The intracellular distribution of liver cell calcium in normal rats and one hour after administration of carbon tetrachloride

Evaluation of the carcinogenicity of carbon tetrachloride

Carbon tetrachloride (CCl4) has been extensively used and reported to produce toxicity, most notably involving the liver. Carbon tetrachloride metabolism involves CYP450-mediated bioactivation to trichloromethyl and trichloromethyl peroxy radicals, which are capable of macromolecular interaction with cell components including lipids and proteins. Radical interaction with lipids produces lipid peroxidation which can mediate cellular damage leading to cell death. Chronic exposure with CCl4 a rodent hepatic carcinogen with a mode of action (MOA) exhibits the following key events: 1) metabolic activation; 2) hepatocellular toxicity and cell death; 3) consequent regenerative increased cell proliferation; and 4) hepatocellular proliferative lesions (foci, adenomas, carcinomas). The induction of rodent hepatic tumors is dependent upon the dose (concentration and exposure duration) of CCl4, with tumors only occurring at cytotoxic exposure levels. Adrenal benign pheochromocytomas were also increased in mice at high CCl4 exposures; however, these tumors are not of relevant importance to human cancer risk. Few epidemiology studies that have been performed on CCl4, do not provide credible evidence of enhanced risk of occurrence of liver or adrenal cancers, but these studies have serious flaws limiting their usefulness for risk assessment. This manuscript summarizes the toxicity and carcinogenicity attributed to CCl4, specifically addressing MOA, dose-response, and human relevance.

Postulated carbon tetrachloride mode of action: a review

Under the 2005 U.S. EPA Guidelines for Carcinogen Risk Assessment (1), evaluations of carcinogens rely on mode of action data to better inform dose response assessments. A reassessment of carbon tetrachloride, a model hepatotoxicant and carcinogen, provides an opportunity to incorporate into the assessment biologically relevant mode of action data on its carcinogenesis. Mechanistic studies provide evidence that metabolism of carbon tetrachloride via CYP2E1 to highly reactive free radical metabolites plays a critical role in the postulated mode of action. The primary metabolites, trichloromethyl and trichloromethyl peroxy free radicals, are highly reactive and are capable of covalently binding locally to cellular macromolecules, with preference for fatty acids from membrane phospholipids. The free radicals initiate lipid peroxidation by attacking polyunsaturated fatty acids in membranes, setting off a free radical chain reaction sequence. Lipid peroxidation is known to cause membrane disruption, resulting in the loss of membrane integrity and leakage of microsomal enzymes. By-products of lipid peroxidation include reactive aldehydes that can form protein and DNA adducts and may contribute to hepatotoxicity and carcinogenicity, respectively. Natural antioxidants, including glutathione, are capable of quenching the lipid peroxidation reaction. When glutathione and other antioxidants are depleted, however, opportunities for lipid peroxidation are enhanced. Weakened cellular membranes allow sufficient leakage of calcium into the cytosol to disrupt intracellular calcium homeostasis. High calcium levels in the cytosol activate calcium-dependent proteases and phospholipases that further increase the breakdown of the membranes. Similarly, the increase in intracellular calcium can activate endonucleases that can cause chromosomal damage and also contribute to cell death. Sustained cell regeneration and proliferation following cell death may increase the likelihood of unrepaired spontaneous, lipid peroxidation- or endonuclease-derived mutations that can lead to cancer. Based on this body of scientific evidence, doses that do not cause sustained cytotoxicity and regenerative cell proliferation would subsequently be protective of liver tumors if this is the primary mode of action. To fulfill the mode of action framework, additional research may be necessary to determine alternative mode(s) of action for liver tumors formed via carbon tetrachloride exposure.

