Role of calcium in cytotoxic injury of cultured hepatocytes

Hepatocytes in Primary Culture: An Alternative to LD50 Testing? Validation of a Predictive Model by Multivariate Analysis

The cytotoxicity of 30 chemicals was assessed in rat hepatocyte primary cultures using four methods: lactate dehydrogenase release, neutral red uptake, the MTT assay, and measurement of total protein content.

Comparison of the data obtained in vitro (IC50 values) and in vivo (LD50 values) resulted in a significant correlation (p<0.001) between IC50 values and intravenous LD50 values. The validity, as well as the predictability of the model, were determined by multivariate analysis (principal component analysis and correspondence analysis). The predictability area, expressed in IC50 values, was in the range of 0–l,500μg/ml and reached 95%, with a 75–100% confidence interval (p = 0.05).

Assessment of the cytotoxicity of 54 additional chemicals would provide a more accurate predictability limit around l,500μg/ml and the estimated predictability confidence interval could be reduced to 90–100%.

Calcium-deficient diet attenuates carbon tetrachloride-induced hepatotoxicity in mice through suppression of lipid peroxidation and inflammatory response

The aim of this study is to investigate whether a Ca-deficient diet has an attenuating effect on carbon tetrachloride (CCl4)-induced hepatotoxicity. Four-week-old male ddY mice were fed a Ca-deficient diet for 4 weeks as a part of the experimental protocol. While hypocalcemia was observed, there was no significant change in body weight. The CCl4-exposed hypocalcemic mice exhibited a significant decrease in alanine aminotransferase and aspartate aminotransferase activities at both 6 h and 24 h even though markers of renal function remained unchanged. Moreover, lipid peroxidation was impaired and total antioxidant power was partially recovered in the liver. Studies conducted in parallel with the biochemical analysis revealed that hepatic histopathological damage was attenuated 24 h post CCl4 injection in hypocalcemic mice fed the Ca-deficient diet. Finally, this diet impaired CCl4-induced inflammatory responses. Although upregulation of Ca concentration is a known indicator of terminal progression to cell death in the liver, these results suggest that Ca is also involved in other phases of CCl4-induced hepatotoxicity, via regulation of oxidative stress and inflammatory responses.

Effects of extracellular Ca2+ and HCO3- on epidermal growth factor-induced DNA synthesis in cultured rat hepatocytes

BACKGROUND/AIMS

The elevation of cytosolic free calcium concentration ([Ca2+]i) and intracellular pH mediate the growth factor-initiated proliferation of many cells, but it is not known if they trigger mitosis in resting hepatocytes. The maintenance of [Ca2+]i and intracellular pH depends partly on extracellular calcium concentration ([Ca2+]e) and extracellular bicarbonate concentration ([HCO3-]e). Therefore, the effects of [Ca2+]e and [HCO3-]e on hepatocyte proliferation were examined.

METHODS

Epidermal growth factor induced proliferation in primary cultures of rat hepatocytes. [3H]thymidine incorporation into DNA and nuclear labeling indices were measured.

RESULTS

Between 0.2 and 0.9 mmol/L of [Ca2+]e, the proliferative response to epidermal growth factor increased, and total hepatocellular Ca2+ content was increased. Increasing [HCO3-]e also stimulated DNA synthesis in a concentration-dependent manner, maximal at 35 mmol/L. Using optimal [Ca2+]e (0.9 mmol/L) and [HCO3-]e (35 mmol/L), a synergistic stimulation of hepatocellular DNA synthesis was shown. Voltage-dependent Ca2+ channel blockers failed to inhibit hepatocyte proliferation when administered in concentrations that inhibit proliferation in other cell types.

CONCLUSIONS

[Ca2+]e and [HCO3-]e are both essential for hepatocyte proliferation, and their effects are synergistic. The entry of extracellular Ca2+ is critical for epidermal growth factor-induced DNA synthesis in hepatocytes, but this is not mediated by voltage-dependent Ca2+ channels.

