Hydrogen peroxide and methyl mercury are primary stimuli of eicosanoid release in human platelets

Hydrogen peroxide (H2O2) and methyl mercury induced the liberation of arachidonate and its metabolites from human washed platelets. [14C]Eicosanoids were extracted from the supernatants of [14C] arachidonate-prelabelled platelets and analysed by thin layer chromatography and radioscanning. Thromboxane B2 (TXB2), 12(S)-hydroxy-5,8,10-heptadecatrienoic acid (HHT) and 12(S)-hydroxy-5,8,10,14-eicosatetraenoic acid (12-HETE) were found as stable metabolites, together with unreacted arachidonate. In the presence of dazoxiben, a shift in eicosanoid metabolism was observed towards prostaglandin E2 (PGE2), prostaglandin D2 (PGD2) and prostaglandin F2 alpha (PGF2 alpha), while in the presence of indomethacin there was a shift towards 12-HETE and unmetabolized arachidonate. The concentration pattern of those metabolites resembled that found with the physiological agonist, thrombin. H2O2 and methyl mercury also induced platelet shape change, aggregation and secretion. The EC50 values for the induction of shape change and aggregation were 27 and 850 mumol/l for H2O2 and 0.33 and 2.7 mumol/l for methyl mercury, respectively. The [3H]serotonin release required higher stimulus concentrations and amounted to 45% with 2 mumol/l H2O2 and to 16% with 3 mumol/l methyl mercury. These effects on platelet function were absent in platelets exposed to acetylsalicylic acid and prevented by indomethacin, the prostaglandin H2 (PGH2)/thromboxane A2 (TXA2) receptor antagonist, daltroban, and the functional antagonist, iloprost. In contrast, none of these drugs suppressed the formation of [14C]eicosanoids, indicating that the platelet activation by H2O2 and methyl mercury essentially requires previous PGH2/TXA2 formation. As expected, the thromboxane synthase inhibitor, dazoxiben, did not prevent, but instead potentiated the activation by H2O2 and methyl mercury through accumulated PGH2.(ABSTRACT TRUNCATED AT 250 WORDS)

The in vitro effect of methylmercuric chloride on superoxide dismutase activity of pig platelets

Incubation of the suspension of washed pig platelets with methylmercuric chloride (MeHg) caused decrease of superoxide dismutase (SOD) activity at the concentrations of MeHg 10(-5)-10(-4) mol/l. We also observed the increase of enzyme activity at low concentrations of MeHg (10(-7)-10(-6) mol/l). MeHg affects the SOD activity similarly to other mercurials which are known to react with sulfhydryl groups, and differentially influence enzymatic systems.

The action of organic mercury compounds on the function of isolated mammalian heart muscle

The effects of four organic mercury compounds (methylmercuric chloride; bromomercurihydroypropane, BMHP; chlormerodrin; p-chloromercuribenzoic acid, PCMB) on mechanical and electrical functions of guinea-pig papillary muscles were investigated. An initial decline in contraction force was followed by a transient positive inotropic response. The first was accompanied by a shortening of the action-potential duration and by a reduction of the depolarization velocity and the duration of the Ca2+-dependent slow response. The latter was characterized by an indirect component (release of noradrenaline) and by a direct component, which was dependent on the stimulation rate and on the extracellular concentration of Na+ and K+. The direct positive effect, therefore, was likely to have resulted from inhibition of the sarcolemmal Na+ + K+-ATPase. This notion was confirmed by experiments with isolated membrane particles. The prevalence of the negative or positive inotropic action of these compounds could be ascribed to their lipophilic or hydrophilic properties, respectively.

Brain, kidney and liver 203Hg-methyl mercury uptake in the rat: relationship to the neutral amino acid carrier