Potential role of regucalcin as a specific biochemical marker of chronic liver injury with carbon tetrachloride administration in rats

The potential sensitivity of liver specific protein regucalcin as a biochemical marker of chronic liver injury with carbon tetrachloride (CCl4) administration in rats was investigated. CCl4 (10%; 1.0 ml/100 g body wt) was orally given 5 times at 3-day intervals to rats, and the animals were killed by bleeding at 3, 6, 18, and 30 days after the first administration of CCl4. The body weight of rats was significantly lowered 3 and 6 days after CCI4 administration as compared with that of control rats administered with corn oil, and then the weight was restored at 18 and 30 days. Serum glutamate-oxaloacetate transaminase (GOT) and glutamate-pyruvate transaminase (GPT) activities were significantly increased 3 days after the administration, while a significant increase in serum y-glutamyltranspeptidase (gamma-GTP) activity was seen at 3 and 6 days after the administration. Serum GOT, GPT, and gamma-GTP activities were restored to control levels at 18 and 30 days after CCl4 administration. Serum albumin, alpha-fetoprotein, and ammonium levels were not changed by CCl4 administration. Meanwhile, serum regucalcin concentration was markedly increased 3 and 6 days after CCl4 administration, and a significant increase in serum regucalcin concentration was observed 18 and 30 days after the administration. Liver regucalcin mRNA and liver cytosolic regucalcin levels were significantly decreased 18 and 30 days after CCl4 administration. Liver content of calcium, which intracellular calcium homeostasis is maintained, was significantly increased between 3 and 30 days after CCl4 administration. Hepatic mitochondrial succinate dehydrogenase activity was significantly increased 30 days after the administration. The present study demonstrates that serum regucalcin has a potential sensitivity as a specific biochemical marker of chronic liver injury with CCl4 administration in rats.

Decrease in protein kinase and phosphatase activities in the liver nuclei of rats exposed to carbon tetrachloride

The alteration in protein kinase and phosphatase activities in the liver nuclei of rats administered carbon tetrachloride (CCl(4)) was investigated. Rats received a single oral administration of CCl(4) (1 ml/100 g body wt of 5, 10, and 25% CCl(4) in corn oil), and 5, 24, and 48 h later they were euthanized by bleeding. The administration of CCl(4) (10 and 25%) caused a significant decrease in protein kinase activity in the liver nuclei. The enzyme activity in the liver nuclei from normal and CCl(4)-administered rats was significantly increased by the addition of Ca(2+) (0.5 mM) and calmodulin (10 microg/ml) in the reaction mixture, suggesting that Ca(2+)/calmodulin-dependent protein kinase activation is not suppressed by CCl(4) treatment. Liver nuclear phosphatase activity toward phosphotyrosine, but not phosphoserine and phosphothreonine, was markedly decreased by CCl(4) (5, 10, and 25%) administration. This decrease was seen 5 h after CCl(4) administration. The presence of vanadate (10(-4) M) in the reaction mixture caused a significant decrease in phosphotyrosine phosphatase activity in the liver nuclei from normal and CCl(4)-administered rats, whereas the enzyme activity was not decreased by okadaic acid (10(-5) M) or sodium fluoride (10(-3) M). The effect of anti-regucalcin antibody (100 ng/ml) in increasing phosphotyrosine phosphatase activity was seen in the liver nuclei of CCl(4)-administered rats, suggesting that regucalcin-sensitive phosphatase activity is decreased by CCl(4) administration. The present study demonstrates that CCl(4) administration induces a decrease in protein kinase and tyrosine phosphatase activities, which are involved in signaling factors in the liver nuclei of rats.

Alteration in calcium content and Ca2+-ATPase activity in the liver nuclei of rats orally administered carbon tetrachloride

The alteration in calcium transport in the liver nuclei of rats orally administered carbon tetrachloride (CCl4) was investigated. Rats received a single oral administration of CCl4 (5, 10, and 25%, 1.0 ml/100 g body weight), and 5, 24 and 48 h later the animals were sacrificed. The administration of CCl4 (25%) caused a remarkable elevation of calcium content in the liver tissues and the nuclei of rats. Liver nuclear Ca2+-ATPase activity was markedly decreased by CCl4 (25%) administration. The presence of dibutyryl cyclic AMP(10(-4) and 10(-3) M) or inositol 1,4,5-trisphosphate (10(-6) and 10(-5) M) in the enzyme reaction mixture caused a significant decrease in Ca2+-ATPase activity in the liver nuclei obtained from normal rat, while the enzyme activity was significantly increased by calmodulin (1.0 and 2.0 microg/ml). These signaling factor's effects were completely impaired in the liver nuclei obtained from CCl4 (25%)-administered rats. DNA fragmentation in the liver nuclei obtained from CCl4-administered rats was significantly decreased by the presence of EGTA (2 mM) in the reaction mixture, suggesting that the endogenous calcium activates nuclear DNA fragmentation. The present study demonstrates that calcium transport system in the liver nuclei is impaired by liver injury with CCl4 administration in rats.