A digitized fluorescence imaging study of intracellular free calcium, mitochondrial integrity and cytotoxicity in rat renal cells exposed to ionomycin, a calcium ionophore

The objective of this study was to explore the role of extracellular Ca2+ and mitochondrial integrity in ionomycin-induced cytotoxicity in primary cultures of rat kidney cortical epithelial cells using digitized fluorescence imaging (DFI), which is a powerful tool for continuously observing the dynamic intracellular biochemistry of single living cells. Using DFI, intracellular free calcium ion concentration ([Ca2+]i), mitochondrial membrane potential and loss of cell viability in individual rat renal cortical epithelial cells were examined temporally by fura-2, rhodamine 123 (Rh-123) and propidium iodide (PI), respectively. Images were taken within 10 min after exposure to 5 and 10 microM ionomycin. These three parameters, [Ca2+]i, mitochondrial membrane potential and cell viability, were also measured in populations of cells by a multiwell fluorescence scanner with fluo-3, Rh-123 and PI, respectively. Cytotoxicity was also assessed by two colorimetric cytotoxicity tests (LDH leakage and mitochondrial MTT reduction). Using DFI, the fluorescence scanner and the colorimetric cytotoxicity tests, we found that exposure of primary cultures of rat kidney cortical epithelial cells to high concentrations of ionomycin (5 and 10 microM) caused a rapid and sustained rise in [Ca2+]i, which preceded dissipation of the mitochondrial membrane potential and loss of cell viability and that chelation of extracellular Ca2+ with EGTA attenuated these responses. We demonstrated the value of using DFI to continuously observe the dynamic intracellular biochemistry of single living cells by establishing a sequence of elevated [Ca2+]i, dissipation of mitochondrial membrane potential and cytotoxicity. We conclude that a combination of the influx of extracellular Ca2+ and loss of mitochondrial integrity may be responsible for the cytotoxicity observed in individual renal cells and populations of renal cells after treatment with ionomycin.

Xenobiotic-modulated expression of hepatic glutathione S-transferase genes in primary rat hepatocyte culture

CYP 2B1/B2 and 1A1 expression in primary rat hepatocytes plated on a substratum of Vitrogen using Chee's Essential Medium has been reported to be responsive to xenobiotic treatment (Jauregui, H.O., Ng, S.F., Gann, K.L. and Waxman, D.J. (1991) Xenobiotica 21, 1091-1106). Class alpha, mu and pi glutathione S-transferase (GST) gene expression in response to xenobiotic treatment using this primary hepatocyte culture system was examined and the results compared with those obtained for P4502B1/B2 and 1A1 expression. Cytosolic GST activity decreased approx. 75% during the first 48 h of culture relative to freshly isolated hepatocytes and subsequently, increased, attaining a level at 96 h that was 134% of the activity at 48 h post-plating. Treatment of the hepatocyte cultures with phenobarbital (2 mM) or 3-methylcholanthene (5 microM) for 24, 48, or 72 h, beginning 24 h after plating, resulted in significant increases in glutathione S-transferase activity relative to control, with maximal increases of 158 and 164% measured at 72 h following phenobarbital or 3-methylcholanthrene treatment, respectively. SDS-PAGE analysis of cytosolic proteins showed a substantial increase in the intensities of protein bands migrating in the region of the GSTs following phenobarbital, beta-naphthoflavone or 3-methylcholanthrene treatment. Immunoblot analysis of cytosolic fractions using affinity-purified class-specific GST IgGs confirmed that alpha, mu and pi-class GST isozymes were elevated approx. 1.5- to 2-fold following phenobarbital, or beta-naphthoflavone treatment; 3-methylcholanthrene was less effective in enhancing GST expression in cultured hepatocytes as compared to phenobarbital or beta-naphthoflavone. Although GST pi was below the limit of detection in freshly-isolated hepatocytes, enhanced expression of this form was observed in untreated hepatocytes cultured for longer than 72 h. Immunoblot analysis of microsomal fractions revealed that cytochrome P-4502B1/2B2 and 1A1 levels were increased significantly in hepatocyte cultures treated with phenobarbital or 3-methylcholanthrene, respectively, relative to the undetectable levels found in untreated controls. Northern blot analysis of poly(A)+ mRNA isolated from cultures that had been treated with phenobarbital or 3-methylcholanthrene showed an approx. 2- and 4-fold increase in the expression of alpha and pi class glutathione S-transferase mRNAs, respectively, as compared to untreated cells. The level of P-4501A1 or 2B1 mRNA was also markedly elevated following 3-methylcholanthrene or phenobarbital treatment, respectively. The results of this study demonor the first time, that expression of alpha, mu and pi-class glutathione S-transferase genes is effectively modulated in primary yet culture system by different classes of xenobiotics.