To investigate the effect of L-neutral amino acids on tissue levels of methyl mercury in the adult animal, rats were infused into the external jugular vein with solutions containing a) 0.05 mM 203Hg-MeHgCl and saline, b) 0.05 mM 203Hg-MgHgCl-0.1 mM L-cysteine, c) 0.05 mM 203Hg-MeHgCl-0.1 mM L-cysteine-0.1 mM L-cysteine-0.1 mM L-methionine, d) 0.05 mM 203Hg-MeHgCl-0.1 mM L-leucine, or e) 0.05 mM 203Hg-MeHgCl-0.1 mM L-cysteine-0.1 mM L-leucine, Groups of animals were sacrificed at 3 min. 7 hr, and 96 hr. Brain, kidney, and liver 203Hg radioactivity was measured by means of gamma-scintillation spectrometry. Brain 203Hg concentrations L-cysteine treated animals were significantly higher compared with saline treated animals (P less than 0.05) at 3 min., 7 hr and 96 hr. The coinjection or coinfusion of methyl mercury with L-cysteine and L-methionine abolished the L-cysteine-mediated brain 203Hg uptake (P less than 0.05), at each sacrifice time. Kidney and liver 203Hg concentrations were not significantly different in any of the treatment groups compared with controls, irrespective of the sacrifice time. Furthermore, the percentage of diffusible 203Hg (non-protein bound) at each sacrifice time was not statistically different irrespective of the treatment assigned. These results suggest that methyl mercury L-cysteine conjugates in the plasma may share a common transport step with the L-neutral amino acid carrier transport system and indicate the presence in brain capillaries of a transport system capable of selectively mediating methyl mercury uptake across the capillary endothelial cell membrane.

Behavioral teratogenic insult of methylmercury assessed by using sets of measures: reanalysis of data from the Collaborative Behavioral Teratology Study of National Center for Toxicological Research

The data of the Collaborative Behavioral Teratology Study were analyzed by using sets of measures simultaneously. A set of preweaning measures offered far more information as to treatment effects than a set of postweaning measures. The treatment effect observed in behavioral measures was also explained considerably by physical measures. However, the effect explained by uniquely behavioral measures (i.e., magnitude of semipartial R2) was also observed in some degree. A sex-related difference was observed in auditory startle response habituation appearing only later or earlier in life. The usefulness of using a set of measures and assessing the relationship among sets in behavioral teratology, in addition to conventional single measure analysis, was discussed.

Enhanced full-length transcription of Sindbis virus RNA by effective denaturation with methylmercury hydroxide

49-S Sindbis virus RNA was reverse transcribed into a complementary DNA. The RNA templates were denatured by three different methods prior to DNA synthesis. Efficient full-length transcription was only achieved after treatment with methylmercury hydroxide.

Differential effects of methylmercuric chloride and mercuric chloride on the histochemistry of rat thyroid peroxidase and the thyroid peroxidase activity of isolated pig thyroid cells

This study was designed to characterize the interaction of CH3HgCl or HgCl2 with thyroid peroxidase (TPO). Two types of experiments were performed. First, the thyroids from rats that were given 5.6 mg/kg/day of either CH3HgCl or HgCl2 for 2 weeks by intubation were subjected to histochemical treatment and then to electron microscopy. TPO activities in all cell compartments were inhibited by HgCl2 but not by CH3HgCl. Morphological observation showed that taller epithelia were induced by HgCl2, whereas flattened epithelia forming large follicles were induced by CH3HgCl. The serum thyrotropin level was substantially lowered by CH3HgCl but was unchanged by HgCl2. Second, the guaiacol oxidation by TPO in isolated and ruptured pig thyroid cells was spectrophotometrically monitored in the presence of either CH3HgCl or HgCl2. The TPO was not inhibited by CH3HgCl but was inhibited by HgCl2. These results indicated that CH3HgCl induced a hypothyroid state without affecting TPO, whereas HgCl2 inhibited TPO and induced a hypertropic state owing to compensation for loss of enzyme activity, and that the lack of inhibitory activity of CH3HgCl was not due to the inability to penetrate the cells. Therefore, there appeared to be a differential interaction of organic and inorganic forms of mercurials with the thyroid.

Block of 45Ca uptake into synaptosomes by methylmercury: Ca++- and Na+-dependence