Activatory effect of regucalcin on hepatic plasma membrane (Ca(2+)-Mg2+)-ATPase is impaired by liver injury with carbon tetrachloride administration in rats

The alteration of the plasma membrane (Ca(2+)-Mg2+)-ATPase activity in the liver of rats administered orally carbon tetrachloride (CCl4) solution was investigated. Rats received a single oral administration of CCl4 (10, 25 and 50%, 1.0 ml/100 g body weight), and 3 or 24 h later they were sacrificed. CCl4 administration caused a remarkable elevation of liver calcium content and a corresponding increase in liver plasma membrane (Ca(2+)-Mg2+)-ATPase activity, indicating that the increased Ca2+ pump activity is partly involved in calcium accumulation in liver cells. Moreover, the participation in regucalcin, which is an intracellular activating factor on the enzyme, was examined by using anti-regucalcin IgG. The plasma membrane (Ca(2+)-Mg2+)-ATPase activity increased by CCl4 administration was not entirely inhibited by the presence of anti-regucalcin IgG (1.0 and 2.5 ug/ml) in the enzyme reaction mixture. However, the effect of regucalcin (0.25-1.0 uM) to activate (Ca(2+)-Mg2+)-ATPase in the liver plasma membranes of normal rats was not revealed in the liver plasma membranes obtained from CCl4-administered rats. Also, the effect of regucalcin was not seen when the plasma membranes were washed with 1.0 mM EGTA, indicating that the disappearance of regucalcin effect is not dependent on calcium binding to the plasma membranes due to liver calcium accumulation. Now, the presence of dithiothreitol (5 mM) or heparin (20 ug/ml) caused a remarkable elevation of the plasma membrane (Ca(2+)-Mg2+)-ATPase activity in the liver obtained from CCl4-administered rats. Thus, the regucalcin effect differed from that of dithiothreitol or heparin. The present study suggests that the impairment of regucalcin effect on Ca2+ pump activity in liver plasma membranes is partly contribute to hepatic calcium accumulation induced by liver injury with CCl4 administration.

Ca-efflux, from direct membrane injury by CCl4, is elicited by amphiphilic vehicles in vitro

Direct membrane injury by CCl4, in situations excluding metabolic activation, was evaluated in saponin-permeabilized hepatocytes and in microsomes by measuring immediate Ca2+ efflux. A good correlation appears between the Ca2+ efflux and the level of CCl4 in the membrane and also the variations in fluidity. Mixtures of CCl4 with water-soluble vehicles were used to improve the dispersion of CCl4 in the medium. The mixtures varied in their ability to elicit the membrane effects of CCl4. The performance of ethanol and, to a lesser degree, other alcohols, suggests the existence of a water stable structural organization between CCl4 and these amphiphilic vehicles, facilitating the transfer of CCl4 to the membrane.

In vivo and in vitro evidence concerning the role of lipid peroxidation in the mechanism of hepatocyte death due to carbon tetrachloride

Isolated rat hepatocytes exposed to CCl4 showed a stimulated formation of malonaldehyde after only 30-60 min incubation. Conversely, the onset of hepatocyte death was a relatively late event, being significant only after 2-3 h of treatment. A cause-effect relationship between the two phenomena has been demonstrated by using hepatocytes isolated from rats pretreated with alpha-tocopherol. Comparable results were obtained in vivo where supplementation with alpha-tocopherol 15 h before CCl4 dosing induced a partial or complete protection against the drug's necrogenic effect, depending on the concentration of the haloalkane used. Moreover, the vitamin supplementation prevented the CCl4-induced increase of liver total calcium content, probably by blocking alterations in the liver cell plasma membranes due to lipid peroxidation.