Role of lipid metabolism in cell killing by calcium plus ionophore A23187

Cultured fibroblasts treated with divalent cation ionophore A23187 in the presence of extracellular calcium provide a useful model system for studying mechanisms of cell death associated with elevated intracellular calcium concentrations. Cell death induced by A23187 plus calcium can be conveniently monitored as membrane permeabilization to Trypan blue dye. Because lipids are a major component of cell membranes and play an important role in determining membrane permeability, the present study was initiated to identify changes in cell lipid composition that occur during membrane permeabilization induced by calcium plus A23187. The percent label in each of the major structural lipids in biosynthetically labeled NIH3T3 fibroblasts changed less than 10% during the time course of membrane permeabilization. During the course of membrane permeabilization there was significantly increased label in lysophosphatidylinositol and lysophosphatidylcholine and reduced label in phosphatidylinositol 4,5-bisphosphate. The time course of these changes corresponded to that of the arachidonic acid release response stimulated by calcium plus A23187, not to the time course of membrane permeabilization, which occurs later. These observations are consistent with lipid metabolism induced by A23187 plus calcium playing only a possible regulatory or intermediatory role in membrane permeabilization, rather than causing direct permeabilization of the lipid phase of the membrane.

Cell calcium, vitamin E, and the thiol redox system in cytotoxicity

The controversial role of extracellular Ca2+ in toxicity to in vitro hepatocyte systems is reviewed. Recent reports demonstrate that extracellular Ca2+-related cytotoxicity is dependent on Ca2+-influenced vitamin E (alpha-tocopherol) content of isolated hepatocytes. Based on a Ca2+-omission model of in vitro oxidative stress, the role of vitamin E in cytotoxicity is further explored. This model demonstrates the interdependence of the GSH redox system and vitamin E as protective agents during oxidative stress. Following chemical oxidant-induced depletion of intracellular GSH, cell morphology and viability are maintained by the continuous presence of cellular alpha-tocopherol above a threshold level of 0.6-1.0 nmol/10(6) cells. alpha-Tocopherol threshold-dependent cell viability is directly correlated with the prevention of the loss of cellular protein thiols in the absence of intracellular GSH. Potential mechanisms for this phenomenon are explored and include a direct reductive action of alpha-tocopherol on protein thiyl radicals, and the prevention of oxidation of protein thiols by scavenging of lipid peroxyl radicals by alpha-tocopherol. It is suggested that in light of the threshold phenomenon of vitamin E prevention of potentially severe oxidative stress-induced cytotoxicity, its use as a protective agent against an oxidative challenge in vivo should be reassessed.

Reactive intermediates and their toxicological significance

Many chemicals that cause toxicity do so via metabolism to biologically reactive metabolites. However, the nature of the interaction between such reactive metabolites and various cellular components, and the mechanism(s) by which these interactions eventually lead to cell death are poorly understood. The relative importance of macromolecular alkylation (covalent binding), lipid peroxidation, alterations in thiol, calcium and energy homeostasis are discussed with reference to specific toxicants. It is concluded that the cytotoxic effects of reactive metabolites are a consequence of simultaneous and/or sequential alterations in several cellular processes. Further studies are required to determine the relationship between these alterations and cell death.