Block of Ca++ influx into isolated nerve terminals by the neurotoxicant methylmercury (MeHg) was studied for its dependence on extracellular Ca++ and Na+. Depolarization-independent entry of 45Ca++ was determined in rat forebrain synaptosomes incubated in 5 mM K+ solution. 45Ca++ uptake was similarly measured after 1 ("fast" phase) or 10 sec ("total") of elevated K+ (41.25 mM)-induced depolarization or after 10 sec of elevated K+-induced depolarization after synaptosomes had been predepolarized for 10 sec in Ca++- and MeHg-free solutions ("slow" phase). In 5 mM K+ solutions, MeHg concentrations of 125 microM and greater significantly reduced synaptosomal 45Ca++ uptake measured during 1 or 10 sec of incubation. In K+-depolarized synaptosomes, the estimated IC50 for block of total, fast and slow 45Ca++ uptake by MeHg is 75 microM; 250 microM MeHg reduced uptake by approximately 90%. The reversibility of block by extracellular Ca++ was tested by increasing the extracellular Ca++ concentration from 0.01 to 1.15 mM. When compared to control, 50 microM MeHg reduced total uptake of 45Ca++ by greater than or equal to 70% and reduced fast uptake by 20 to 60% at all concentrations of extracellular Ca++ tested. At Ca++ concentrations of 0.01 to 0.15 mM, MeHg (50 microM) reduced slow uptake by 75 to 90%, but did not affect slow uptake at higher Ca++ concentrations (greater than or equal to 0.30 mM). When the dependence of block of 45Ca++ uptake on extracellular Na+ was tested, equivalent levels of inhibition were caused by MeHg (25 microM) for fast uptake by synaptosomes in Na+-containing and Na+-free solutions.(ABSTRACT TRUNCATED AT 250 WORDS)

Rapid cell death induced by methyl mercury in suspension of cerebellar granule neurons

We have further investigated the cytotoxicity of methyl mercury (MeHg) in cerebellar granule neurons isolated from 5-12-day-old rats. At 20 microM MeHg adenosine triphosphate (ATP) levels were reduced to 30% of control within 15 minutes and 1% of control at three hours (h), while cell viability assayed by trypan blue exclusion was reduced to approximately 80% and 20% of control, respectively. When potassium cyanide (KCN) was used to reduce ATP levels greater than 95%, virtually no change in cell viability was observed during three h incubation. Potassium cyanide combined with cycloheximide and actinomycin D to inhibit ATP and macromolecule synthesis simultaneously caused substantially less cell death than that produced by MeHg. Comparable rates of cell death were obtained when the free-radical generating system, hypoxanthine plus xanthine oxidase, was included with KCN in the incubation. Murine hybridoma MHY206 cells, representing a non-neuronal cell type, were less sensitive to cell killing by MeHg compared to granule neurons at equivalent cell protein concentrations. A three h exposure to 20 microM MeHg resulted in the death of 96% of the granule neurons while only 27% of the hybridoma cells were permeable to trypan blue. The results suggest that additional cytotoxic mechanisms beyond perturbations of the main metabolic pathways are involved in the neurotoxic mechanism of action of MeHg in cerebellar granule neurons. The results also indicate that oxidative or free-radical-generating systems are capable of reproducing the temporal pattern of neuronal cell destruction manifested by MeHg.

The inhibitory effects of nitrous oxide and methylmercury in vivo on methionine synthase (EC 2.1.1.13) activity in the brain, liver, ovary and spinal cord of the rat

1. Baseline levels of methionine synthase (MS) activity in spinal cord and ovary from female rats were comparable to activities seen in brain and liver. 2. Exposure (1 hr) of female rats to 50% nitrous oxide (N2O) caused significant decreases in MS in brain, liver, and spinal cord and a sharp drop in ovary. 3. Four daily injections of 6 mg/kg methylmercury (MM) caused a significant decrease in MS activity in brain and a decrease in mean MS activity in ovary and spinal cord. 4. Results from a combined treatment with MM and N2O suggests additive inhibition of MS by these agents.

Methyl mercury stimulates chain elongation by purified HeLa RNA polymerase II

Methyl mercury (MeHg) inhibited the overall RNA synthetic reaction of HeLa RNA polymerase II. However, when RNA chain initiation was allowed to occur in its absence, MeHg stimulated the rate of the subsequent elongation stage of the reaction. Chain elongation with both double-stranded and single-stranded DNA templates was stimulated. This stimulatory effect was specific for MeHg; both p-hydroxymercuribenzoate and HgCl2 inhibited chain elongation (to about the same degree as they inhibited the overall reaction). The stimulatory effect was also specific for the HeLa polymerase; with Escherichia coli RNA polymerase, MeHg inhibited elongation (to the same degree as it inhibited the overall reaction).