Calcium staining by the glyoxal-bis-(2-hydroxyanil)-method in the livers of rats treated with CCl4, diltiazem, and with both agents together

We studied the histochemistry of Ca in livers treated with CCl4, diltiazem (one of the Ca antagonists), and both agents together to determine whether hepatocytes or other parts of the liver lesions show Ca staining and whether the grade or location of Ca in these injuries varies. For Ca staining, cryostat sections were treated by the glyoxal-bis-(2-hydroxyanil) (GBHA)-method using O.C.T. imbedding compound instead of paraffin. Diltiazem-treated rats showed Ca granules in the bile canaliculi around the terminal hepatic veins and Kupffer cells 6 h after intragastric injection. Rats treated with CCl4 showed fine red granules in the cytoplasm of the hepatocytes around the terminal hepatic veins as soon as 5 min mildly and 2 h moderately after intraperitoneal injection. Hepatocytes around the terminal hepatic veins showed positive Ca granules 6 to 30 h after intragastric injection of CCl4. Hepatocytes stained by Ca showed acidophilic degeneration and coagulative necrosis. The hepatocytes of rats treated with both diltiazem and CCl4 revealed fewer Ca granules than those treated with CCl4 alone. In summary, Ca was stained by the GBHA method from the early stage of liver injury by CCl4 and was closely involved in acidophilic degeneration and coagulative necrosis of hepatocytes. The Ca staining in liver cells in CCl4-treated rats was decreased by diltiazem.

Mechanism of the lethal interaction of chlordecone and CCl4 at non-toxic doses

There is significant interest in the possibility of unusual toxicity due to interaction of toxic chemicals upon environmental or occupational exposures even though such exposures may involve levels ordinarily considered harmless individually. While many laboratory and experimental models exist for such interactions, progress in this area of toxicology has suffered for want of a model where the interactants are individually non-toxic. We developed such a model where prior exposure to non-toxic levels of the pesticide Kepone (chlordecone) results in a 67-fold amplification of CCl4 lethality in experimental animals. The mechanism(s) by which chlordecone amplifies the hepatotoxicity of halomethanes such as CCl4, CHCl3, and BrCCl3 has been a subject of intense study. The biological effects of this interaction include extensive hepatotoxicity characterized by histopathological alterations, hepatic dysfunction, and perturbation of related biochemical parameters. Close structural analogs of chlordecone such as mirex and photomirex do not share the propensity of chlordecone to potentiate halomethane toxicity. Mechanisms such as induction of microsomal cytochrome P-450 by chlordecone and greater lipid peroxidation are inadequate to explain the remarkably powerful potentiation of toxicity and lethality. Time-course studies in which liver tissue was examined 1-36 h after CCl4 administration were conducted. While animals receiving a normally nontoxic dose of CCl4 alone show limited hepatocellular necrosis by 6 h, proceeding to greater injury after 12 h, recovery phase ensues as revealed by greatly increased number of mitotic figures. Such regeneration and hepatic tissue repair processes are totally suppressed in animals exposed to chlordecone prior to CCl4. Thus, the arrested hepatocellular repair and renovation play a key role in the potentiation of CCl4 liver injury by chlordecone. These findings have allowed us to propose a novel hypothesis for the mechanism of chlordecone amplification of halomethane toxicity and lethality. While limited injury is initiated by the low dose of CCl4 by bioactivation followed by lipid peroxidation, this normally recoverable injury permissively progresses due to arrested hepatocellular regeneration and tissue repair processes. Recent studies designed to test this hypothesis have provided additional supporting evidence. Hepatocellular regeneration stimulated by partial hepatectomy was unaffected by 10 ppm dietary chlordecone, while these animals were protected from the hepatotoxic and lethal actions of CCl4 if administered at the time of maximal hepatocellular regeneration. The protection was abolished when CCl4 was administered upon cessation of hepatocellular regeneration.

