Methyl mercury exposure via placenta and milk impairs natural killer (NK) cell function in newborn rats

The effect of methyl mercury (MeHg) exposure (3.9 micrograms/g diet) on the development of immune function was studied in the newborn Sprague-Dawley rat after MeHg exposure via placenta and/or milk. No consistent alterations were observed between control and treated offspring (at the age of 15 days) on the following parameters: body weights, lymphoid organ weights or cell number, and the lymphoproliferative response to B-cell mitogen. The lymphoproliferative response to T-cell mitogen was increased in thymocytes (by 30-48%), but decreased in splenocytes (by 30-32%). This decreased activity was only observed in the groups exposed during lactation. White blood cell counts (WBC) were increased in all groups. Natural killer (NK) cell activity was reduced (by 42%, P less than 0.01) in the group that was exposed both via placenta and milk. These results indicate that placental and lactational transfer of MeHg does adversely affect the developing immune system of the rat.

Disruption of the potential across the synaptosomal plasma membrane and mitochondria by neurotoxic agents

The effect of a neurotoxic organo-metal, methyl mercuric iodide, and an aromatic solvent, toluene, upon the transmembrane potential (psi), across both the limiting membrane of isolated nerve terminals and their mitochondria, has been studied. Exposure of nerve endings to either of these toxicants in vitro resulted in a dose-dependent diminution of psi that was especially pronounced in the case of mitochondria. This was not prevented by a concurrent exposure to an antioxidant (alpha-tocopherol), or an iron chelator (deferoxamine), or ganglioside GM1. No significant changes were detected in synaptosomal potentials derived from cortices of rats exposed to methyl mercury or toluene at levels known to increase the rate of formation of reactive oxygen species within this region. The special vulnerability of mitochondrial psi to these agents may be due to the disruption of oxidative phosphorylation and may be related to the increase in intrasynaptosomal free ionic calcium that both of these chemicals can induce.

Taxol protects the microtubules of concanavalin A-activated lymphocytes from disassembly by methylmercury, but DNA synthesis is still inhibited

We have shown previously that there is a good correlation between the degree of microtubule disassembly by methylmercury (MeHg) and the extent of inhibition of DNA replication in Concanavalin A (Con A)-stimulated mouse splenic lymphocytes. The purpose of this study was to determine if these two events are causally related and to examine the effects of MeHg-induced microtubule disassembly on earlier events of the stimulation process. We show that early steps constituting the activation pathway, such as the Con A-induced increase in Ca2+ influx and the expression of interleukin 2 receptor, are not inhibited by concentrations of MeHg that disassemble microtubules. RNA synthesis is not affected by short-term (3 h) treatment with MeHg, but longer treatment (24 h) inhibits RNA synthesis. In contrast, DNA synthesis is effectively inhibited by a 3-h treatment with MeHg. In lymphocytes treated with taxol, microtubules are not disassembled by MeHg; however, the inhibition of RNA and DNA synthesis persists. We conclude that the inhibition of nucleic acid synthesis by MeHg is not causally related to MeHg-induced microtubule disassembly.

Interactions of methylmercury with rat primary astrocyte cultures: methylmercury efflux

Methylmercury (MeHg) efflux from rat astrocyte cultures was studied to complement our previous studies on uptake of MeHg in these cells. Exchange with extracellular MeHg was not obligatory for the efflux of [203Hg]MeHg into the extracellular media, because efflux occurred into MeHg-free extracellular media, but stimulation of [203Hg]MeHg net efflux was shown when astrocytes were equilibrated in the presence of 'cold' MeHg and graded concentrations of L-cysteine. Net efflux of MeHg was most rapid for the first 5 min, and approximately 20% of preloaded [203Hg]MeHg was lost from the astrocytes by 60 min. Uptake of [203Hg]MeHgCl was maximal by 30 min and did not increase when the loading period was extended up to 4 h. However, the total amount of intracellular 203Hg that was available for net efflux gradually decreased as the duration of the preloading period increased. MeHg net efflux from astrocytes was unchanged when [203Hg]MeHgCl preloaded astrocytes were equilibrated in hypotonic buffer, suggesting that unlike ions and amino acids swollen astrocytes remain impervious to MeHg efflux. Thus, the main MeHg efflux transport system is apparently specific for the MeHg-L-cysteine conjugate and represents transport by the same neutral amino acid System L that facilitates its uptake.

