Formation of N-7-(2-carbamoyl-2-hydroxyethyl)guanine in DNA of the mouse and the rat following intraperitoneal administration of [14C]acrylamide

Acrylamide is an alkylating agent which reacts very slowly in direct reactions with DNA and is negative in the Ames test, but is carcinogenic in mice and rats. In order to explain the cancer-initiating properties of acrylamide we have studied DNA adduct formation in vitro with a metabolizing system and in vivo in mice and rats following i.p. administration of 14C-labeled acrylamide. A major adduct found in both species was N-7-(2-carbamoyl-2-hydroxy-ethyl)guanine, formed by reaction of the DNA with the epoxide metabolite glycidamide. The levels of this adduct were similar in the different organs of the two rodent species, which supports the notion that glycidamide is relatively evenly distributed among tissues and that the organ-specificity in acrylamide carcinogenesis cannot be explained by a selective accumulation of the DNA-reactive metabolite in target organs.

Evaluation of the neurotoxicity of glycidamide, an epoxide metabolite of acrylamide: behavioral, neurochemical and morphological studies

Acrylamide is an important chemical used in the synthesis of polyacrylamides, which have a wide variety of industrial applications. The principal toxic effect of acrylamide, both in animals and in humans, is neurotoxicity. Peripheral nervous system effects are most prominent, but central nervous system effects have also been reported. Acrylamide is metabolized to the epoxide glycidamide, whose adducts to hemoglobin and to DNA have been identified in animals and humans. This metabolite may be involved in the reproductive and carcinogenic effects of acrylamide. In the present study we investigated whether glycidamide would exert neurotoxic effects similar to those caused by its parent compound. Male rats were injected i.p. with acrylamide (25 or 50 mg/kg) or glycidamide (50 or 100 mg/kg) daily for 8 days. Reduced weight gain was evident in animals exposed to glycidamide or to the higher dose of acrylamide. Both compounds induced lethargy and ataxia, but the posture of glycidamide-treated rats differed from that of animals treated with acrylamide. At the high doses, both compounds significantly affected rats' behavior in the rotarod test; on the other hand, only acrylamide was effective in the hindlimb splay test. Acrylamide inhibited activity of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) in sciatic and tibial nerves, as well as in brain. Glycidamide inhibited GAPDH activity only in brain and activity of creatine kinase in both peripheral and central tissues. Acrylamide also caused profound urinary retention and distended bladders, while the effects of glycidamide were minimal. Morphological abnormalities were seen in sciatic nerves and dorsal root ganglion cells of rats treated with acrylamide (50 mg/kg x 12), but not in rats exposed to glycidamide (100 mg/kg x 11). These results indicate that the toxicities of acrylamide and glycidamide differ and suggest that acrylamide itself may be primarily responsible for its peripheral neurotoxicity.

Paraoxonase protects against chlorpyrifos toxicity in mice

Paraoxonase can hydrolyze paraoxon (PO), chlorpyrifos-oxon (CPO) and other organophosphates. Previous studies have indicated that the levels of serum paraoxonase can influence the toxicity of PO and CPO. In the present study we have investigated whether exogenous paraoxonase administered to mice would offer protection toward the acute toxicity of a phosphorothioate, chlorpyrifos (CPS). Paraoxonase was purified from rabbit serum and injected i.v., or i.v. plus i.p., in mice. Inhibition of acetylcholinesterase (AChE) in brain, diaphragm, plasma and red blood cells was measured as an index of CPS (100 mg/kg) toxicity. Administration of paraoxonase 30 min before CPS increased plasma enzyme activity toward CPO by 35-fold, and protected against its toxicity; protection was still present at 24 h, when enzyme activity was still 20-fold over basal. When paraoxonase was given 30 min after CPS, a significant protection against CPS toxicity was still observed, while after 3 h the protective effect was decreased. To mimic conditions of severe acute poisoning, a higher dose of CPS (150 mg/kg) was also administered. Administration of paraoxonase 30 min after this exposure abolished cholinergic signs and significantly protected against AChE inhibition. These results indicate that exogenous paraoxonase offers significant protection against CPS toxicity when administered both before and after the organophosphate, suggesting that it may be considered as a potential additional treatment of organophosphate poisoning.