Effect of acetaminophen hepatotoxicity on hepatic mitochondrial and microsomal calcium contents in mice

The effect of toxic doses of acetaminophen on hepatic intracellular calcium compartmentation were studied in mice. No effects on the calcium contents of the mitochondria, microsomes or cytosol were observed 4 h after the administration of 175 and 375 mg/kg acetaminophen when compared to saline-treated controls. However, doses of 500 and 750 mg/kg of acetaminophen increased mitochondrial calcium contents at this time. Also, the 750 mg/kg dose caused marked alterations in the calcium contents of microsomal and cytosolic compartments. The time-course of the onset of these effects was examined using a 500 mg/kg dose. No changes in either mitochondrial, microsomal or cytosolic calcium contents were observed in the livers of mice treated with acetaminophen compared to saline-treated controls at either 1 or 2 h after dose administration. However, at 3, 4 and 24 h after acetaminophen, mitochondrial and cytosolic calcium contents were significantly increased above control values. The increases in mitochondrial and cytosolic calcium contents observed in the acetaminophen-intoxicated mouse liver appear to occur at the same time as the appearance of plasma membrane damage, as measured by sorbitol dehydrogenase leakage. The data suggest that a perturbation in hepatic calcium compartmentation is not an early event in acetaminophen-induced hepatotoxicity in the mouse.

Early disturbance of calcium translocation across the plasma membrane in toxic liver injury

An increased influx and/or a decreased extrusion of calcium across the plasma membrane resulting in an increase in cytosolic-free calcium could play an important role in the initiation of irreversible cell injury. Therefore, the translocation of calcium across the plasma membrane was probed in the perfused rat liver using multiple indicator dilution methodology. The sucrose space corresponding to the extracellular space amounted to 0.35 +/- 0.13 ml per gm liver, and the water space corresponding to the extra- and intracellular spaces was 0.97 +/- 0.08 ml per gm. The calcium space was always slightly larger (0.42 +/- 0.10 ml per gm) than the sucrose space. The calcium space further increased during perfusion with the calcium ionophore A 23187, whereas the sucrose space remained unchanged. Two hours after administration to intact rats of acetaminophen (2 gm per kg) and carbon tetrachloride (2 ml per kg), respectively, the calcium space had increased markedly relative to the sucrose space and relative to the water space, indicating an increased accessibility of the cells to extracellular calcium. Similarly, reperfusion of livers after 90 min of ischemia was associated with an increase in calcium space relative to the sucrose and water spaces. These studies indicate that, in three models of acute liver injury, the net influx of calcium across the plasma membrane is increased early in the evolution of the injury before irreversible damage occurs.

Cytosolic calcium after carbon tetrachloride, 1,1-dichloroethylene, and phenylephrine exposure. Studies in rat hepatocytes with phosphorylase a and quin2

Carbon tetrachloride (CCl4) and 1,1-dichloroethylene (DCE), both hepatotoxins, inhibit sequestration of Ca2+ by rat liver endoplasmic reticulum (ER) both in vivo and in vitro. It is possible that, as a result, cytosolic Ca2+ concentrations rise in liver cells. In experiments presented here, isolated hepatocytes were exposed to CCl4, DCE, and phenylephrine (PE), a non-hepatotoxic alpha 1-adrenergic agent that mobilizes Ca2+. Cytoplasmic Ca2+ concentrations were evaluated by two methods: indirectly by assaying the activity of glycogen phosphorylase a, and directly by monitoring the fluorescence of quin2. In primary hepatocyte cultures, CCl4, DCE, and PE exposure increased the activity of phosphorylase a at 5 min from 39 +/- 2 to 130 +/- 12, 80 +/- 13, and 97 +/- 10 nmoles PO4(3-)/mg protein/min respectively. In rat hepatocyte suspensions loaded with quin2 and exposed to CCl4, DCE, or PE, cytosolic Ca2+ concentrations were elevated within 20 sec to 0.83 +/- 0.13, 0.59 +/- 0.06 and 0.99 +/- 0.14 microM Ca2+ respectively. Basal Ca2+ levels in these cells averaged 0.25 +/- 0.03 microM. Thus, CCl4 and PE apparently increased cytosolic Ca2+ levels to approximately the same extent, whereas DCE was somewhat less effective. The durations of the effects of CCl4 and PE were examined further by determining their time courses of elevated phosphorylase a activity. In hepatocyte cultures, increased phosphorylase a activity persisted through at least 60 min following CCl4 exposure. In contrast, phosphorylase a activity returned to basal levels by 20 min after PE. Increases in cytoplasmic Ca2+ levels that are sustained rather than transient may distinguish these hepatotoxic chlorinated aliphatic hydrocarbons from non-toxic hormonal agents.