Extracellular Ca2+-dependent elevation in cytosolic Ca2+ potentiates HgCl2-induced renal proximal tubular cell damage

While normal fluctuations of cytosolic Ca2+ ([Ca2+]i) occur physiologically, the deregulation of cellular Ca2+ homeostasis leads to cellular injury. The contribution of [Ca2+]i to the process of cellular damage was assessed in a model system where HgCl2 was used to induce plasma membrane damage in renal tubular cells. In the presence of 1.37 mM extracellular Ca2+, HgCl2 (10-50 microM) induced a slow, dose-dependent, 4-6 fold increase in [Ca2+]i (as measured by Quin 2) by 10 min of exposure, which could be abolished by prior incubation of the cells with dithiothreitol. Correlates of cellular injury, i.e., decrease in cell viability, change in cellular morphology, such as bleb formation, membrane distortion and mitochondrial swelling, were induced after HgCl2 addition. The rate and dose-responses of these changes were similar to that of [Ca]i elevation. When cells were exposed to HgCl2 in the absence of added extracellular Ca2+, there was no increase in [Ca2+]i and both the rate and extent of cell damage were reduced. When Ca2+ was readded to the extracellular medium after HgCl2, there was a rapid elevation of [Ca2+]i, increased cell killing and bleb formation. The observed correlation between [Ca2+]i elevation, decreased cell viability and morphological aberrations in terms of (i) dose-dependency for HgCl2, (ii) requirement for high extracellular Ca2+, and (iii) rate of change, suggests that HgCl2-induced renal cell damage involves the entry of Ca2+ from the extracellular milieu which potentiates the progression of cellular injury.

HgCl2-induced changes in cytosolic Ca2+ of cultured rabbit renal tubular cells

Fura 2 was used to measure changes in cytosolic [Ca2+] ([Ca2+]i) in cultured rabbit kidney proximal tubule cells exposed to HgCl2. Treatment with 2.5-10 microM HgCl2 resulted in an extracellular [Ca2+] ([Ca2+]e)-independent 2- to 12-fold increase in [Ca2+]i above resting levels of about 100 nM. Treatment with 25-100 microM HgCl2 caused a rapid [Ca2+]e-independent 10- to 12-fold increase in [Ca2+]i within 1 min followed by a recovery to about 2-fold steady state by 3 min. With 25-100 microM HgCl2, both magnitude and rate of Ca2+ increase were similar, but recovery was greater with increasing doses. A slower, secondary increase in [Ca2+]i followed which varied with HgCl2 concentration and required [Ca2+]e. The first increase in [Ca2+]i represents release from intracellular pools. Calcium channel blockers, calmodulin inhibitors, and mitochondrial inhibitors do not alter the patterns of [Ca2+]i changes due to HgCl2. The recovery response with higher HgCl2 concentrations appears to be triggered by Hg2+ and not by the increased [Ca2+]i. Sulfhydryl modifiers N-ethylmaleimide, PCMB and PCMBS produced [Ca2+]e-independent [Ca2+]i increases similar to those induced by low HgCl2 concentrations. Cell killing with HgCl2 was about 50% greater with normal [Ca2+]e than with low [Ca2+]e, suggesting that [Ca2+]e influx is important in accelerating injury leading to cell death.

Relationship between cellular calcium and vitamin E metabolism during protection against cell injury