Methylmercury blocks N- and L-type Ca++ channels in nerve growth factor-differentiated pheochromocytoma (PC12) cells

Effects of methylmercury (MeHg) on whole-cell Ba++ currents in rat pheochromocytoma (PC12) cells were examined. Based on biophysical characteristics and sensitivity to omega-conotoxin GVIA and dihydropyridine agonists and antagonists, voltage-activated Ba++ currents (IBa) in PC12 cells were mediated by N- and L-type Ca++ channels. Addition of MeHg (10 microM) to the extracellular solution caused a rapid and complete block of current carried by 20 mM Ba++. The rate of block of IBa by MeHg increased in a concentration-dependent manner between 1 and 20 microM. Increasing the frequency of stimulation from 0.1 to 0.4 Hz facilitated block of IBa by MeHg. A 2-min application of 10 microM MeHg in the absence of stimulation also reduced IBa by approximately 80%. Thus, block of IBa by MeHg is not state-dependent. Additionally, MeHg blocked IBa when the membrane holding potential was -40, -70 and -90 mV, indicating that both N- and L-type Ca++ channels are blocked by MeHg. Block of IBa by MeHg was voltage-dependent at a membrane holding potential of -40 mV, but not at holding potentials of -70 and -90 mV. Decreasing the extracellular concentration of Ba++ ([Ba++]e) from 20 mM to 10 mM increased the magnitude of block by MeHg from 45.6 to 77.3%. Increasing [Ba++]e to 30 mM caused no further antagonism of block. Block of IBa by MeHg was not reversed by washing with MeHg-free solution. The ionic permeability of PC12 cell Ca++ channels was Ca++ = Sr++ greater than Ba++. In the presence of MeHg, all three divalent cations were equally permeant.(ABSTRACT TRUNCATED AT 250 WORDS)

Mercury modulation of GABA-activated chloride channels and non-specific cation channels in rat dorsal root ganglion neurons

The effects of mercuric chloride and methylmercury chloride on the rat dorsal root ganglion neurons in primary culture were studied by the whole-cell patch clamp technique. gamma-Aminobutyric acid-induced chloride currents were augmented by mercuric chloride in a potent and efficacious manner; at concentrations of 1 and 10 microM, the current amplitude was increased to 130% and 200% of the control. Methylmercury even at 100 microM did not augment but rather decreased the GABA-induced chloride current. Both mercuric chloride and methylmercury generated slow inward currents by themselves. These currents are not mediated by the GABA-activated chloride channels or by voltage-activated sodium, potassium or calcium channels, and are likely to be due to non-specific cation channels.

Effects of mercury and lead on rubidium uptake and efflux in cultured rat astrocytes

Astrocytes readily sequester lead and mercury (8, 10, 19, 22). Accordingly, studies were undertaken to assess the effects of lead and mercury on homeostatic functions in neonatal rat brain primary astrocyte cultures. Both inorganic and organic mercury, but not lead, significantly inhibited the initial rate (5 min) of uptake of 86RbCl, used as a tracer for K+, at concentrations of 10-100 microM. Mercury and to a lesser extent lead also stimulated the efflux of intracellular 86Rb+ at 10-500 microM. These observations suggest that the astrocyte plasma membrane may be an important target for lead and mercury, and that relatively low concentrations of these heavy metals should inhibit the ability of astrocytes to maintain a transmembrane K+ gradient.