Long-lasting microencephaly following exposure to cocaine during the brain growth spurt in the rat

In utero exposure to cocaine has been shown to produce somatic and behavioral effects. As microencephaly is often present in children born from cocaine-addicted mothers, aim of the present study was to develop an animal model for cocaine-induced microencephaly. Rats were treated with cocaine (20, 30 or 50 mg/kg/day, s.c., each dose divided in two equal doses given 3 h apart) from postnatal day 4 through 10. None of the doses had any effect on growth, however, at 50 mg/kg, cocaine caused a significant decrease in brain weight, measured on day 12. The effect of cocaine was similar in male and female rats, and microencephaly was still present in 45-day-old animals. When the same dose of cocaine was given as a single daily injection, long-lasting microencephaly was also present, but it was accompanied by a decrease in body weight and significant toxicity. Ethanol (4 g/kg), used as a positive control, also caused microencephaly without affecting body weight, but, differently from cocaine, its effect was more pronounced in female animals. Blood and brain levels of cocaine and its metabolites norcocaine and benzoylecgonine were measured by HPLC during treatment (postnatal day 8). After administration of the 50 mg/kg dose, concentrations of cocaine were 1.92 micrograms/g in brain and 0.94 microgram/ml in blood. These levels are encountered in cases of cocaine overdoses and have been found in meconium of newborns from crack-addicted mothers.(ABSTRACT TRUNCATED AT 250 WORDS)

Platelet monoamine oxidase B activity in workers exposed to styrene

A cross-sectional study was conducted to evaluate monoamine oxidase type B (MAO-B) activity in platelets as a biomarker of effect of styrene and perchloroethylene exposures. MAO-B is an enzyme system involved in dopamine catabolism, the impairment of which has been postulated as a mechanism of styrene-induced neurotoxicity. We previously observed an inverse association between blood styrene and MAO-B among reinforced plastics manufacturing workers. The present study included 59 male boat plant workers exposed to styrene (exposure range < 1-144 ppm, 8-h TWA). Two comparison groups comprised six male dry cleaning workers exposed to perchloroethylene (PCE; exposure range < 2-37 ppm) and 14 male laundry workers not exposed to either agent. Respiratory protection was not used by any of the styrene- or PCE-exposed workers; thus, air concentrations were regarded as valid exposure indicators. MAO-B activity (pmol/10(8) cells/h) was measured in peripheral blood platelets, using phenylethylamine as substrate. Only small overall mean differences in MAO-B were observed among the three groups; mean values were 4.21, 4.51, and 4.12 for the styrene-exposed, PCE-exposed, and laundry workers, respectively. Despite the absence of gross differences among the groups, styrene exposure was inversely related to MAO-B. Mean values for four increasing exposure group quartiles were: 5.60, 4.13, 3.69, and 3.44. The Spearman rank correlation coefficient for styrene with MAO-B was -0.41. Adjustment for age, medication use, smoking, and alcohol consumption had only a minimal effect on this trend.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of styrene oxide on differentiation and viability of rodent embryo cultures

Epidemiological investigations on the reproductive and developmental outcomes of rubber and plastic industry workers have focused on the potential developmental toxicity of styrene, an important occupational chemical, and its metabolite, styrene oxide using two in vitro culture systems: micromass cell cultures and whole embryo culture (WEC). Further, we have compared the effects of styrene oxide in both culture systems to the developmental toxicity of styrene in the micromass system. The ability of styrene oxide to affect the differentiation of rat embryo midbrain (CNS) and limb bud (LB) cells was compared to general cytotoxicity over 5 days in high-density micromass cultures. The IC50 for differentiation was 9.2 micrograms/ml (76 microM) for CNS and 6.7 micrograms/ml (56 microM) for LB. The LC50 for cytotoxicity was 9.6 micrograms/ml (80 microM) and 27.5 micrograms/ml (228 microM) for CNS and LB, respectively. The values for CNS sensitivities suggest that inhibition of differentiation is probably a consequence of high levels of cytotoxicity. In contrast, effects of styrene oxide on LB endpoints of differentiation were evident at concentration levels which had minimal effects on cell viability. Styrene alone or in the presence of an exogenous monooxygenase system had minimal effects when tested at concentrations 4-12 times the highest IC50 values seen with styrene oxide alone. In whole embryo culture experiments, styrene oxide produced growth retardation and embryo malformations (primarily neural) with an MC50 value of 20 micrograms/ml (167 microM). The LC50 value for styrene oxide was approximately 1.7-fold higher (33.2 micrograms/ml, 276 microM). Our in vitro studies suggest that further evaluations of the relationship of developmental toxicity and generalized cytotoxicity of styrene and its metabolite are needed, especially given the low concentrations at which effects were seen.