The extent of chemically induced injury to isolated hepatocytes has been previously shown to depend on the content of alpha-tocopherol in the cells, the levels of which are influenced by the concentration of extracellular calcium. Investigations into the effect of calcium on the alpha-tocopherol content of nonchemically exposed cells demonstrated that incubation of isolated hepatocytes in a calcium-deficient medium decreased cell calcium content to 10% of initial levels, and resulted in the depletion of endogenous alpha-tocopherol. This loss in alpha-tocopherol was not accounted for by alpha-tocopherylquinone formation. After supplementation of the cell incubation medium with alpha-tocopheryl succinate, the decreased cell calcium content was associated with higher levels of cellular alpha-tocopherol than in calcium-adequate cells. This was the result of greater intracellular hydrolysis of the tocopheryl ester in the calcium-depleted cells, and not an effect of extracellular calcium concentration on the uptake of alpha-tocopheryl succinate into the cells or on the extracellular hydrolysis of the ester. Uptake studies indicated a much greater achievable level of alpha-tocopherol in hepatocytes after incubation with alpha-tocopherol than with the alpha-tocopheryl ester. These data provide substantial support for the hypotheses that the content of extracellular calcium per se is not the determinant in toxic injury to hepatocytes, but that cell calcium content affects the intracellular metabolism of alpha-tocopherol and its esters, which may subsequently govern the outcome of a toxic challenge.

Stimulation of enzyme secretion from isolated pancreatic acini by Clostridium difficile toxin B

Exposure of isolated rat pancreatic acini to increasing concentrations (10 ng - 800 ng/ml) of toxin B from Clostridium difficile produced a biphasic effect on the rate of secretion of amylase, trypsinogen, and chymotrypsinogen. Whereas doses of toxin B from 10-30 ng/ml increased enzyme secretion by 15-20%, doses between 30 ng and 60 ng/ml showed a regression of this effect, whereafter the rate of secretion of amylase, trypsinogen, and chymotrypsinogen increased with increasing concentrations of the toxin. Toxin B concentration of 800 ng/ml enhanced amylase, trypsinogen and chymotrypsinogen secretion by 119%, 185% and 195%, respectively, when compared with the basal level. Stimulation of enzyme secretion by toxin B was not affected by the presence of either actinomycin-D or cycloheximide, at a concentration which inhibited acinar RNA or protein synthesis by 80-90%. Although toxin B as well as CCK8, carbachol and secretin by themselves caused significant stimulation in amylase, trypsinogen and chymotrypsinogen secretion from isolated pancreatic acini, toxin B together with either CCK8, carbachol or secretin produced no further augmentation in enzyme secretion than what was observed with the secretagogues alone. It is concluded that toxin B of Cl. difficile exerts a direct effect on pancreatic acinar cells as evidenced by stimulation of enzyme secretion.

The role of specific ions in glutamate neurotoxicity

When the chick embryo retina is incubated in balanced salt solution containing glutamate (Glu) in 1 mM concentration, a neurodegenerative reaction occurs within 30 min. Here we report that the neurotoxic action of Glu on retinal neurons is dependent on the presence of Na+ and Cl-, but not Ca2+, in the incubation medium. Also, we report that depolarizing concentrations of K+ can induce a severe cytotoxic reaction in chick retina which, like the depolarization-linked neurotoxicity of Glu, is a Cl- dependent phenomenon.

Calcium in lymphotoxin-mediated cytolysis: cellular pools, fluxes, and role of extracellular concentration

This study explores, by kinetic analysis, the movement of calcium among cellular pools of target cells in which cytotoxicity is induced by human lymphotoxin, and evaluates the requirement for calcium in this reaction. We employed the kinetic model of Borle to quantitate flux rates and pool sizes. It was found that the rate of flux between the surface (plasma membrane-glycocalyx) compartment and the intracellular compartment was greatly increased. The size of the total exchangeable intracellular calcium pool was not altered, but there was an apparent decrease in the size of the surface calcium pool. This latter phenomenon may be related to the blebbing and exfoliation of plasma membranes under the influence of the lymphokine. Lymphotoxin-induced cytotoxicity is observed in calcium-free medium and over a range of calcium concentrations. These results argue against cell death due to a massive in rush of calcium into the cell under these circumstances.