Flow cytometric analysis of the mechanism of methylmercury cytotoxicity

Flow cytometric analysis of murine erythroleukemic cells (MELC) exposed in vitro to 2.5 to 7.5 mumol/l (micromolar) methylmercury (MeHg) reveals a dose-dependent decrease in the rate of DNA synthesis (rate of passage through the S phase of the cell cycle), manifested as the accumulation of most of the cells in the S phase, and a modest accumulation of cells in the G2/M phase of the cycle. Light microscopy reveals a progressive increase in chromosomal damage (condensation, pulverization). At or above 10 mumol/l MeHg, progression through all the phases of the cell cycle is blocked and mitotic cells are arrested irreversibly in anaphase, with most exhibiting arrangement of chromosomes in a wreathlike ring formation. Also the cells exhibit both nuclear propidium iodide (PI) fluorescence (indicative of loss of viability) and concurrent cytoplasmic carboxyfluorescein (CF) fluorescence (viable cells exhibit CF fluorescence and exclude PI). In addition, there is a dose-dependent increase in cellular refractive index (90 degrees light scatter), an apparent decrease in cell volume (axial light loss), and progressive resistance to detergent (NP-40)-mediated cytolysis. Resistance to detergent-mediated cytolysis is indicative of fixation (protein denaturation, cross-linking, and so on) of the plasma membrane/cytoplasm complex. Our findings indicate that DNA synthesis is the primary target of MeHg cytotoxicity and that apparent targets and degree of cytotoxicity are a complex function of dose.

Interactions of methylmercury with rat primary astrocyte cultures: inhibition of rubidium and glutamate uptake and induction of swelling

The ability of astrocytes to sequester MeHg may indicate an astrocyte-mediated role in MeHg's neurotoxicity. Hence, studies were undertaken to assess the effects of MeHg on metabolic functions in cultured astrocytes. MeHg (10(-5) M) significantly inhibited the initial rate (5 min) of uptake of 86RbCl, used as a tracer for K+. 86RbCl uptake was also sensitive to the omission of medium Na+. MeHg (10(-5) M) also markedly inhibited the initial rate of uptake (1 min) of the Na(+)-dependent uptake of [3H]L-glutamate. A second neurotoxin, MnCl2 (0-5 x 10(-4) M), did not alter [3H]glutamate or 86RbCl uptake. MeHg, but not MnCl2, also stimulated the release of intracellular 86Rb+ in a dose-dependent fashion. This effect could be prevented by the administration of MeHg as the glutathione conjugate. These observations support the hypothesis that the astrocyte plasma membrane is an important target for MeHg's toxic effect and specifically that small concentrations of this organometal inhibit the ability of astrocytes to maintain a transmembrane K+ gradient. This would be expected to compromise the ability of astrocytes to control extracellular K+ either by spatial buffering or active uptake, resulting in cellular swelling. We therefore studied volume changes in astrocytes using uptake of [14C]3-O-methyl-D-glucose, in attached cells in response to exposure to MeHg. Exposure to MeHg (0-5 x 10(-4) M) caused a marked increase in the cell volume that was proportional to concentrations of MeHg.

Effects of cadmium and mercury on Na(+)-K+, ATPase and uptake of 3H-dopamine in rat brain synaptosomes

Effects in vivo of cadmium (Cd), mercury (Hg) and methylmercury (CH3Hg) on Na(+)-K+ ATPase and uptake of 3H-dopamine (DA) in rat brain synaptosomes were studied. These heavy metals significantly inhibited the Na(+)-K+ ATPase activity in a dose-dependent manner. Similarly, inhibition of DA uptake by synaptosomes was also observed in rats treated with these metals. Intraperitoneal route of metal administration was found to be more effective than per os treatment. Mercuric compounds compared to Cd elicited a higher inhibition of Na(+)-K+ ATPase and DA uptake in rat brain synaptosomes.

Differential effects of methylmercuric chloride and mercuric chloride on oxidation and iodination reactions catalyzed by thyroid peroxidase

Thyroid peroxidase (TPO), the major enzyme in the thyroid hormone synthesis, multifunctionally catalyzes (1) iodide oxidation, (2) iodination of the precursor protein, and (3) a coupling reaction of iodotyrosyl residues. The present study was carried out to examine the mercurial effects on the iodination, the second step of TPO. Purified porcine thyroglobulin or bovine serum albumin as acceptor protein was iodinated with [125I]NaI and H2O2 by purified porcine TPO. Iodinated protein was separated by acid precipitation on membrane filter or paper chromatography. Both CH3HgCl and HgCl2 dose-dependently inhibited the iodination, but HgCl2 was more potent to inhibit the iodination than CH3HgCl. These mercurial effects on the second step resemble the effects on the third step which were already reported; but are in marked contrast to the effects on the first step, where TPO was inhibited by HgCl2 but never by CH3HgCl.

