium competes with these substrates to bind at the same sulphydryl site.

Effect of methyl isocyanate on rabbit cardiac Na+, K(+)-ATPase

The present study describes the effect of methyl isocyanate (MIC) on rabbit cardiac microsomal Na+, K(+)-ATPase. Addition of MIC in vitro resulted in dose-dependent inhibition of Na+, K(+)-ATPase, Mg(2+)-ATPase and K(+)-activated p-nitrophenyl phosphatase (K(+)-PNPPase). Activation of Na+, K(+)-ATPase by ATP in the presence of MIC showed a decrease in Vmax with no change in Km. Similarly, activation of K+ PNPPase by PNPP in the presence of MIC showed a decrease in Vmax with no change in Km. The circular dichroism spectral studies revealed that MIC interaction with Na+, K(+)-ATPase led to a conformation of the protein wherein the substrates Na+ and K+ were no longer able to bind at the Na(+)- and K(+)-activation sites. The data suggest that the inhibition of Na+, K(+)-ATPase was non-competitive and occurred by interference with the dephosphorylation of the enzyme-phosphoryl complex.

Haemodynamic and electrophysiological acute toxic effects of mercury in anaesthetized rats and in langendorff perfused rat hearts

The acute toxic effects of HgCl2 on the cardiovascular system were studied in Langendorff-perfused rat hearts and in anaesthetized rats. Isovolumic systolic pressure (ISP), atrial and ventricular rates, and atrioventricular conduction (PR-interval) were studied in the hearts perfused with bicarbonate buffer Krebs solution, at 31 degrees C, under a constant pressure of 75 mmHg. Eight hearts were studied at a fixed rate (200 bpm) under control conditions and at different HgCl2 concentrations (0.1, 1 and 10 microM). In these preparations the left ventricular function curves showed that Hg2+ reduces ISP development in a concentration-dependent manner whilst the myocardial response to increasing diastolic pressure is preserved. Ten additional spontaneously beating hearts were studied also under control conditions and at several HgCl2 concentrations (0.5, 1, 2 and 10 microM). ISP and ECG were recorded. Again, ISP decreased after Hg2+ treatment, but all HgCl2 concentrations produced effects of the same magnitude. The reduction of heart rate that also occurs during Hg2+ treatment is the possible explanation for the different effects of Hg2+ on the ISP obtained from the driven and spontaneously beating preparations. Hg2+ also decreased the atrial and ventricular rate driven by atria and increased the PR-interval. Several arrhythmias were induced, such as extrasystoles, A-V blocks, brady- and tachyarrhythmias and ventricular fibrillation without a clear relationship with Hg2+ concentrations. The possibility of an increased activity of autonomic neurotransmitters was also investigated. Cholinergic activity was evaluated in 14 preparations and adrenergic activity in eight by blocking their effects with atropine (0.2 micrograms ml-1) and propranolol (0.1 microgram ml-1), respectively. Atropine reduced Hg2+ effects on ISP, heart rate and PR-interval while propranolol enhanced the cholinergic effects. In the anaesthetized rats the changes in mean arterial blood pressure (MBP), heart rate (HR), and atrioventricular conduction (PR-interval) were recorded and followed for 120 min. In five rats acute poisoning was achieved using a high dose of HgCl2 (50 mg kg-1). MBP and HR decreased and PR-interval increased. Arrhythmias developed followed by ventricular fibrillation and all the animals died after 1 min. In nine other rats a lower dosage (5 mg kg-1) was used. MBP and HR decreased progressively and the PR-interval increased after 40 min. Using the same protocol, six other rats were pretreated with propranolol (2 mg kg-1), and seven with atropine (1 mg kg-1). Propranolol delayed the reduction in MBP caused by HgCl2. HR decreased after propranolol injection but did not change thereafter. The PR-interval, however, increased significantly within the first minute after HgCl2 injection. Atropine blocked the changes in MBP, HR and PR interval produced by HgCl2 during 120 min of observation. Another group treated with 0.5 mg kg-1 was also studied but no changes of the parameters analysed were observed. The results suggest that, in addition to the reduction of mechanical activity, Hg2+ affects heart rate and atrioventricular conduction, has arrhythmogenic effects, decreases arterial blood pressure and increases autonomic neurotransmitter activity.

Similarities of the in vivo and in vitro effects of mercuric chloride on [3H]ouabain binding and potassium activation of Na+/K(+)-ATPase in isolated rat cerebral microvessels

A previous study revealed that a single i.p. administration of 6 mg/kg body wt. of mercuric chloride (MC) durably inhibits the rat cerebral microvascular Na+/K(+)-ATPase activity [1]. In this study, cerebral microvessels isolated 18 h after MC treatment were compared to those obtained from control rats and subsequently treated or not treated with MC in vitro, with regard to: (a) [3H]ouabain binding to, and (b) K(+)-activation kinetics of, the Na+/K(+)-ATPase. Microvessels from MC-treated rats showed a decrease of [3H]ouabain binding down to 62% of the control binding, and the same degree of inhibition was attained in microvessels treated in vitro with 5 microM MC. Analysis of the K(+)-activation kinetics of Na+/K(+)-ATPase revealed a decrease of Vmax from the control value of 13.1 to 7.67 mumol/mg/h in microvessels from MC-treated rats and 6.07 mumol/mg/h in microvessels treated in vitro with 5 microM MC, with no change in Km in either case. The similarity of the effects of in vivo and in vitro treatments suggests that the inhibition of the cerebromicrovascular Na+/K(+)-ATPase following in vivo administration of MC results from a direct interaction of Hg2+ with the enzyme.