Expression of cholinergic muscarinic receptor subtypes mRNA in rat blood mononuclear cells

The cholinergic system plays a role in the neuroimmune network. White blood cells have been found to express muscarinic and nicotinic cholinergic receptors as well as acetylcholinesterase on their surface, and cholinergic agents have been shown to modulate immune functions. Although the presence of muscarinic receptors on white blood cells has been well documented by binding and functional studies, the subtype(s) of muscarinic receptor present on these cells is still unknown. We have previously shown the absence of m2 muscarinic receptor subtype mRNA in rat mononuclear cells (Costa et al., 1994). The expression of m1, m3, m4 and m5 mAChR mRNA was analyzed in this study in rat peripheral blood mononuclear cells by Northern blotting hybridization experiments and reverse transcription-polymerase chain reaction. Only mRNA of the m3 subtype was detected in rat mononuclear cells, at levels about 100 times lower than in brain. Traces of m4 subtype mRNA were found in lymphocytes, at levels about 10,000 times lower than the cerebral cortex, while m5 mRNA was undetectable.

Interaction of organophosphorus compounds with muscarinic receptors in SH-SY5Y human neuroblastoma cells

Human neuroblastoma cells (line SH-SY5Y) were used to examine the interaction of single exposure to organophosphorus compounds (OPs) with muscarinic receptors. In this study, SH-SY5Y cells were exposed for 30 min to concentrations of paraoxon, diisopropyl phosphorofluoridate (DFP), phenyl saligenin cyclic phosphate (PSP), and mipafox (N,N'-diisopropyl phosphorodiamide fluoridate) that ranged between 10(-9) M and 10(-3) M (10(-2) M for mipafox). Ability to interfere with muscarinic receptor binding was determined by change in the binding of the nonspecific antagonist [3H]-N-methylscopolamine (3H-NMS). Concentrations of paraoxon > 0.5 x 10(-3) M and PSP 1 x 10(-3) M significantly inhibited the binding of a saturating concentration of 3H-NMS. Concentrations of > 10(-5) M paraoxon or PSP could significantly inhibit the binding of a half-saturating concentration of 3H-NMS. Studies using specific antagonists for muscarinic subtypes (pirenzepine for M1, AFDX-116 for M2, and 4-DAMP for M3) indicated that SH-SY5Y cells have muscarinic receptors most sensitive to the specific antagonist for the M3 subtype (IC50 of 10(-8) M for 4-DAMP compared to 2.5 x 10(-6) M and 2.7 x 10(-5) M for pirenzepine and AFDX-116, respectively). As M3 receptor stimulation results in formation of inositol phosphates from membrane phosphoinositides the capability of OPs to alter levels of inositol phosphates and agonist-stimulated increases in inositol phosphate formation was examined. Intact cells were prelabeled with [3H]myo-inositol and then incubated for 15 min with the OPs before addition of 10(-5) M to 10(-3) M carbachol. Levels of inositol phosphates were determined as the amount of aqueous soluble radiolabeled product extracted from the reaction mixture. Paraoxon and PSP, but not mipafox or DFP, decreased basal levels of inositol phosphates in a concentration-related manner. This could be overcome in cells stimulated with carbachol, a muscarinic agonist, and with sodium fluoride, which does not act at muscarinic receptors. These results indicate that certain OPs, upon acute exposure, interact with muscarinic receptors, but that they also have effects on levels of inositol phosphates that may be associated with another site of action in SH-SY5Y cells.