Role of glutathione depletion in the cytotoxicity of acetaminophen in a primary culture system of rat hepatocytes

A primary culture system of postnatal rat hepatocytes was utilized to study the cytotoxicity of acetaminophen and the toxicological significance of glutathione (GSH) depletion. The relative time of onset and magnitude of GSH depletion, lipid peroxidation and cytotoxicity were contrasted in order to gain insight into their interrelationships. Exposure of the hepatocytes to acetaminophen resulted in time- and dose-dependent depletion of cellular GSH. The acetaminophen-induced GSH depletion and ensuing lactate dehydrogenase (LDH) leakage were quite modest and delayed in onset, in contrast to that caused by iodoacetamide (IAA) and by diethylmaleate (DEM), 2 well-known depletors of GSH. There was comparable LDH leakage, irrespective of drug treatment, when GSH levels decreased to about 20% of normal. Reduction of GSH levels below the 20% threshold by IAA treatment resulted in marked LDH leakage and loss of viability. Maximal LDH leakage in response to IAA and acetaminophen preceded maximal malondialdehyde (MDA) formation, suggesting that lipid peroxidation may be a consequence of cell damage as well as GSH depletion. IAA and DEM produced a comparable, modest accumulation of MDA, yet IAA was much more cytotoxic. These findings indicate that lipid peroxidation does not play a central role in hepatocellular injury by compounds which deplete GSH, although it may contribute to degeneration of the cell. As events in the cultured postnatal hepatocytes paralleled those reported in vivo, the system can be a useful and valid model with which to study mechanisms of chemical toxicity.

Erythromycin estolate-induced toxicity in cultured rat hepatocytes

Primary cultures of rat hepatocytes were exposed to several concentrations of erythromycin estolate (EE). Hepatotoxicity was evaluated using lactate dehydrogenase (LDH) leakage and morphometric analysis of representative populations of cells examined optically. Results of the two techniques provided parallel information: cells exposed to the higher concentrations of EE had significantly greater LDH release and higher percentages of morphologically damaged cells. Planimetric analysis of a second set of hepatocytes showed increasing swelling of cells with increasing concentration of EE. Severe cellular swelling preceded disintegration, as hepatocytes became progressively more damaged by EE.

An in vitro approach to the study of target organ toxicity of drugs and chemicals

A major goal of our laboratory has been the development of primary culture systems that retain differentiated functions and responses characteristic of intact tissues in vivo. Specifically, we have developed cellular models of primary cultures of rat heart, liver, and kidney cells to explore the mechanisms by which drugs or chemicals may be toxic to key organs of the body and to develop new techniques by which xenobiotics may be evaluated or identified as potential toxicants to living systems. The purpose of this paper is to describe our rationale and approach to the study of target organ toxicology with in vitro cellular systems.

Mechanism of chemical-induced toxicity. II. Role of extracellular calcium

Previous studies disagree as to if chemical-induced cell death is caused by the influx and accumulation of extracellular Ca2+. To determine the role of extracellular Ca2+ in toxic cell death, the viability (leakage of intracellular K+ and lactate dehydrogenase) and total Ca2+ content of isolated hepatocytes incubated in the presence or absence of extracellular Ca2+ were determined during a toxic insult with bromobenzene, ethyl methanesulfonate (EMS), Ca2+ ionophore A23187, and adriamycin (ADR) in combination with 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). The present study utilized the dibutyl phthalate separation technique which enabled the analysis of only viable hepatocytes for changes in intracellular Ca2+ and K+ content during toxic cell injury. The three chemical treatments, bromobenzene, EMS, and ADR-BCNU, each caused an accelerated loss of viability in hepatocytes incubated without extracellular Ca2+ as compared to cells incubated with Ca2+. Furthermore, the total Ca2+ content of viable hepatocytes incubated in the presence of extracellular Ca2+ did not increase during chemically induced cell injury as compared to control cells. In fact, a significant decline in total cellular Ca2+ was observed in viable hepatocytes incubated in Ca2+-free medium during toxic cell injury. Treatment with Ca2+ ionophore A23187 was also toxic to hepatocytes incubated in the presence or absence of extracellular Ca2+. At high concentrations of ionophore (20 microM or 4 micrograms/10(6) cells), cell death was accelerated in hepatocytes incubated with Ca2+ as compared to cells incubated in Ca2+-free medium. In contrast, after treatment with lower concentrations of ionophore (10 microM or 2 micrograms/10(6) cells), the rate of cell death was reversed with hepatocytes incubated without extracellular Ca2+ dying first. Thus, depending on the concentration of A23187 and the time of exposure, the presence of extracellular Ca2+ can be shown either to accelerate or protect against cell death. Surprisingly, reversible and irreversible cell injury were not observed in hepatocytes incubated with extracellular Ca2+ and 2 microM A23187 though this treatment resulted in an 800% increase in total intracellular Ca2+ content. We conclude that chemical-induced hepatic cell death is not caused by an increase in total cellular Ca2+ resulting from the influx of extracellular Ca2+.

