Endogenous thiols enhance thallium toxicity

The acute systemic toxicity of thallium in rats produces oxidative stress: attenuation by metallothionein and Prussian blue

Thallium (TI) is one of the most toxic heavy metals. Human exposure to Tl occurs through contaminated drinking water and from there to food, a threat to health. Recently, environmental contamination by Tl has been reported in several countries, urging the need for studies to determine the impact of endogenous and exogenous mechanisms preventing thallium toxicity. The cytoprotective effect of metallothionein (MT), a protein with high capacity to chelate metals, at two doses (100 and 600 µg/rat), was tested. Prussian blue (PB) (50 mg/kg) was administered alone or in combination with MT. A dose of Tl (16mg/kg) was injected i.p. to Wistar rats. Antidotes were administered twice daily, starting 24h after Tl injection, for 4 days. Tl concentrations diminished in most organs (p < 0.05) by effect of PB, alone or in combination with MT, whereas MT alone decreased Tl concentrations in testis, spleen, lung and liver. Likewise, brain thallium also diminished (p < 0.05) by effect of PB and MT alone or in combination in most of the regions analyzed (p < 0.05). The greatest diminution of Tl was achieved when the antidotes were combined. Plasma markers of renal damage increased after Tl administration, while PB and MT, either alone or in combination, prevented the raise of those markers. Only MT increased the levels of reduced glutathione (GSH) in the kidney. Finally, increased Nrf2 was observed in liver and kidney, after treatment with MT alone or in combination with PB. Results showed that MT alone or in combination with PB is cytoprotective after thallium exposure.

Evaluation of cytogenetic and DNA damage caused by thallium(I) acetate in human blood cells

Although thallium is detrimental to all living organisms, information regarding the mutagenic and genotoxic effects of this element and its compounds remains scarce. Therefore, we tested the genotoxic and cytotoxic effects of thallium(I) acetate on human peripheral blood cells in vitro using structural chromosomal aberrations (SCAs), sister chromatid exchanges (SCEs), and single-cell gel electrophoresis (at pH >13 or 12.1) analysis. Whole blood samples were incubated with 0.5, 1, 5, 10, 50, or 100 µg/mL thallium salt. Exposure to this metal compound resulted in a clear dose-dependent reduction in the mitotic and replicative indices. An increase in SCAs was evident in the treated group compared with the control group, and significant differences were observed in the percentage of cells with SCAs when metaphase cells were treated with 0.5-10 µg/mL of thallium(I). The SCE test did not reveal any significant differences. We observed that a 1-h treatment with thallium(I) at pH > 13 significantly increased the comet length for all the concentrations tested; however, at pH 12.1, only the two highest concentrations affected the comet length. These results suggested that thallium(I) acetate induces cytotoxic, cytostatic, and clastogenic effects, as well as DNA damage.

Misdiagnosis and long-term outcome of 13 patients with acute thallium poisoning in China

PURPOSE

To analyze clinical feature and evaluate long-term outcome of patients with thallium poisoning.

MATERIALS AND METHODS

An observational series of cases with acute thallium poisoning was analyzed retrospectively in West China Hospital of Sichuan University between 2000 and 2010. The clinical data including symptom, determination of thallium level, treatment, neurophysiological examination, and neuropsychological evaluation were analyzed. The patients were followed up until December 2012.

RESULTS

Seven men and six women were enrolled in the study. The median patient age was 37 years (range: 15-53 years). The median duration of hospitalization was 44 days (range: 7-72). All the patients were misdiagnosed initially. One patient died in the hospital. The other 12 patients were followed for a median of 7 years (range: 1-12 years) after discharge from hospital. One patient died from leukemia in the first year of follow-up. Long-term outcome results showed peripheral neuropathy improved substantially. However, many patients have mild or moderate sequelae in sensory nerve fibers of distal lower extremity. A sural nerve biopsy in one patient revealed shrunken axons, distorted myelin sheath, and myelinated fibers loss. During follow-up period, problem of intelligence (4/12 patients, 33%), memory impairment (4/12, 33%), anxiety (6/12, 50%), and depression (5/12, 42%) were demonstrated.

CONCLUSIONS

Neurological symptoms may lead to misdiagnosis of thallium poisoning. Mild or moderate neurological sequelae may last for a long time after thallium poisoning.