Inhibition of muscarinic receptor-stimulated phosphoinositide metabolism by cocaine, norcocaine and cocaethylene in rat brain

The interaction of cocaine, its metabolites norcocaine and benzoylecgonine, and cocaethylene, which is formed following a combined cocaine and ethanol exposure, with muscarinic receptor binding and phosphoinositide metabolism was investigated in brain from immature rats. Cocaine and norcocaine inhibited binding of [3H]telenzepine and carbachol-stimulated phosphoinositide metabolism in cerebral cortex, while benzoylecgonine was devoid of any inhibitory activity. Cocaethylene was the most potent inhibitor of both binding and phosphoinositide metabolism. The effect of cocaine was more pronounced at the muscarinic receptors, but a small inhibition of histamine--and serotonin--stimulated phosphoinositide metabolism was also observed.

Biomarkers in environmental medicine: alterations of cell signalling as early indicators of neurotoxicity

Many environmental and occupational chemicals are known to affect the central and/or peripheral nervous system, causing changes that may result in neurological and psychiatric disorders. Because of the limited accessibility of the mammalian nervous tissue, new strategies are being developed to identify biochemical parameters of neuronal cell function, which can be measured in easily obtained tissues, such as blood cells, as potential markers of the chemically-induced alterations occurring in the nervous system. This review includes a comparative analysis of the effects of mercurials on calcium signalling in the neuroadrenergic PC12 cells and rat splenic T lymphocytes in an attempt to characterize this second messenger system as a potential indicator of subclinical toxicity. The suitability of neurotransmitter receptors in blood cells, such as the sigma binding sites, as biological markers of psychiatric disorders is also discussed.

Developmental neurotoxicity of ethanol: further evidence for an involvement of muscarinic receptor-stimulated phosphoinositide hydrolysis

Various lines of evidence suggest that muscarinic receptor-stimulated phosphoinositide hydrolysis during postnatal development in the rat brain may play a relevant role in glial cell proliferation and neuronal differentiation. We have previously shown that administration of ethanol to developing rats during the brain growth spurt causes microencephaly and selectively decreases muscarinic receptor-stimulated phosphoinositide hydrolysis. In the present study we have investigated the sensitivity of the phosphoinositide system coupled to muscarinic receptors to ethanol inhibition during distinct stages of the brain growth spurt. Different groups of rats were treated for 3 days with ethanol (4 g/kg per day) on postnatal days 2-4 (initial), 6-8 or 10-12 (peak), 13-15 (final stage of the brain growth spurt). The results show that the period of maximal sensitivity to ethanol of muscarinic receptor-stimulated phosphoinositide hydrolysis coincides with the peak of the brain growth spurt and with the period of maximal efficacy of muscarinic receptor agonists to induce inositol phosphates accumulation. Interestingly, only when muscarinic receptor-stimulated phosphoinositide hydrolysis was inhibited, a significant reduction of brain weight was observed. The close parallel between inhibition of this second messenger response and reduction of brain weight suggests that the phosphoinositide system coupled to muscarinic receptors may represent a target for the neurotoxic effects of ethanol during this stage of brain development.

Lack of m2 muscarinic acetylcholine receptor mRNA in rat lymphocytes

The presence of muscarinic acetylcholine receptors on lymphocytes has been demonstrated by radioligand binding experiments. Although the specific subtype(s) of muscarinic acetylcholine receptors expressed in lymphocytes is still unknown, some reports suggest the presence of the m2 subtype. In this study we analyzed the expression of m2 subtype mRNA in rat mononuclear cells, B lymphocytes and T lymphocytes by Northern blot hybridization and reverse transcription-polymerase chain reaction. Positive signals for the presence of m2 mRNA were found in rat heart, brainstem, cerebral cortex, corpus striatum and hippocampus, which were used as positive controls. On the other hand, no expression of m2 was detected in lymphocytes. These results indicate that mRNA for the m2 subtype is absent in rat lymphocytes and that one or more other subtypes may be responsible for the reported results in binding experiments.

