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

The transcriptional regulation of regucalcin gene expression

Regucalcin, which is discovered as a calcium-binding protein in 1978, has been shown to play a multifunctional role in many tissues and cell types; regucalcin has been proposed to play a pivotal role in keeping cell homeostasis and function for cell response. Regucalcin and its gene are identified in over 15 species consisting of regucalcin family. Comparison of the nucleotide sequences of regucalcin from vertebrate species is highly conserved in their coding region with throughout evolution. The regucalcin gene is localized on the chromosome X in rat and human. The organization of rat regucalcin gene consists of seven exons and six introns and several consensus regulatory elements exist upstream of the 5'-flanking region. AP-1, NF1-A1, RGPR-p117, β-catenin, and other factors have been found to be a transcription factor in the enhancement of regucalcin gene promoter activity. The transcription activity of regucalcin gene is enhanced through intracellular signaling factors that are mediated through the phosphorylation and dephosphorylation of nuclear protein in vitro. Regucalcin mRNA and its protein are markedly expressed in the liver and kidney cortex of rats. The expression of regucalcin mRNA in the liver and kidney cortex has been shown to stimulate by hormonal factors (including calcium, calcitonin, parathyroid hormone, insulin, estrogen, and dexamethasone) in vivo. Regucalcin mRNA expression is enhanced in the regenerating liver after partial hepatectomy of rats in vivo. The expression of regucalcin mRNA in the liver and kidney with pathophysiological state has been shown to suppress, suggesting an involvement of regucalcin in disease. Liver regucalcin expression is down-regulated in tumor cells, suggesting a suppressive role in the development of carcinogenesis. Liver regucalcin is markedly released into the serum of rats with chemically induced liver injury in vivo. Serum regucalcin has a potential sensitivity as a specific biochemical marker of chronic liver injury with hepatitis. Regucalcin has been proposed to be a key molecule in cellular regulation and metabolic disease.

Protective effect of selenium-enriched lactobacillus on CCl4-induced liver injury in mice and its possible mechanisms

AIM: To study the protective effects and mechanisms of Se-enriched lactobacillus on liver injury caused by carbon tetrachloride (CCl₄) in mice.

METHODS: Seventy-two ICR mice were randomly divided into four groups: normal group, CCl₄-induced model group, low Se-enriched lactobacillus treatment group (L-Se group), and high Se-enriched lactobacillus treatment group (H-Se group). During a 3-wk experimental period, the common complete diet was orally provided daily for normal group and model group, and the mice in L-Se and H-Se groups were given a diet with 2 and 4 mg of organoselenium from Se-enriched lactobacillus per kg feed, respectively. From the 2nd wk of experiment, the model group, L-Se group, and H-Se group received abdominal cavity injection of olive oil solution containing 500 mL/L CCl₄ (0.07 mL/100 g body mass) to induce liver injury, and the normal group was given olive oil on every other day for over 2 wk. In the first 2 wk post injection with CCl₄, mice in each group were killed. The specimens of blood, liver tissue, and macrophages in abdominal cavity fluid were taken. Then the activities of the following liver tissue injury-associated enzymes including glutathione peroxidase (GSH-Px), superoxide dismutase (SOD), alanine aminotransferase (ALT) and aspartate aminotransferase (AST) as well as malondialdehyde (MDA) content were assayed. Changes of phagocytic rate and phagocytic index in macrophages were observed with Wright-Giemsa stain. Plasma TNF-α level was measured by radioimmunoassay. The level of intracellular free Ca²⁺ ([Ca²⁺]i) in hepatocytes was detected under a laser scanning confocal microscope.

RESULTS: During the entire experimental period, the AST and ALT activities in liver were greatly enhanced by CCl₄ and completely blunted by both low and high doses of Se-enriched lactobacillus. The Se-enriched lactobacillus-protected liver homogenate GSH-Px and SOD activities were higher or significantly higher than those in model group and were close to those in normal group. CCl₄ significantly increased MDA content in liver homogenates, while administration of Se-enriched lactobacillus prevented MDA elevation. Phagocytic rate and phagocytic index of macro-phages decreased after CCl₄ treatment compared to those in normal control, but they were dramatically rescued by Se-enriched lactobacillus, showing a greatly higher phagocytic function compared to model group. CCl₄ could significantly elevate plasma TNF-α and hepatocyte [Ca²⁺]ilevel, which were also obviously prevented by Se-enriched lactobacillus.