Water hyacinth (Eichhornia crassipes) to biomonitor genotoxicity of low levels of mercury in aquatic environment

The mitotic cell-cycle duration of root meristematic cells of Eichhornia crassipes as determined by the colchicine labelling method was approximately 24 h at 30 +/- 1 degrees C. In one experiment the intact root meristems of E. crassipes were subjected to 1 h acute exposure to water contaminated with maleic hydrazide (MH), 56 ppm, or methyl mercuric chloride (MMCl), 0.1-0.5 ppm, followed by recovery in tap water for 4-48 h. In a second experiment the roots were subjected to 96 h exposure to water contaminated with MH, 56 ppm, or MMCl, 0.0001-0.1 ppm. In both experiments the cytological end-point measured was the frequency of cells with micronuclei (MNC). In the first experiment, while in the MH-exposed root meristems the frequency of MNC was significant at 40 h of recovery, MMCl induced significant MNC at 12, 20, 24, 40, and 40 h of recovery depending on the concentration. In the second experiment both test chemicals induced MNC which was concentration-dependent in case of MMCl. The highest ineffective concentration tested (HICT) and lowest effective concentration tested (LECT) for MMC determined in this experiment were 0.0005 ppm and 0.001 ppm, respectively. The present work provides evidence that E. crassipes could be a promising in situ environmental biomonitoring assay system.

Differential effects of mercurial and methylmercurial ions on the phrenic nerve and diaphragm of the mouse

Both Hg2+ and CH3Hg+ exert a depressive action on the twitch amplitude elicited by nerve stimulations more rapidly than that by muscle stimulation. CH3Hg+ was relatively more potent than Hg2+ in the nerve depressive action but was weaker on the muscle. Hg2+ induced a rapid phase and a slow phase of contracture in the diaphragm, while CH3Hg+ induced only a slow one. The biphasic contracture induced by Hg2+ could be inhibited by repetitive washout of the diaphragm with low Ca2+ (10(-3) mM) Krebs and also by treatment with glycerol to close T-tubule, while the slow contracture induced by CH3Hg+ was little affected by glycerol treatment. None of denervation, d-tubocurarine and tetrodotoxin inhibited the contracture induced by Hg2+ and CH3Hg+, indicating that the contracture was induced by a direct action on the muscle. Hg2+, but not CH3Hg+, rapidly caused a decrease of the membrane potential, decreased the amplitude and prolonged the duration of the muscle action potential. Hg2+ also initially decreased and then increased 45Ca2+ uptake of the diaphragm, but CH3Hg+ had no effect, except a prolonged incubation at a high concentration. The action of Hg2+ could be antagonized by glutathione, and CH3Hg+ was more susceptible than Hg2+ to the antagonistic action of glutathione. Both Hg2+ and CH3Hg+ did not change the ATP content of the diaphragm. All of these findings indicate that the monovalent organic CH3Hg+, with a higher lipophilicity, possesses a higher affinity to the nerve while the divalent cationic Hg2+ exerted more effects than CH3Hg+ on the sarcolemma of the mouse diaphragm.

Abnormal neuronal distribution within the cerebral cortex after prenatal methylmercury intoxication

C57BL/6J mice were poisoned with methylmercury during pregnancy, and the location of heavily labeled neurons generated at embryonic day 16 was determined by tritiated thymidine autoradiography of the cerebral cortex of offspring at postnatal day 10. Camera lucida plotting of the distribution of radioactively labeled cortical neurons revealed statistically significant differences between control and methylmercury-treated groups. While control animals showed regular and tight packing of labeled neurons within the upper part of the cortical layer II, in methylmercury-treated animals such neurons were irregularly distributed throughout cortical layers II and III. Short-term intermittent and long-term low-dosage regimens of intoxication produced similar results. These findings support the hypothesis that prenatal methylmercury poisoning results in abnormal neuronal migration and anomalous cortical cytoarchitectonic patterning within the developing brain and provide a possible morphological basis for some of the neurobehavioral abnormalities that may be observed in association with sublethal prenatal intoxication in humans.

The enhanced rate of transcription of methyl mercury-exposed DNA by RNA polymerase is not sufficient to explain the stimulatory effect of methyl mercury on RNA synthesis in isolated nuclei