Altered patterns of protein phosphorylation and synthesis caused by methyl mercury in cerebellar granule cell culture

In the preceding report we demonstrated a dose-dependent increase in 32P-phosphoprotein labeling after 24-h exposure of cultured cerebellar granule neurons to methyl mercury (MeHg), a response that was not observed in glial cultures. In the present study we have examined 32P-labeled phosphoproteins by two-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis. At concentrations of 0.5 and 1 microM, which were not extensively cytotoxic, MeHg enhanced phosphorylation of numerous acidic proteins, particularly a cluster of proteins with Mr approximately 28,000 and pI approximately 5.7-5.9 (pp 28/5.7-5.9) and a protein with Mr approximately 58,000 and pI approximately 5.6. The pp28 cluster displayed considerable two-dimensional pattern variability from one experiment to the next, suggesting susceptibility to subtle structural modifications. Time course studies revealed that increased 32P phospholabeling of pp28/5.7-5.9 was detectable after 12-h exposure to 3 microM MeHg and reached values of 300-500% of control by 24 h. These studies also showed that among the 21 proteins analyzed by two-dimensional densitometry, 32P phospholabeling of four proteins increased by 20-50% and of two proteins decreased by 20-50% after 24-h treatment. However, exposure to 10 microM MeHg produced stimulation of pp28/5.7-5.9 32P phospholabeling within 2 h. Under these conditions a relatively high stimulation (sevenfold) of pp28/5.7 phospholabeling occurred, while pp28/5.9 32P phospholabeling was only moderately (5-20%) enhanced. 35S and 32P double-label analysis of cells treated with 0, 0.5, and 1 microM MeHg indicated specific stimulation of 32P phospholabeling of these proteins without increased polypeptide synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Methyl mercury stimulates protein 32P phospholabeling in cerebellar granule cell culture

Cultures of cerebellar granule neurons have been utilized to examine morphological and biochemical consequences of methyl mercury (MeHg). Exposure to MeHg for 24 h was found to exert toxic effects at concentrations below 1 microM characterized by neuron degeneration and neuritic varicosities. Dose-response and time course profiles for cell death were established using the 51Cr release assay, which revealed that 1 microM MeHg produced 15% cell death at 24 h, progressing to 50% at 48 h. Labeling of cultures with [32P]orthophosphate following 24-h exposure to 1 microM MeHg disclosed abnormalities in both protein and lipid phosphorylation. After 24-h exposure to 5 microM MeHg, phospholabeling of protein and lipid increased 174 and 128%, respectively, compared with controls. This stimulation of phosphorylation appeared to be neuron specific since cultures enriched in cerebellar glial cells and devoid of granule neurons displayed dose-dependent inhibition of total phosphorylation. Measurement of 32P labeling of ATP using a cyclic AMP-dependent protein kinase assay in conjunction with the firefly luciferase assay for ATP indicated no significant change in either total ATP levels or [32P]ATP specific activity at 1 or 4 h as a function of [MeHg]. Studies measuring 32P-phosphoprotein turnover indicated that MeHg had no effect on intracellular protein phosphatase activity. We conclude that one of the manifestations associated with in vitro cerebellar granule cell neurotoxicity is an abnormality in protein phosphorylation that is independent of [32P]ATP specific activity and protein phosphatase activity.

The early effects of methylmercury on the developing rat brain

The effects of organic mercury compounds on the development of the brain are well known since the exposure of people at a large scale to methylmercury in the Minamata Bay area and in Iraq. The neuropathological examination of the brains of children prenatally exposed revealed dysplasia of the cerebral and cerebellar cortex, neuronal ectopia and several other developmental disturbances. In this experimental study we examined developmental mechanisms involved in methylmercury-induced cerebral anomalies. By examining the fetuses soon after treatment we concentrated in the initial effects of the treatment. The pregnant rats were given 10 mg/kg methylmercury chloride i.p. on day 18. Already at 2 h after administration mitochondrial degeneration occurred in the endothelium of the cerebral capillaries. Subsequently hemorrhages developed interfering with the cellular arrangement in the ventricular zone, with neuronal migration in the intermediate zone and with the development of the cortical cytoarchitecture. Macrophages and cavities appeared in the hemorrhagic areas. It is suggested that the abnormalities seen in the experiments can be considered as the initial methylmercury-induced effects which, in combination with various other toxic effects, ultimately result in the anomalies that have been observed in the brains of children prenatally exposed to methylmercury.