CONCLUSION: Se-enriched lactobacillus can intervene in CCl₄-induced liver injury in mice by enhancing macrophage function activity to keep normal and beneficial effects, elevating antioxidant-enzyme activities and reducing lipid peroxidation reaction, inhibiting excessive release of TNF-α, preventing the dramatic elevation of [Ca²⁺]i in hepatocytes.

Evidence That 1,1-Dichloroethylene Induces Apoptotic Cell Death in Murine Liver

1,1-Dichloroethylene (DCE) causes dysfunction of hepatic mitochondria. As mitochondria have been implicated in apoptosis through opening of the permeability transition pore (PTP), we have undertaken studies to test the hypothesis that DCE induces apoptosis, in addition to necrosis, in murine liver. Our primary objective was to identify the biochemical events associated with DCE-induced apoptosis. Female CD-1 mice were treated with a mildly hepatotoxic dose of DCE (125 mg/kg, i.p.). Using the fluorescent dye JC-1 (5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolocarbocyanine iodide), decreased hepatic mitochondrial membrane potential was detected at 2 h. Western blotting of liver cytosolic proteins showed greater immunoreactivity for cytochrome c in fractions from mice treated with DCE for 4 h than in controls. Furthermore, caspase-9 activity was significantly increased 6 h after DCE exposure. Immunohistochemical studies with an antibody to activated caspase-3 and terminal deoxynucleotidyl transferase dUTP nick-end labeling staining were used to detect apoptotic cells. In both experiments, positive reactivities were observed in centrilobular hepatocytes 12 and 24 h after DCE. Additionally, centrilobular hepatocytes showing morphological criteria of apoptosis were observed at 24 h. Apoptosis and all apoptotic events were inhibited by pretreatment for 20 min with cyclosporine A (CyA) (50 mg/kg), a specific inhibitor of the mitochondrial PTP. To determine a major role for mitochondrial permeability transition (MPT) in DCE hepatotoxicity, serum alanine aminotransferase (ALT) activity was evaluated. ALT activity was significantly elevated 2 to 24 h after DCE, and CyA failed to inhibit this activity. These data suggested that DCE produces apoptosis by inducing MPT, causing release of cytochrome c into the cytosol and caspase activation.

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.

Cisplatin-induced nephrotoxicity in vitro: increases in cytosolic calcium concentration and the inhibition of cytosolic and mitochondrial protein kinase C

Exposure of rat kidney cortical slices to cisplatin (2 mM) significantly increased the activity of cytosolic phosphorylase a, indicating that the concentration of cytosolic Ca2+ was increased. Dithiothreitol (DTT), a sulphydryl reducing agent, markedly reversed this effect but N,N'-diphenylphenylenediamine (DPPD), an antioxidant, did not. Cisplatin inhibited protein kinase C (PKC) activity in both mitochondrial and cytosolic fractions after the slices were exposed to cisplatin. Both DTT and DPPD prevented these inhibitory effects of cisplatin on PKC but diethylmaleate, a glutathione depletor, potentiated this inhibitory effect. These results suggest that the increase in cytosolic Ca2+ is related to depletion of SH-groups, but independent of lipid peroxidation, whereas inhibition of PKC may be associated with cisplatin-induced depletion of thiols and with lipid peroxidation.