Effects of cadmium and calcium on the fluidity of plasma membranes

The fluidity of plasma membranes was assessed by steady-state fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH), a fluorescent probe. The presence of increasing concentrations of calcium (Ca) (0.5-4 mM), cadmium (Cd) (50-500 microM), or both decreased the motional freedom of the fluorescent probe molecules in plasma membranes derived from both human erythrocytes and rat hepatocytes. The effects of Cd were 3-10 times greater than those of Ca. Increasing concentrations of Cd in the presence of Ca increased the anisotropy parameter, which plateaued at lower Cd concentrations. The presence of Ca diminished the overall effects of Cd on these membranes.

In vivo effects of 1,3-bis(2-chloroethyl)-1-nitrosourea and doxorubicin on the cardiac and hepatic glutathione systems

Doxorubicin and 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) are anti-cancer drugs which have been used together in combination therapy of certain cancers. Each drug has been reported to affect intracellular glutathione stores and together, doxorubicin and BCNU have been shown to exert synergistic toxicity and to deplete completely the glutathione content of isolated hepatocytes. Cardiac and hepatic glutathione reductase activity was significantly inhibited following treatment in vivo with BCNU. Treatment of mice with both doxorubicin and BCNU resulted in increased mortality compared to either drug alone. There was, however, no depletion of hepatic or cardiac glutathione levels in vivo beyond that seen with either BCNU or doxorubicin alone. Diethyl maleate, a known glutathione depletor whose effects are enhanced by BCNU in vitro, also was unable to increase GSH depletion after BCNU in vivo. These discrepancies between in vivo and in vitro studies may be due to the presence of more effective compensatory mechanisms in the whole animal, or to differences in the metabolism and inactivation of these drugs.

Calcium influx accompanies but does not cause excitotoxin-induced neuronal necrosis in retina

Several authors have recently proposed that excessive calcium (Ca++) influx into postsynaptic cells may be the mechanism by which excitotoxins such as glutamate (Glu), N-methylaspartate (NMA) and kainic acid (KA) cause neuronal necrosis. Here we have undertaken both in vivo and in vitro studies to explore this hypothesis. Our findings indicate that Ca++ does accumulate selectively in neural elements undergoing degeneration in the in vivo mouse hypothalamus following subcutaneous administration of NMA. However, pretreatment with the putative Ca++ channel blocker nimodipine resulted in augmentation rather than suppression of the toxic action of NMA and Glu on the mouse hypothalamus and eliminating Ca++ from the incubation medium did not interfere with the toxic action of Glu, NMA or KA on the chick embryo retina in vitro. We conclude, therefore, that Ca++ influx is an unlikely explanation for excitotoxin-induced degeneration of retinal or hypothalamic neurons.

Vitamin E reversal of the effect of extracellular calcium on chemically induced toxicity in hepatocytes

Isolated rat hepatocytes were incubated in the presence or absence of extracellular calcium and alpha-tocopherol succinate with three different toxic chemicals; namely, adriamycin in combination with 1,3-bis(2-chloroethyl)-1-nitrosourea, ethyl methanesulfonate, and the calcium ionophore A23187. In the absence of extracellular calcium these three compounds were far more toxic to the cells than in its presence. The addition of vitamin E to calcium-free medium, however, protected hepatocytes against toxic injury, whereas cells incubated in medium containing calcium were not protected. Hepatocyte viability during each toxic insult correlated well with the cellular alpha-tocopherol content but not with the presence or absence of extracellular calcium. These results suggest that cellular alpha-tocopherol maintains the viability of the cell during a toxic insult and that the presence or absence of vitamin E in the incubation medium probably explains the conflicting reports on the role of extracellular calcium in toxic cell death.