Changes in synaptosomal pH and rates of oxygen radical formation induced by chlordecone

The resting pH of 7.14 +/- 0.02 within rat cortical synaptosomes is elevated in vitro by the insecticide chlordecone, in a dose-dependent manner. Chlordecone also reduces the rate of oxygen radical formation within synaptosomes. Both of these changes can also be demonstrated following in vivo treatment of rats with chlordecone (75 mg/kg body wt). Although chlordecone increases the permeability of the plasma membrane, the increase in pH observed is unlikely to be caused by this, since in vivo administration of chlordecone does not appreciably alter membrane order as evaluated by both a lipophilic probe, and a probe with an ionic segment. Another xenobiotic agent, methyl mercuric chloride, and a free radical generating system, an ascorbic acid-ferrous sulfate mixture, did not modulate synaptosomal pH, although membrane permeability was increased. Other evidence of the ability of synaptosomes to maintain homeostasis was the failure of mitochondrial inhibitors to significantly reduce pH. The drop in synaptosomal pH effected by amiloride, an inhibitor of Na+/H+ exchange, and the transient rise in pH caused by ammonium chloride further suggested that synaptosomes may be a good model in the study of the regulation of intracellular pH. The elevation of cytosolic pH, and depression of oxygen radical formation by chlordecone, may result from both the attenuation of respiratory metabolism and an impaired capacity of the plasma membrane to maintain ionic gradients.

Distribution of mercury and evaluation of testicular steroidogenesis in mercuric chloride and methylmercury administered rats

Intraperitoneal administration of methylmercury chloride (MMC) and mercuric chloride (MC) to male rats in doses of 5, 10 micrograms MMC/kg or 50, 100 micrograms MC/kg for 90 days induced cellular disintegration of Leydig cells which was conspicuous on day 30 and onwards in the exposed groups. Progressive degeneration of Leydig cells and decrease in their nuclear diameter and population were associated with gradual increase in deposition of mercury. Gradual diminution of 3 beta-hydroxy-delta 5-steroid dehydrogenase activity in Leydig cells after MMC or MC treatment was correlated with different structural deformations of the cells over 90 days. Moreover, a significant decrease in serum testosterone levels by day 90 confirmed steroidogenic impairment after MMC or MC treatment.

Prevention of chemically induced changes in synaptosomal membrane order by ganglioside GM1 and alpha-tocopherol

Synaptosomal membrane order has been studied by analysis of light depolarization by fluorescent dyes intercalated within membranes following exposure to various environmental toxicants. Two probes were explored: 1,6-diphenyl-1,3,5-hexatriene (DPH), signaling predominantly from the lipid-rich membrane core, and 1-[4-(trimethylamino)phenyl]-6-phenyl-1,3,5-hexatriene (TMA-DPH), reporting from the more hydrophilic membrane surface. Chlordecone, a neurotoxic insecticide, decreased the anisotropy of either dye and this change could be prevented by prior treatment of synaptosomes with ganglioside GM1 but not alpha-tocopherol. Exposure to an iron-ascorbic acid oxidizing mixture enhanced synaptosomal membrane order and this effect was blocked by preincubation with alpha-tocopherol but not ganglioside GM1. While these interactions may have partially reflected additive anisotropy changes, the protective agents were also effective at concentrations where they did not in themselves modulate membrane order. Methyl mercuric chloride at concentrations up to 100 microM had no discernable effect upon membrane order. It is suggested that these changes in membrane order may underlie some of the previously reported variations in the content of ionic calcium and in the leakiness of synaptosomes.

Characterization of interactions of methylmercury with Ca2+ channels in synaptosomes and pheochromocytoma cells: radiotracer flux and binding studies