Serum paraoxonase status: a major factor in determining resistance to organophosphates

A number of lines of evidence suggest that serum paraoxonase is protective against poisoning by organophosphorus substrates of this enzyme. Birds that have very low levels of paraoxon hydrolyzing activity in their sera are very susceptible to parathion poisoning. Rabbits, which have a sevenfold higher enzyme level compared with rats, have a fourfold higher resistance to paraoxon poisoning than rats. Rabbit paraoxonase hydrolyzes chlorpyrifos-oxon with a much higher turnover number than does rat paraoxonase, resulting in a very high resistance of rabbits to chlorpyrifos toxicity. Direct tests of paraoxonase protection have been carried out by injecting purified rabbit enzyme into rats. The protection achieved was higher for chlorpyrifos-oxon than for paraoxon, probably due to the high hydrolytic activity of the rabbit enzyme for chlorpyrifos-oxon. In humans, a substrate-dependent polymorphism of serum paraoxonase is observed, where one isoform of paraoxonase has a high turnover number for paraoxon and the other a low turnover number. Both isoforms appear to hydrolyze chlorpyrifos-oxon and phenylacetate at the same rate. Cloning and sequencing of the human paraoxonase cDNAs has elucidated the molecular basis of the polymorphism. Arginine at position 192 determines high paraoxonase activity, and glutamine at this position, low paraoxonase activity. In addition to the polymorphism, a 13-fold variation in serum enzyme levels within a given genetic class is seen. The experiments reported here demonstrate that rabbit paraoxonase injected into mice provides protection against the parent insecticide chlorpyrifos as well as the toxic oxon. These results suggest that serum paraoxonase status may serve as a biomarker for insecticide susceptibility in humans.

Interaction of ethanol with muscarinic receptor-stimulated phosphoinositide metabolism during the brain growth spurt in the rat: role of acetaldehyde

The developing brain is extremely sensitive to the neurotoxicity of ethanol; however, the mechanism(s) of its developmental neurotoxicity are still elusive. In the developing rat brain, ethanol exerts an age-, brain region-, and receptor-specific inhibitory effect on muscarinic receptor-stimulated phosphoinositide metabolism, which may be linked to some of the neurotoxic effects of ethanol found in children with fetal alcohol syndrome. Since some studies have suggested that the ethanol metabolite acetaldehyde may mediate, at least in part, the developmental effects of ethanol, in the present study we have examined whether acetaldehyde would inhibit carbachol-stimulated phosphoinositide metabolism in brain slices from immature rats. We also tested propionaldehyde, the corresponding aldehyde of n-propanol, another alcohol shown to cause microencephaly and to affect phosphoinositide metabolism in the developing rat. Neither acetaldehyde nor propionaldehyde, at concentrations up to 1 mM, had any inhibitory effect on this system, while the two alcohols did, as previously reported. These results suggest that ethanol itself may be the primary agent responsible for its developmental neurotoxicity.

Human and rabbit paraoxonases: purification, cloning, sequencing, mapping and role of polymorphism in organophosphate detoxification

Human and rabbit paraoxonases/arylesterases were purified to homogeneity by chromatographic and gel electrophoretic/isofocusing procedures coupled with activity stains. N-terminal and peptide sequence analysis suggested retention of the secretion signal sequence and allowed design of oligonucleotide probes. The probes were used to isolate a 1294-bp rabbit paraoxonase cDNA clone, which, in turn, was used to isolate three human cDNA clones. Comparison of rabbit and human protein and cDNA sequences indicated a high degree of sequence conservation (approximately 85% identity) and verified that paraoxonase retains its signal sequence (except for the N-terminal Met). The rabbit cDNA encodes a protein of 359 amino acids and the human a protein of 355 amino acids. In situ hybridization demonstrated, as expected, that the paraoxonase gene maps to the long arm of human chromosome 7. Arginine at position 192 specifies high activity paraoxonase and glutamine low activity human paraoxonase. Variation in protein levels explains the variation of enzyme activity observed within a genetic class. Toxicity studies showed that raising rat plasma paraoxonase levels by i.v. administration of partially purified rabbit paraoxonase protected animals against cholinesterase inhibition by paraoxon and chlorpyrifos oxon. Protection correlated with the relative rates of hydrolysis of these two compounds.