Previous work demonstrated two stimulatory effects of methyl mercury on nucleic acid synthesis: (1) in isolated nuclei, methyl mercury stimulates RNA synthesis which is catalyzed by RNA polymerase II [Frenkel and Randles, J. Biol. Chem. 257, 6275-6279 (1982)]. (2) Brief exposure of purified DNA to methyl mercury increases the rate of its transcription by purified RNA polymerase II [Frenkel, Cain, and Chao, Biochem. Biophys. Res. Commun. 127, 849-856 (1985)]. The latter effect was considered as a possible mechanism of the former. Two lines of evidence are presented here which demonstrate that the latter effect is not a sufficient explanation for the former. (1) Mercuric perchlorate has been found to increase the rate of DNA transcription by purified polymerase and the template properties of the mercuric perchlorate-exposed DNA have been found to resemble those of methyl mercury-exposed DNA. Nevertheless, mercuric perchlorate has been shown not to stimulate RNA synthesis in isolated HeLa nuclei. (2) In isolated nuclei of the B50 rat neuroblastoma cell line, RNA synthesis has been found to be stimulated only minimally by methyl mercury. Nevertheless, RNA polymerase II purified from the B50 cells has been found to transcribe methyl mercury-exposed DNA at a higher rate than unexposed control DNA.

Selenium-mercury interaction during intestinal absorption of 75Se compounds in chicks

The effects of inorganic (HgCl2) and organic (CH3HgCl) mercury on the intestinal absorption of Se compounds [Na2(75)SeO3, Na2(75)SeO4, L-[75Se]methionine ([75Se]Met)] were determined in 3-wk-old White Leghorn cockerels by the in vivo ligated duodenal loop procedure. The intraduodenal dose contained 0.05 microCi 75Se, 0.01 mM Se, 150 mM NaCl and 0-1.0 mM Hg. In the presence of 1 mM inorganic Hg in the intraduodenal dose, the absorption of the inorganic 75Se compounds was only about 65% of that in the control group, whereas only a slight inhibitory effect on [75Se]Met absorption was observed. Methylmercury had no effect on [75Se]selenite absorption. Precipitation of the 75Se-selenite in the intestinal lumen partly explained the direct interaction between inorganic Hg and Se compounds. Absorption of [75Se]Met and [75Se]selenite was also determined in chicks fed after hatching a purified diet supplemented with varying amounts of Hg (0-500 mg/kg) and Se (0-4 mg/kg). Dietary Hg significantly reduced the transfer of [75Se]selenite to body by enhancing the accumulation of the isotope in the intestinal tissue. Dietary Hg did not affect the absorption of [75Se]Met, but altered the whole-body distribution of this Se compound. Because interaction between Se and Hg was observed mainly between the inorganic compounds and with use of a manyfold excess of Hg over Se, the data suggest that intestinal interaction between these metals is not of great nutritional importance.

In vivo incorporation of [14C]leucine into brain protein of mice treated with methylmercury and thiol complexes of methylmercury

The effect of methylmercury and thiol complexes of methylmercury on inhibition of protein synthesis was evaluated. Mice were injected (i.p.) with the following treatments: methylmercuric chloride, methylmercury-glutathione, methylmercury-cysteinylglycine and control (vehicle) for 10 days. Ten animals from each group were injected with [14C]leucine 90 min prior to death. The brains were removed and the extracted protein was subjected to liquid scintillation analysis. Mice receiving the methylmercury and methylmercury-glutathione treatments exhibited significantly greater weight loss than the control while the methylmercury-cysteinylglycine treatment was not significantly different than the control. Incorporation of [14C]leucine into brain protein was significantly depressed in the methylmercury (81% of control) and the methylmercury-glutathione (79% of control) treatments. Protein synthesis in mice receiving the methylmercury-cysteinylglycine complex although not significantly different than the methylmercury treatments was only 92% of the control mice.

Effects of methylmercury on the spontaneous and potassium-evoked release of endogenous amino acids from mouse cerebellar slices

The effects of methylmercury on the spontaneous and potassium-evoked release of endogenous amino acids from mouse cerebellar slices have been examined. Methylmercury induced a concentration-dependent increase in the spontaneous release of glutamate, aspartate, gamma-aminobutyric acid, and taurine from mouse cerebellar slices. Glycine release was slightly increased, but not in a concentration-dependent manner. The spontaneous release of glutamine from mouse cerebellar slices was not altered by any concentration of methylmercury examined (10, 20, and 50 microM). The tissue content of glutamate, gamma-aminobutyric acid, glutamine, and taurine decreased after exposure to methylmercury. Exposure of cerebellar slices to 20 microM methylmercury resulted in a significant enhancement in glutamate release during stimulation with 35 mM K+. This increase could be accounted for by the methylmercury-induced increase in spontaneous glutamate release. The increase in spontaneous release of glutamate and gamma-aminobutyric acid was independent of the availability of extracellular calcium. These results suggest that methylmercury increases the release of neurotransmitter amino acids, particularly gamma-aminobutyric acid and glutamate, by acting at intracellular sites to increase release from a neurotransmitter pool. The increase in the potassium-stimulated release of glutamate may reflect an increased sensitivity of the cerebellar granule cell to the effects of methylmercury. It is suggested that alterations in amino acid neurotransmitter function in the cerebellum may contribute to some of the neurological symptoms of methylmercury intoxication.