Calcium amelioration of cadmium-induced cytotoxicity in cultured rat hepatocytes

Parenchymal hepatocytes from neonatal rats were isolated, cultured about 24 h, exposed to cadmium with or without calcium, and processed for scanning electron microscopy. To assess the severity of cadmium-induced changes, exposed hepatocytes were categorized based upon the extent of morphological damage. Differences in surface blebbing, alterations in microvilli, variations in the degree of swelling, and changes in cell shape were used to categorize the severity of cell damage. A double-blind morphometric analysis (a geometricostatistical processing of two-dimensional data for the collection of three-dimensional information) of cellular changes was conducted for each exposure time and for each concentration of cadmium in the presence or absence of calcium. Significant decreases occurred in the percent relative volume of normal, flattened cells present in cultures exposed for 30 min to 50 or 100 microM cadmium in the absence of calcium. In contrast, the percent relative volume of severely damaged spherical cells was significantly increased after exposure to solutions containing 50 or 100 microM cadmium and lacking calcium. Percent relative volume of intermediate cells (which were slightly swollen and showed changes in microvillar number) was significantly increased following a 30 min exposure to all cadmium concentrations in the absence of calcium. The examination of hepatocytes exposed for 60 min showed that the degree of cadmium-induced cytotoxicity was more severe in the absence of calcium than was the case for the hepatocyte cultures exposed for 30 min: approximately 30% more spherical cells and 30% fewer flattened cells were present if cultures were exposed in the absence of calcium for 60 min compared to those exposed for 30 min. The degree of blebbing was significantly greater at all cadmium concentrations in the absence of calcium. The presence of calcium, therefore, reduced cadmium-induced cytotoxicity in primary cultures of rat hepatocytes subjected to morphometric analysis after scanning electron microscopy.

Cadmium-induced hepatotoxicity in cultured rat hepatocytes as evaluated by morphometric analysis

Freshly isolated hepatocytes from neonatal rats were cultured for approximately 24 h; incubated for 5, 30, or 60 min in solutions containing 0, 50, 100, or 200 microM cadmium; embedded in plastic; and sectioned for optical microscopy. The extent of cadmium-induced hepatotoxicity was evaluated by double-blind morphometric analysis (a geometricostatistical processing of two-dimensional data for the collection of three-dimensional information) whereby hepatocytes were classified on the basis of the severity of morphologic damage at the optical level. Both time and concentration effects were studied. Cultures exposed to 200 microM cadmium, for various intervals of time from 5 to 60 min, showed statistically significant reductions in the relative volume percent of normal hepatocytes, elevations (then reductions) in the relative volume percent of slightly damaged hepatocytes, increases in the relative volume percent of moderately damaged cells, and increases in the relative volume percent of severely damaged liver cells. As the concentration of cadmium was increased from 50 to 200 microM cadmium (during both 30 and 60-min exposures), significant trends were observed in cellular distribution patterns based on relative volume percent. Morphologically normal cells decreased, both slightly damaged and moderately damaged cells increased, and severely damaged cells remained unchanged. These results indicated that morphometric analysis at the optical level provided quantitative estimates for the evaluation of time- and concentration-effects of cadmium on cultured hepatocytes.

Extracellular calcium protects isolated rat hepatocytes from injury

The incubation of isolated rat hepatocytes in calcium-free medium resulted in a pronounced increase in lipid peroxidation, mitochondrial and cytoplasmic glutathione depletion, glutathione disulfide formation and efflux of reduced glutathione as compared with hepatocytes incubated in calcium containing medium. These data suggest that extracellular calcium ions serve a protective role in isolated rat hepatocytes against cell injury.