Interaction of the neurotoxic pesticides ivermectin and lindane with the enteric GABAA receptor-ionophore complex in the guinea-pig

In isolated segments of guinea-pig small intestine, gamma-aminobutyric acid (GABA) (3-300 microM), the GABAA receptor agonist 3-aminopropane sulphonic acid (3-APS) (3-300 microM) and ivermectin (1-300 microM) caused concentration-dependent nerve-mediated cholinergic contractions sensitive to tetrodotoxin (1 microM) and hyoscine (1 microM). The EC50 values were 30.2 +/- 4.3, 24.6 +/- 3.1 and 4.8 +/- 0.6 microM, respectively. Picrotoxinin (10 microM), an allosteric blocker of the Cl- channel associated with GABAA receptors, non-competitively antagonized the contractile response caused by each agonist. Like picrotoxinin, lindane (10, 30 microM) caused a dose-related shift to the right of the concentration-response curve to GABA, 3-APS and ivermectin with depression of the maximum response. SR 95531 (3 microM), a competitive antagonist of GABAA receptors, caused a parallel dextral shift of the concentration-response curve to ivermectin with an apparent single point pA2 value of 6.5. Our results suggest that ivermectin and lindane, two neurotoxic pesticides interfering with central GABAErgic transmission, exert agonist and non-competitive antagonist properties at the enteric GABAA receptor-ionophore complex. This peripheral complex can thus be considered as an additional target for the action of both these compounds.

Determination of hemoglobin adducts in humans occupationally exposed to acrylamide

Hemoglobin (Hb) adduct determinations were used to monitor occupational exposure to acrylamide (AA) and acrylonitrile (AN). Forty-one workers in a factory in the People's Republic of China who were involved in the synthesis of AA by catalytic hydration of AN and the manufacturing of polyacrylamides were studied. Ten nonexposed workers in the same city served as controls. AA and AN exposures were monitored using the modified Edman degradation procedure for the determination of their respective Hb adducts to N-terminal valine. The adduct levels in the exposed workers were 0.3-34 nmol/g Hb for AA and 0.02-66 nmol/g Hb for AN, as determined by gas chromatography-mass spectrometry (GC-MS). The formation of glycidamide (GA), the epoxide metabolite of AA, in humans was demonstrated by GC-MS analysis of its Hb adduct to N-terminal valine following acid hydrolysis, ion-exchange chromatography, and derivatization. The GA adduct was detected in samples from the exposed persons with levels of 1.6-32 nmol/g Hb. There was a linear relationship between the AA and GA adduct levels (r = 0.96) and the ratio of the in vivo doses of GA and AA was 3:10. These results suggest that AA is metabolized to GA in humans, as had previously been shown in the rat. The high AA adduct levels in the exposed workers, as compared to those expected from air concentrations, indicate that dermal exposure may contribute significantly to the total uptake of AA. The average daily in vivo doses of AA and GA in the highest exposed workers were comparable to the in vivo doses in rats injected with 3 mg/kg AA. Since a regimen of 2 mg/kg/day is known to cause a significant increase of tumors in rats, preventive measures may be necessary for humans exposed to high levels of AA in industrial settings.

Cyclic GMP formation induced by muscarinic receptors is mediated by nitric oxide synthesis in rat cortical primary cultures

In rat primary cortical cultures, carbachol caused a time- and concentration-dependent increase in guanosine cyclic 3',5'-monophosphate (cGMP) levels, which was antagonized by the muscarinic antagonist atropine. Glutamate and sodium nitroprusside also caused large increases in cGMP levels, as previously reported. Two nitric oxide (NO) synthase inhibitors, L-NG-nitroarginine and L-NG-monomethylarginine, were tested for their ability to inhibit the carbachol- and the glutamate-induced cGMP formation. The cGMP response to carbachol was decreased by both compounds in a dose-dependent fashion. The effect of L-NG-nitroarginine was competitively reversed by addition of an excess of L-arginine. Similarly, the stimulatory effect of glutamate on cGMP levels was antagonized by L-NG-nitroarginine and L-NG-monomethylarginine. Hemoglobin, a scavenger of NO, also blocked the carbachol-stimulated cGMP production. These results indicate that muscarinic receptor-stimulated cGMP formation in rat cerebral cortex is mediated by NO.

Effects of the muscarinic agonist oxotremorine on membrane fluidity in rat lymphocytes

The muscarinic agonist oxotremorine produced a concentration-dependent increase in membrane fluidity in intact viable rat splenic lymphocytes in vitro. This effect was antagonized by atropine, but only at high concentrations (1 mM), while scopolamine was ineffective. Two other muscarinic agonists, carbachol and pilocarpine, did not affect membrane fluidity in lymphocytes. The fluidizing effect of oxotremorine occurred at both 10 and 37 degrees C with a similar time-course. Oxotremorine also increased membrane fluidity in liposomes of DMPC in gel phase, although its effect was less pronounced than in lymphocytes. The data suggest that the fluidization caused by oxotremorine is primarily nonreceptor-mediated and associated with a nonspecific physicochemical effect.