The interaction of methylmercury (MeHg) with neuronal Ca2+ channels in rat forebrain synaptosomes and dihydropyridine (DHP)-sensitive Ca2+ channels in rat pheochromocytoma (PC12) cells was examined using radiotracer flux assays and radioligand binding analyses. In synaptosomes, the influx of 45Ca2+ was used to examine the voltage and state dependence of block of Ca2+ channels by MeHg, as well as the effects of MeHg on apparent inactivation of 45Ca2+ influx. In addition, the differential influx of 45Ca2+, 85Sr2+, and 133Ba2+ was used to examine the effect of MeHg on the ionic selectivity of synaptosomal Ca2+ channels. The ability of MeHg to block 45Ca2+ influx via a DHP-sensitive Ca2+ channel was examined in PC12 cells. Effects of MeHg on binding of [3H]nitrendipine in synaptosomes and 125I-omega-conotoxin GVIA (CgTx) in synaptosomes and PC12 cells were measured. In synaptosomes, MeHg blocked 45Ca2+ influx in a voltage-dependent manner, inasmuch as increasing the extracellular K+ concentration increased the magnitude of block by 100 microM MeHg. When synaptosomes were incubated for 10 sec in either a nondepolarizing or a depolarizing solution before measurement of 1 sec of depolarization-induced 45Ca2+ influx, the potency and efficacy of the block of 45Ca2+ influx by MeHg were similar. Thus, block of Ca2+ channels by MeHg does not appear to be state dependent. To determine the kinetics of apparent inactivation of 45Ca2+ influx, synaptosomes were predepolarized in Ca2(+)-free high [K+] solution, for intervals varying from 1 to 10 sec, before measurement of 1 sec of K(+)-induced 45Ca2+ influx. When compared with control, MeHg (100 microM) altered the rate constant for apparent inactivation and decreased the fraction of 45Ca2+ influx that does not inactivate. Influx of 45Ca2+, 85Sr2+, and 133Ba2+ during 1 sec of depolarization was blocked in a dose-dependent manner by MeHg, with estimated IC50 values of 125, 150, and greater than 150 microM for 45Ca2+, 85Sr2+, and 133Ba2+, respectively. In triple-label experiments, the relative flux of radiolabeled Ca2+:Sr2+:Ba2+ was altered from approximately 6:2:3 to 6:1:3 in the presence of 100 microM MeHg. In undifferentiated and nerve growth factor-differentiated PC12 cells, K(+)-induced 45Ca2+ influx was blocked by the DHP nifedipine, with an approximate IC50 value of 5 nM. MeHg reduced 45Ca2+ influx in PC12 cells with an estimated IC50 value of 50 microM, and 125 microM MeHg reduced uptake by greater than 90%. [3H]Nitrendipine bound to synaptosomes with high affinity in normal and elevated [K+] solutions.(ABSTRACT TRUNCATED AT 400 WORDS)

Metal inhibition of ferrochelatase activity in human lymphocytes

The activity of the terminal enzyme of haem biosynthesis, ferrochelatase (EC 4.99.1.1) was measured in sonicates of human lymphocytes. We used a sensitive method with zinc and mesoporphyrin as substrates and quantification of the product zinc-mesoporphyrin by HPLC. A variety of metal ions and organometal compounds were examined as possible inhibitors of ferrochelatase activity. Inhibition was observed with copper and mercury (but not with lead) and with tributyltin and methylmercury. The kinetics of ferrochelatase inhibition were examined for each of the four inhibitors identified.

Prevention of chemically induced synaptosomal changes

The ability of altered environmental conditions to modulate some properties of synaptosomes has been studied. Incubation conditions used included the presence of methyl mercury or an organochlorine insecticide: chlordecone. Other adverse chemical conditions during incubation were the absence of calcium salts from the incubation medium or the addition of agents bringing about enhanced oxidative conditions. Synaptosomal parameters studied were the cytosolic level of free, ionic calcium, [Ca2+]i, the extent of depolarization-induced uptake of radioactive calcium, and the permeability of the limiting membrane. In addition, peroxidative activity was estimated by quantitation of thiobarbituric acid-reactive material. All these facets of synaptosomal function were responsive to the presence of these potentially deleterious changes in the incubation medium. While the response of [Ca2+]i was potentially in either direction, all adverse conditions increased synaptosomal permeability as evaluated by leakage of fura-2 into the extracellular compartment. Pretreatment with ganglioside GM1 in some situations or alpha-tocopherol in others could either wholly or partially prevent the onset of such altered synaptosomal characteristics.