Subcellular calmodulin distribution in rat liver after CCl4 poisoning

Disturbed cellular calcium homeostasis has been observed during CCl4 poisoning, with an increase in calcium content 1 h after administration. Intracellular increase of calcium may be expected to alter membrane/cytosol distribution of calmodulin (CaM). This paper investigates changes in rat liver subcellular CaM distribution 30 min, 1 h and 2 h after CCl4 intoxication. The whole liver value remained unchanged, whereas the nuclear fraction increased and the microsomal and cytosolic fraction decreased. This may suggest that CaM is involved in the several liver cell alterations caused by CCl4 poisoning.

Ca2+ ions, an allosteric activator of calcium uptake in rat liver mitochondria

In a previous investigation, I have shown that the kinetics of the Ca uniporter change fundamentally when mitochondria have transitorily lost their membrane potential. The sigmoidal kinetics, usually observed in liver mitochondria, became almost hyperbolic. This means an increase in the affinity for calcium, and hence a considerable acceleration of Ca uptake in the range of low, e.g., physiological calcium concentration. In this investigation I show that extramitochondrial calcium released from the deenergized mitochondria causes the allosteric activation of the Ca uniporter. The dependence of the allosterical activation on the extramitochondrial Ca2+ concentration and on time is described. It is also reported that it is possible to activate allosterically the Ca uniporter of energized mitochondria by a short-term elevation of the extramitochondrial Ca2+ concentration. The process of activation is reversible. It is quickly reversed by the addition of chelators for Ca2+, and it is slowly reversed when the activating Ca2+ has to be removed by the mitochondrial Ca uniporter, though the bulk of extramitochondrial calcium is taken up by it very quickly. Several kinetics of the Ca uniporter are described. The implications of continually changing kinetics of the Ca uniporter are considered for carbon tetrachloride intoxication and the action of alpha 1-adrenergic agonists in liver cells.

Inhibition of liver endoplasmic reticulum calcium pump by CCl4 and release of a sequestered calcium pool

One of the earliest effects observed in rat liver after CCl4 administration is inhibition of an ATP-dependent calcium pump found at the endoplasmic reticulum. This report confirms that the amount of calcium associated with the microsomal fraction is reduced after CCl4 administration and, for the first time, demonstrates time-, dose-, and metabolism-dependent relationships between inhibition of the liver microsomal calcium pump and the amount of calcium found in the microsomal fraction. Furthermore, release of calcium from the endoplasmic reticulum is shown to cause activation of a cytoplasmic enzyme that responds to increases of ionized calcium, glycogen phosphorylase. This suggests that the endoplasmic reticulum calcium pump sequesters an intracellular pool of calcium within the endoplasmic reticulum. This pool of calcium may be released into the cytoplasm as a consequence of inhibition of the calcium pump by CCl4.

Mechanisms responsible for carbon tetrachloride-induced perturbation of mitochondrial calcium homeostasis

Incubation of isolated hepatocytes with CCl4 results in early reduction of the intracellular calcium content, mostly due to loss from the mitochondrial compartment. CCl4 treatment directly affects mitochondrial functions as indicated by the inhibition of Ca2+ uptake in cells permeabilized to the ion by digitonin exposure and by the reduction of intracellular ATP content in hepatocytes incubated in a glucose-free medium. Such mitochondrial damage is not caused by CCl4-induced stimulation of lipid peroxidation since it is not prevented by alpha-tocopherol, used at a concentration able to inhibit completely peroxidative reactions without interfering with CCl4 activation. All data together are in favour of a direct action of CCl4-reactive metabolites on liver cell calcium homeostasis.