A nonlinear dosimetric model for hemoglobin adduct formation by the neurotoxic agent acrylamide and its genotoxic metabolite glycidamide

Hemoglobin (Hb) adducts, formed by the neurotoxic agent acrylamide (AA) and its genotoxic metabolite glycidamide (GA), were measured in the rat by means of a method for simultaneous determination of the adducts formed to cysteine. A novel, nonlinear dosimetric model was developed to describe Hb adduct formation. This model incorporates the saturable kinetics of the metabolic conversion in vivo of AA to GA. The pharmacokinetic parameters Vmax and Km and the first-order rates of elimination, k1 and k2, for AA and GA from all processes except conversion of AA to GA, were estimated directly from Hb adduct data to 19 M hr-1, 66 microM, 0.21 hr-1, and 0.48 hr-1, respectively. At low concentrations, approximately 60% of AA was metabolized to GA. The nonlinear dosimetric model for adduct formation has potential general applicability in high-to-low-dose extrapolation of genotoxic effects.

Calcium entry blockade as a mechanism for chlordimeform-induced inhibition of motor activity in the isolated guinea-pig ileum

Central and peripheral alpha 2-adrenoceptors, including those of the gastrointestinal tract, have been indicated as a toxicity target of formamidine pesticides in mammals. In this study, the inhibitory effect of chlordimeform on twitch contractions from electrically-stimulated longitudinal muscle-myenteric plexus preparations (LMMPs) of the guinea-pig ileum was found to be resistant to the action of the alpha 2-adrenoceptor antagonist idazoxan. This drug was also ineffective on chlordimeform-induced inhibition of peristalsis recorded in whole ileal segments. As expected, idazoxan antagonized the inhibitory effect of the alpha 2-adrenoceptor agonist clonidine on twitch contractions and peristaltic activity. Chlordimeform reduced the amplitude of direct mechanical responses to a variety of spasmogens such as acetylcholine, histamine and substance P, suggesting a muscular site of action. Moreover, Ca(2+)-free, K(+)-depolarized LMMPs, chlordimeform inhibited submaximal contractions caused by addition of exogenous calcium, through an action apparently similar to that of the Ca2+ entry blocker nifedipine. Both chlordimeform- and nifedipine-induced inhibition of calcium contractions were reversed by the calcium channel activator BAY K 8644. This compound also partially prevented the inhibitory action of chlordimeform on peristaltic activity. On the whole, these results indicate that chlordimeform-induced depression of motor activity in the guinea-pig ileum is, at least in part, related to inhibition of transmembrane Ca2+ fluxes responsible for smooth muscle contraction.

Effects of inorganic and organic mercury on intracellular calcium levels in rat T lymphocytes

The importance of cytosolic free calcium level ([Ca2+]i) in lymphocyte activation prompted us to investigate changes in [Ca2+]i in T cells caused by mercury compounds, which have been shown to have immunomodulatory and immunotoxic properties. Using fura-2 as fluorescent Ca2+ indicator, we found that both methyl-mercury (MeHg; 0.02-2 microM) and inorganic mercury (HgCl2; 0.01-1 microM) increased [Ca2+]i in lymphocytes from rat spleen in a concentration-dependent manner. The effect of MeHg was rapid and the increase of Ca2+ level was sustained in time, while HgCl2 caused a slow rise in [Ca2+]i. The effects of mercury compounds did not appear to be associated with alterations of membrane integrity, since there was no significant difference in the extent of MnCl2 quench between control and mercury-treated cells. However, HgCl2 (1 microM) and MeHg (2 microM) appeared to cause membrane damage at longer incubation times (15 min). When cells were incubated in Ca(2+)-free medium (in the presence of 1 mM EDTA) MeHg still increased [Ca2+]i, though to a lesser extent, while HgCl2 had no effect. Heparin, an inhibitor of inositol 1,4,5-trisphosphate-induced Ca2+ mobilization partially blocked this rise of [Ca2+]i, while carbonyl cyanide m-chlorophenylhydraxone (CCCP), an inhibitor of mitochondrial function, had a lesser effect. When added together, heparin and CCCP almost completely block the response to MeHg. These results suggest that MeHg and HgCl2 exert their effects of [Ca2+]i in different ways: MeHg-induced increases in [Ca2+]i are due to influx from outside the cells as well as to mobilization from intracellular stores, possibly the endoplasmic reticulum, and, to a minor extent, the mitochondria; on the other hand, HgCl2 causes only Ca2+ influx from the extracellular medium.