Primary stimuli of icosanoid release inhibit arachidonoyl-CoA synthetase and lysophospholipid acyltransferase. Mechanism of action of hydrogen peroxide and methyl mercury in platelets

Icosanoid formation in platelets depends on the concentration of free arachidonate that is mainly liberated from membrane phospholipids by phospholipase A2. The concentration of free arachidonate is also controlled by the activities of the reacylating enzymes arachidonoyl-CoA synthetase and lysophospholipid acyltransferase. In human platelet microsomes we determined the high enzyme activities of 5.9 nmol.min-1.(10(9) platelets)-1 for the arachidonoyl-CoA synthetase and 37 nmol.min-1.(10(9) platelets)-1 for the lysophospholipid acyltransferase. The activities of these reacylating enzymes were strongly reduced by hydrogen peroxide (H2O2) and methyl mercury that are primary stimuli of arachidonate release in intact platelets. H2O2 inhibited the arachidonoyl-CoA synthetase with an IC50 of 3.3 mmol/l without affecting the lysophospholipid acyltransferase. Sulfhydryl group protection by 3-mercapto-1,2-propanediol did not overcome the inhibition but glutathione prevented the inhibition of the arachidonoyl-CoA synthetase by H2O2. This suggests that glutathione by virtue of the glutathione peroxidase reduces H2O2 rather than that it protects free sulfhydryl groups of the arachidonoyl-CoA synthetase. Methyl mercury left the arachidonoyl-CoA synthetase activity unaffected but inhibited the lysophospholipid acyltransferase activity with an IC50 of 3.4 mumol/l. The inhibition is probably evoked by the blockade of sulfhydryl groups of the lysophospholipid acyltransferase because it disappeared when 3-mercapto-1,2-propanediol was added at a concentration higher than that of methyl mercury. Thrombin as a physiological full agonist, Ca2+ less than or equal to 1 mmol/l, the calcium ionophore A23187 and phorbol 12-myristate 13-acetate (TPA) and 1-oleoyl-2-acetylglycerol as model stimuli of protein kinase C neither influenced arachidonoyl-CoA synthetase nor lysophospholipid acyltransferase. It is concluded that the inhibitory effect of H2O2 and methyl mercury on the arachidonate-reacylating enzymes arachidonoyl-CoA synthetase or lysophospholipid acyltransferase, respectively, are responsible for their capacity to stimulate icosanoid release in intact cells. Thrombin and its intracellular messengers Ca2+ and diacylglycerol do not directly affect arachidonoyl-CoA synthetase and lysophospholipid acyltransferase.

Toxic effects of methylmercury on spermatozoa in vitro

In an in vitro investigation, methylmercury (MeHg) reduced the motility of rat spermatozoa probably by the inhibition of succinate dehydrogenase and ATPase activities. Concomitant morphological changes observed in the spermatozoa were coiled tails and kinks in midpiece and tail regions.

Inhibition of amino acid transport and protein synthesis by HgCl2 and methylmercury in astrocytes: selectivity and reversibility

The previously reported observation that submicromolar concentrations of HgCl2 inhibit glutamate uptake reversibly in astrocytes, without effect on 2-deoxyglucose uptake, suggested that elemental mercury vapor, which is oxidized to mercuric mercury in the brain, might cause neurodegenerative change through the mediation of glutamate excitotoxicity. Here, selectivity is explored further by measuring the inhibition of other amino acid transporters and protein synthesis as a function of HgCl2 concentration. The properties of MeHgCl were compared under identical conditions, and some morphological correlates of function were examined. Inhibition of amino acid transport by HgCl2 was selective, whereas MeHgCl was nonselective. The 50% inhibitory concentrations of HgCl2 for uptake of alpha-aminoisobutyric acid by system A, uptake of alpha-aminoisobutyric acid or kynurenine by a system L variant, and uptake of gamma-aminobutyric acid were all two- to fourfold greater than that for uptake of glutamate. The submicromolar concentrations of HgCl2 that inhibited glutamate transport also inhibited protein synthesis, but in a rapidly reversible fashion, and elicited only discrete ultrastructural changes (heterochromatin, increased numbers of lysosomal bodies, and increased complexity of cell surface). In contrast, inhibition of protein synthesis by MeHgCl was acutely (1-h) irreversible and became marked only at concentrations higher than those that elicited gross morphologic change in the form of "bleb"-like swellings. The results lend support to the proposed excitotoxic mediation of mercury vapor neurotoxicity and reveal a sharp contrast between the effects of HgCl2 and MeHgCl on astrocytes.