Changes of the Na/K ATPase activity in the cerebral cortical microvessels of rat after single intraperitoneal administration of mercuric chloride: histochemical demonstration with light and electron microscopy

Since inorganic mercury salts only poorly penetrate the cerebral microvascular endothelial cells comprising the blood-brain barrier (BBB), their neurotoxicity may be predicted to result from interference with BBB transport enzymes. In the present study, we tested the effect of mercuric chloride (HgCl2) on Na+/K+ ATPase activity, a key enzyme involved in the ion transport in and out of the brain. Routine histochemical staining in conjunction with light and electron microscopy was used to evaluate the changes in the Na+/K+ ATPase activity in cerebral cortical microvessels of rats who received a single intraperitoneal injection of 6 mg/kg HgCl2. At 1 h after HgCl2 administration, light microscopy revealed uniform reduction of the Na+/K+ ATPase reaction in all cortical layers. Electron microscopy confirmed the enzyme reaction to be very weak to completely absent in both the luminal and abluminal endothelial cell membranes, and the luminal plasmalemma showed invaginations and pinocytic vesicles indicative of changes in its transport functions. The enzyme inhibition coincided with, and was likely to contribute to, profound perivascular swelling, involving mainly the astrocytic endfeet. The enzyme activity showed a partial recovery 18 h after HgCl2 treatment, mainly in cortical layers II and III. After 5 days, the recovery of the enzyme activity appeared complete as observed by light and electron microscopy. The recovery of the microvascular Na+/K+ ATPase coincided with the appearance of a strongly positive Na+/K+ ATPase reaction in the adjacent astrocytic processes and with the diminution of perivascular swelling.

In vitro interaction of heavy metals with ouabain receptors in rat brain microsomes

This study investigates the influence of heavy metals on ouabain-binding in presence of thiol (sulfhydryl) compounds. The data on in vitro effects of mercury (Hg), lead (Pb) and cadmium (Cd) showed significant inhibition of 3H-ouabain binding to microsomal membrane in a concentration-dependent manner. Maximum inhibition of 3H-ouabain binding was observed at 1 microM for Hg and 100 microM each for Pb and Cd. Preincubation with monothiol (L-cysteine or glutathione) or dithiol (dithiothreitol) protected inhibition of 3H-ouabain binding to the membranes by Hg or Pb. Dithiol but not monothiols partially protected Cd-inhibition. The present data confirm that the heavy metals interact with ouabain receptors in a manner similar to SH-blocking agents and protection of metal-inhibited 3H-ouabain binding by thiol compounds is metal specific.

Triorganotin inhibition of rat cardiac adenosine triphosphatases and catecholamine binding

Triorganotins have been reported to affect heme metabolism as well as the cardiovascular system. Our recent studies indicated that these organotins inhibit cardiac sarcoplasmic reticulum Ca(2+)-transport and cAMP-stimulated phosphorylation of specific proteins involved in Ca2+ transport, suggesting their interference with cardiac adrenergic function. The present study determines the effect of three organotins--tributyltin bromide (TBT), triethyltin bromide (TET) and trimethyltin chloride (TMT)--on rat cardiac ATPases and catecholamine binding, since these phenomena are involved in cardiac function. Cardiac membrane fraction was prepared from heart ventricles of male Sprague-Dawley rats. All three organotins inhibited cardiac Na+,K(+)-ATPase, [3H]ouabain binding, K(+)-activated p-nitrophenyl phosphatase (K(+)-PNPPase) and oligomycin-sensitive (OS) and oligomycin-insensitive (OI) Mg(2+)-ATPase in a concentration-dependent manner. K(+)-PNPPase was less sensitive to these triorganotins when compared to Na+K(+)-ATPase, suggesting that triorganotins affect the Na(+)-pump activity by acting on the Na(+)-dependent phosphorylation process. OS Mg(2+)-ATPase was more sensitive to these organotins when compared to OI Mg(2+)-ATPase, confirming their potent effect on the enzymes of oxidative phosphorylation. The order of potency is TBT greater than TET greater than TMT. TET and TMT, but not TBT, inhibited [3H]norepinephrine and [3H]dopamine binding to cardiac membranes in a concentration-dependent manner, the effect being more with TET. These results suggest that triorganotins inhibit sodium pump activity as well as ATP synthesis. Since Na+,K(+)-ATPase is involved in the active transport of catecholamines, triorganotins not only inhibited the catecholamine transport but also to some extent affected catecholamine binding, thus interfering with cardiac function.

Inhibition of rat brain microsomal Na+/K(+)-ATPase and ouabain binding by mercuric chloride

This study concerned the effects of mercuric chloride on Na+/K(+)-ATPase and [3H]ouabain binding in rat brain microsomes in vitro. The data showed that HgCl2 inhibited Na+/K(+)-ATPase effectively at micromolar concentrations. The degree of inhibition was decreased with increases in enzyme concentration and incubation time. Variations in the ionic strength of Na+ and K+ did not alter the percent inhibition of Na+/K(+)-ATPase activity by HgCl2. Repeated washings partially restored enzyme activity. The binding of [3H]ouabain to microsomal membranes was inhibited by HgCl2 in a concentration-dependent manner. Cumulative inhibition studies with HgCl2 and ouabain indicated that these inhibitors did not act concurrently and independently on Na+/K(+)-ATPase.