Modulation of muscarinic receptors and acetylcholinesterase activity in lymphocytes and in brain areas following repeated organophosphate exposure in rats

Repeated exposures to organophosphorus (OP) insecticides has been shown to cause a decrease of cholinergic muscarinic receptors (mAChR) in brain and in peripheral tissues. These changes are believed to be involved in the development of tolerance to OP toxicity and may play a role in cognitive dysfunctions observed following repeated OP exposure. Recently, mAChRs identified in circulating lymphocytes have been shown to be modulated similarly to brain mAChRs following repeated OP exposure, suggesting that these peripheral cells may be useful as indicators of mAChR changes in the central nervous system. This study was designed to further investigate whether mAChRs on lymphocytes could serve as a biomarker for changes in brain mAChRs during prolonged OP exposure and during recovery from such exposure. Using the mAChR antagonist [3H]quinuclidinyl benzilate (QNB) to label mAChRs, we found that exposure to disulfoton for 14 days (2 mg/kg/day by gavage) caused a significant decrease (25-35%) in muscarinic receptors density in the cerebral cortex, hippocampus, and striatum, as well as in circulating lymphocytes. The decline of mAChR density in lymphocytes paralleled those observed in brain, particularly in cortex and hippocampus, during exposure to disulfoton; however, while brain mAChR levels recovered slowly after termination of exposure and remained significantly reduced 4 weeks after the last treatment, [3H]QNB binding in lymphocytes recovered rapidly within 1 week. Similarly, lymphocyte acetylcholinesterase (AChE) activity was significantly inhibited and correlated well with brain AChE activity during exposure, but the recovery was rapid relative to AChE activity in brain.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative measurements of vibration threshold in healthy adults and acrylamide workers

The early detection of impaired vibration sensation is necessary in order to monitor the adverse effects in workers occupationally exposed to neurotoxic chemicals such as acrylamide. The conventional neurological examination which assesses vibration sensation by utilizing a tuning fork is relatively insensitive for this purpose. In the present study, the Vibration II, a new device for the quantitative measurement of vibration thresholds, was used in 105 healthy Chinese adults. A new testing procedure combining the "two-alternative forced-choice procedure" and the "yes-or-no method-of-limits procedure" showed good reliability and was less time consuming. The results indicate that significant differences in the vibration threshold of index fingers and great toes were found neither between males and females, nor between the left and the right side. However, there was an age-dependent increase in vibration threshold in nonexposed healthy subjects. The vibration thresholds of 41 workers exposed to acrylamide detected by the Vibration II were significantly higher than those of the healthy adults in the same age group. The quantitative measurement of vibration threshold seems to be potentially useful for screening peripheral nerve dysfunction in field studies.

[Oncogenes as indicators of occupational exposure to carcinogenic agents]

Oncogenes are a class of genes capable of inducing neoplastic alterations of the cell. Recently, oncogenes have received greater attention in environmental medicine, since they may be useful tools for monitoring individuals exposed to carcinogenic chemicals. Their potential use as biomarkers is supported by studies indicating changes in the expression of growth factors and oncoproteins in chemically induced cancer. These alterations may even take place prior to clinical diagnosis. In addition, availability of new immunoblotting techniques, which allow the detection of oncoproteins and growth factors in easily accessible tissues, such as serum and urine, has provided promising preliminary results in humans. In this paper, mechanisms of oncogene activation and their role in occupational carcinogenesis are briefly discussed. Recent human studies suggesting a link between occupational toxicants and oncogenes, as well as a potential use of oncogenes as biomarkers, are also reported. Finally, advantages and limits of oncogenes over more traditional biomarkers are discussed.