Effects of activation of sodium and calcium entry on spontaneous release of acetylcholine induced by methylmercury

The effect of ionophores and channel activators for Ca and Na on the time course and magnitude of methylmercury (MeHg)-induced increase in spontaneous release of neurotransmitter was studied at the murine neuromuscular junction using intracellular microelectrode recording techniques. The goal was to test whether chemicals that increase entry of Na+ or Ca++ into nerve terminals would shorten the latent period that precedes the onset of MeHg-induced increase in MEPP frequency. Administration of MeHg (100 microM) with A23187 (25 microM), a calcium ionophore, caused a more rapid time to peak induced increase in MEPP frequency than "control" MeHg preparations. This effect also occurred in solutions to which no extracellular Ca++ was added. Use of monensin, a Na+ ionophore (25-100 microM), did not shorten the time to peak increase of MEPP frequency. The dihydropyridine Ca++ channel agonist Bay K 8644 (750 nM) produced the most marked shortening of the time to peak MEPP frequency for MeHg. This effect also occurred in solutions deficient in extracellular Ca++. Veratridine (20 microM), a sodium channel activator, decreased the time to peak MEPP frequency when used in conjunction with MeHg in both Ca++-containing and Ca++-deficient solutions. Replacement of sodium in the extracellular perfusion solution with methylamine, which does not penetrate axon sodium channels, did not prevent the MeHg-induced increase in MEPP frequency although it did prolong the time to peak increase and decreased the maximal MEPP frequency induced by MeHg compared with experiments conducted in sodium-containing solutions.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative effects of divalent cations on the methylmercury-induced alterations of acetylcholine release

Bath application of methylmercury (MeHg) at the murine neuromuscular junction blocks synchronous evoked release of acetylcholine (ACh) and then increases spontaneous release of ACh effects observed electrophysiologically as cessation of EPPS, and increased MEPP frequency (MEPPf), respectively. The objectives of the present study were to test whether the effect of MeHg on spontaneous release was Ca++-specific by substituting Sr++ or Ba++ for Ca++, whether the time course of MeHg-induced block of synchronous evoked release was altered by varying Ca++ concentrations or substituting Sr++ and whether the processes involved in the decay of elevated MEPPf after repetitive stimulation (asynchronous evoked release) were altered by MeHg. MEPPf was recorded continuously from the rat hemidiaphragm using conventional methods during pretreatment with 2 mM Ca++, 2 mM Sr++ or 0.5 mM Ba++ and subsequently with the cation plus 100 microM MeHg. The time to peak MEPPf in MeHg was not different under any condition; however, peak MEPPf was lower in Sr++ solutions than in Ca++ or Ba++ solutions. EPPs were recorded from the rat hemidiaphragm cut muscle preparation during pretreatment with either 2, 4 or 8 mM Ca++ or 2 or 4 mM Sr++ and subsequently with the cation plus 100 microM MeHg. The latency to block of the EPP in 4 and 8 mM Ca++ was not significantly different from the latency in 2 mM Ca++. The latency to block in 2 or 4 mM Sr++ was also not different from that in Ca++. In addition, under all conditions EPP amplitude remained virtually unchanged from pretreatment values until block occurred after 8 to 9 min exposure to MeHg.(ABSTRACT TRUNCATED AT 250 WORDS)

Prenatal exposure to methylmercury alters development of adrenergic receptor binding sites in peripheral sympathetic target tissues

In order to assess the impact of prenatal exposure to methylmercury on sympathetic neurotransmission, effects on development of adrenergic receptor binding sites in peripheral tissues were evaluated. In the liver, methylmercury produced a dose-dependent increase in alpha 1-, alpha 2- and beta-receptor binding of radioligands throughout the first 5 weeks of postnatal life. Similarly, renal alpha-receptor subtypes showed increased binding capabilities, but binding to beta-receptor sites was reduced. At least some of the changes in receptors appear to be of functional significance, as physiological reactivity to adrenergic stimulation is altered in the same directions in these two tissues. The actions of methylmercury displayed tissue specificity in that the same receptor populations were largely unaffected in other tissues (lung, heart). These results suggest that methylmercury exposure in utero alters adrenergic responses through targeted effects on postsynaptic receptor populations in specific tissues.