Pathological mechanisms in carbon tetrachloride hepatotoxicity

Liver cell injury induced by carbon tetrachloride involves initially the metabolism of carbon tetrachloride to trichloromethyl free-radical by the mixed function oxidase system of the endoplasmic reticulum. It is postulated that secondary mechanisms link carbon tetrachloride metabolism to the widespread disturbances in hepatocyte function. These secondary mechanisms could involve the generation of toxic products arising directly from carbon tetrachloride metabolism or from peroxidative degeneration of membrane lipids. The possible involvement of radical species such as trichloromethyl (.CCl3), trichloromethylperoxy (.OOCCl3), and chlorine (.Cl) free radicals, as well as phosgene and aldehydic products of lipid peroxidation, as toxic intermediates is discussed. Data do not support the view that an increase in cytosolic free calcium is important in the toxic action of carbon tetrachloride or bromotrichloromethane. In addition, carbon tetrachloride-induced inhibition of very low density lipoprotein secretion by hepatocytes is not a result of elevated levels of cytosolic free calcium.

Evidence against involvement of calcium in carbon tetrachloride-dependent inhibition of lipid secretion by isolated hepatocytes

Carbon tetrachloride (CCl4)-induced inhibition of very low density lipoprotein (VLDL) secretion was studied in isolated hepatocytes. The hypothesis that inhibition of secretion is due to altered calcium homeostasis following CCl4-dependent inhibition of endoplasmic reticulum calcium sequestration was investigated. Inhibition of VLDL secretion by CCl4 was not dependent on extracellular calcium, since inhibition occurred when extracellular calcium was reduced to 0.1 microM. CCl4 inhibited hepatocyte VLDL secretion more rapidly than it inhibited microsomal calcium sequestration. Further, the concentration of CCl4 that produced half-maximal inhibition of VLDL secretion was about one-half the concentration required to produce half-maximal inhibition of microsomal calcium sequestration. The calcium ionophore A23187 did not mimic the action of CCl4 in inhibiting VLDL secretion under conditions in which A23187 altered cellular calcium homeostasis. The results that an alteration of calcium homeostasis is not involved in inhibition of VLDL secretion by carbon tetrachloride.

Excessive hepatic accumulation of intracellular Ca2+ in chlordecone potentiated CCl4 toxicity

The possible role of Ca2+ in chlordecone potentiation of CCl4 hepatotoxicity was examined in male Sprague-Dawley rats. The rats were maintained on a diet containing either 0, 10, 25, 50 or 100 ppm chlordecone for 15 days. On day 15, they received a single i.p. injection of corn oil (1 ml/kg) or CCl4 (100 microliter/kg) in corn oil vehicle. The animals were killed at 1, 6, or 12 h after the oil or CCl4 challenge for hepatic Ca2+ determinations. Ca2+ in whole liver, mitochondria, microsomes or in cytosolic fraction was unaltered in any group of animals receiving chlordecone + oil treatments, indicating that chlordecone alone does not alter whole liver content or hepatic subcellular distribution of Ca2+, even after exposure to toxic levels (50 or 100 ppm). Administration of CCl4 at an otherwise non-toxic dose to chlordecone treated animals resulted in significant increases of whole liver and subcellular Ca2+ as compared to chlordecone alone and CCl4 alone with a characteristic biphasic response. These increases were significant at all 3 time points in whole liver, cytosolic and mitochondrial fractions. Microsomal Ca2+ increased only at 12 h after CCl4. The increases were all progressive with increases in dietary levels of chlordecone, indicating that chlordecone-induced sensitivity is responsible for CCl4 elicited perturbations in whole liver and intracellular Ca2+ levels. This study suggests that chlordecone modifies the liver plasma membrane to amplify the CCl4 elicited perturbations in hepatocellular Ca2+ homeostasis especially during 6-12 h after CCl4 administration. This perturbation of Ca2+ homeostasis may be related to the arrested repair and regeneration of damaged liver tissue leading to progressive deterioration observed in previous histomorphometric studies.