Additive effect of DL-penicillamine plus Prussian blue for the antidotal treatment of thallotoxicosis in rats

DL-penicillamine (DL-P) and Prussian blue (PB) given alone or in combination were tested as possible treatments against acute thallium toxicity. Rats were intoxicated by i.p. injection of thallium (I) acetate at LD(50) (32 mg/kg). A day later, pharmacological treatment was administered until day 4 as follows: (1) vehicles, (2) PB 50mg/kg, by oral route, twice a day, (3) DL-P 25mg/kg i.p. route, twice daily and (4) PB+DL-P. The Estimated Probability Survival (EPS) was recorded during the experiment for each treatment. DL-P alone did not show a significant effect on survival. However, when it was used in combination with PB, it increased the survival significantly (EPS=0.8, P<0.05) as compared to the control group (EPS=0.4). In a different experiment, using 16 mg/kg of Thallium I acetate, the metal levels were analyzed in blood, body organs and brain regions after treatments. DL-P given alone decreased slightly the thallium content in blood, organs and brain. Meanwhile, its administration in combination with PB diminished the thallium levels significantly (P<0.05) in the majority of tissues, at levels lower than those achieved in the PB group. Those results indicate that DL-P administered alone did not prevent the mortality nor accumulation of the metal in body tissues. Its combination with PB could be considered an alternative antidotal treatment in thallium toxicity, because this chelating agent given alone did not cause thallium redistribution to the brain. When given in combination with PB it has an additive effect in the treatment of acute thallotoxicosis.

A comparison of hepatocyte cytotoxic mechanisms for thallium (I) and thallium (III)

Thallium (Tl) is a highly toxic heavy metal though up to now its mechanisms are poorly understood. In this study, we comparatively investigated the cytotoxic mechanisms of Tl(I) and Tl(III) in isolated rat hepatocytes. Both Tl(I) and Tl(III) cytotoxicities were associated with reactive oxygen species (ROS) formation, lipid peroxidation, collapse of mitochondrial membrane potential, activation of caspases cascade, lysosomal membrane leakiness, and cellular proteolysis. Hepatocyte glutathione (GSH) was also rapidly oxidized. GSH-depleted hepatocytes were more resistant to Tl(I)-induced cytotoxicity, ROS formation and lipid peroxidation. This suggests that Tl(I) is reductively activated by GSH. On the other hand, GSH-depleted hepatocytes were much more sensitive to Tl(III)-induced cytotoxicity, ROS formation, and lipid peroxidation. This suggests that GSH only plays an antioxidant role against Tl(III) cytotoxicity. Our results also showed that CYP2E1 involves in Tl(I) and Tl(III) oxidative stress cytotoxicity mechanism and both cations detoxified via methylation. In conclusion, both Tl(I) and Tl(III) cytotoxicities were associated with mutual mitochondrial/lysosomal injuries (cross-talk) initiated by increased ROS formation resulted from metal-CYP2E1 destructive interaction or metal-induced disruption of mitochondrial electron transfer chain.

High-resolution mapping of neuronal activity using the lipophilic thallium chelate complex TlDDC: protocol and validation of the method

In neurons the rate of K(+)-uptake increases with increasing activity. K(+)-analogues like the heavy metal ion thallium (Tl(+)) can be used, therefore, as tracers for imaging neuronal activity. However, when water-soluble Tl(+)-salts are injected systemically only minute amounts of the tracer enter the brain and the Tl(+)-uptake patterns are influenced by regional differences in blood-brain barrier (BBB) K(+)-permeability. We here show that the BBB-related limitations in using Tl(+) for imaging neuronal activity are no longer present when the lipophilic Tl(+) chelate complex thallium diethyldithiocarbamate (TlDDC) is applied. We systemically injected rodents with TlDDC and mapped the Tl(+)-distribution in the brain using an autometallographic (AMG) technique, a histochemical method for detecting heavy metals. We find that Tl(+)-doses for optimum AMG staining could be substantially reduced, and regional differences attributable to differences in BBB K(+)-permeability were no longer detectable, indicating that TlDDC crosses the BBB. At the cellular level, however, the Tl(+)-distribution was essentially the same as after injection of water-soluble Tl(+)-salts, indicating Tl(+)-release from TlDDC prior to neuronal or glial uptake. Upon sensory stimulation or intracortical microstimulation neuronal Tl(+)-uptake increased after TlDDC injection, upon muscimol treatment neuronal Tl(+)-uptake decreased. We present a protocol for mapping neuronal activity with cellular resolution, which is based on intravenous TlDDC injections during ongoing activity in unrestrained behaving animals and short stimulation times of 5 min.