Increased axonal transport in the rat optic system after systemic exposure to methylmercury: differential effects in local vs systemic exposure conditions

Axonal transport was studied by several techniques in the optic system of adult female Long-Evans rats following systemic exposure to methylmercury in 5 mM Na2CO3. Control rats were treated with the buffer alone. Four mg Hg/kg body weight for 4-6 days, or for 12 days, induced significant changes in the rate of protein synthesis in the retinal cells and in the rate of propagation of protein-bound radioactivity along the ganglion cell axons. Axonal transport of particulate material in both groups treated with methylmercury increased to a rate of 147 mm/day compared to 93 mm/day in controls. Methylmercury was distributed evenly throughout the retinogeniculate system. No clinical neuropathy was evident in either mercury-treated group. It is proposed that the increased rates of transport may represent an adaptive compensatory response to distal axonopathy caused by methylmercury. To investigate why systemic dosing produced effects opposite to those observed with local application of MeHg, various doses of MeHg were tested in the local and systemic paradigms, including doses which yielded equal concentrations of Hg in the retina. The results indicate that the differential response between the two treatment conditions is not a function of local dose, per se. Local and systemic application produce different dose-effect curves, which do not coincide at any dose.

Susceptibility to mercurials of clinical Pseudomonas aeruginosa isolated in México

Susceptibility to inorganic mercuric ions and to organomercurials of 237 Pseudomonas aeruginosa clinical strains isolated in Mexico was determined by agar dilution tests. Resistant strains fell into two classes: i) narrow-spectrum resistant strains (27% of total isolates) resistant only to mercuric ions and to merbromin, and most grouped in pyocin type 1; and ii) broad-spectrum resistant strains (5%) with additional resistances to thimerosal, phenylmercury, methylmercury and p-hydroxymercuribenzoate, that belonged mostly to pyocin type 10. Mercurial resistant isolates showed a higher proportion of resistance to antibiotics and metals than did mercurial sensitive isolates, and broad-spectrum resistant strains had the highest frequency of resistance to antibiotics and to tellurite and arsenate.

Translation of messenger RNA of pig kidney D-amino acid oxidase in a cell-free system

In vitro synthesis of D-amino acid oxidase [D-amino acid: O2 oxidoreductase (deaminating), EC 1.4.3.3], one of the peroxisomal flavin enzymes, was performed using a rabbit reticulocyte lysate system in order to elucidate the biosynthetic pathway of the enzyme. The apparent molecular weight of the synthesized enzyme protein was the same as that of D-amino acid oxidase purified from pig kidney. On the other hand, the enzyme protein was not detectable when a wheat germ lysate system was used for the translation. Denaturation of pig kidney poly(A)+ RNA with methylmercury hydroxide prior to the translation was found to enhance the synthesis of the enzyme protein. These results suggest a tight conformational structure of the mRNA used.

Electrophoretic separation of in vitro translation products on giant two-dimensional gels allows detailed analysis of cellular mRNAs

The in vitro translation products of mRNA pretreated with methylmercuric hydroxide were examined by giant two-dimensional gel electrophoresis. In addition to increasing overall translational efficiency approximately 2.5-fold, methylmercuric hydroxide selectively increases the translation of mRNAs coding for higher molecular mass (greater than 45 kDa) proteins, allowing the routine resolution of 1500 [35S]methionine-labeled proteins. This yields 3 to 4-fold the number of translation products seen with smaller size two-dimensional gels. With this method we compare thymus cell proteins synthesized in vivo with the products of in vitro translation of mRNA recovered from thymus cells. Fifty-eight percent of the translation products are qualitatively the same as proteins synthesized in vivo (similar Mr, pI, and neighboring proteins), with 64% of these also being quantitatively similar (less than 5-fold difference). A comparison of thymus mRNA in vitro translation products with those coded for by mRNA from liver reveals only 32% qualitative similarity, with 63% of these also being quantitatively similar. These results are discussed in relation to predictions of mRNA abundance and complexity based on DNA:RNA hybridization data. Giant two-dimensional gel separations of in vitro translation products appear to be useful for detecting less abundant cellular mRNAs, including those that may be regulated by hormones or other physiological mediators.