Regulation of gamma-glutamylcysteine synthetase by protein phosphorylation

We previously reported that the activity of gamma-glutamylcysteine synthetase (GCS; EC 6.3.2.2), the rate-limiting enzyme in GSH synthesis, can be acutely inhibited approximately 20-40% by agonists of various signal transduction pathways in rat hepatocytes [Lu, Kuhlenkamp, Garcia-Ruiz and Kaplowitz (1991) J. Clin. Invest. 88, 260-269]. We have now examined the possibility that GCS is phosphorylated directly by activation of protein kinase A (PKA), protein kinase C (PKC) and Ca2+/calmodulin-dependent kinase II (CMK). Phosphorylation of GCS was studied using both purified rat kidney GCS and cultured rat hepatocytes by immunoprecipitating the reaction product with specific rabbit anti-(rat GCS heavy subunit) (anti-GCS-HS) antibodies. All three kinases, PKA, PKC and CMK, phosphorylated rat kidney GCS-HS in a Mg(2+)-concentration-dependent manner, with the highest degree of phosphorylation occurring at 20 mM Mg2+. The maximum incorporation of phosphate in mol/mol of GCS was 1.17 for PKA, 0.70 for PKC and 0.62 for CMK. The degree of phosphorylation was correlated with the degree of loss of GCS activity, and no additional inhibition occurred when GCS was phosphorylated by all three kinases, suggesting that the kinases phosphorylated the same site(s). Phosphoamino analysis showed that all three kinases phosphorylated serine and threonine residues. Two-dimensional phosphopeptide mapping demonstrated that all three kinases phosphorylated the same five peptides, both PKA and PKC phosphorylated two other peptides, and only PKA phosphorylated one additional peptide. Phosphorylation of GCS decreased its Vmax for cysteine and glutamate without changing its K(m). Finally, treatment of cultured rat hepatocytes with dibutyryl cAMP and phenylephrine significantly increased the phosphorylation of GCS, suggesting a potentially important physiological role. In summary, we have demonstrated that GCS is phosphorylated and suggest that phosphorylation/dephosphorylation may regulate GCS activity.

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.

Effect of carbon tetrachloride on inositol 1,4,5-trisphosphate dependent and independent regulation of rat brain microsomal Ca2+ flux

Carbon tetrachloride (CCl4) is a highly toxic industrial solvent with pronounced effects on the liver and brain. CCl4 is enzymatically cleaved to produce free radicals which attack membrane components, including proteins. Earlier reports indicated that CCl4 affects Ca(2+)-regulated events in the brain. Hence, the present study was initiated to determine whether CCl4 affects inositol 1,4,5-trisphosphate (IP3) receptor binding, free-Ca2+ movements across the microsomal membrane and protein kinase C (PKC) activity in rat brain, since IP3, Ca2+ and PKC are known to be involved in signal transduction. [3H]IP3 binding, free-Ca2+ movements and 45Ca2+ uptake were determined using rat brain microsomes and PKC activity was determined in the cytosolic fraction. CCl4 in vitro decreased [3H]IP3 binding to microsomes. IP3 mediated Ca2+ release from microsomes was inhibited and also the reuptake of IP3-released Ca2+ into microsomes was decreased in the presence of CCl4. CCl4 at concentrations < 2 microM independently released Ca2+ from microsomes. Uptake of total Ca2+ into microsomes was inhibited by CCl4 as observed with 45Ca(2+)-uptake studies. CCl4 at 1 microM inhibited PKC activity by 50%. Thus, perturbations in the binding of IP3 to its receptor sites, changes in the Ca2+ flux across the microsomal membrane and modulation of PKC activity by CCl4 in vitro suggested that CCl4 may exert neurotoxicity by altering signal transduction pathways.

Hepatic calcium-binding protein regucalcin in released into the serum of rats administered orally carbon tetrachloride

The change in calcium-binding protein regucalcin, mainly localized in liver, in the liver and serum of rats received a single oral administration of carbon tetrachloride (50%; 1.0 ml/100 g body weight) was investigated. The change of regucalcin mRNA levels in the liver was analyzed by Northern blotting using liver regucalcin cDNA (0.6 kb). At 10 and 24 h after the administration, liver regucalcin mRNA levels were reduced markedly. Moreover, regucalcin concentrations in the liver and serum was estimated by enzyme-linked immunoadsorbent assay (ELISA) with rabbit-anti-regucalcin IgG. Administration of carbon tetrachloride (CCl4) induced a significant decrease in liver regucalcin concentration and a corresponding elevation of serum regucalcin concentration at 24 h after the administration. An appreciable increase in serum regucalcin concentration was seen at 2 h after the administration. Meanwhile, serum transaminases (GOT and GPT) activities were significantly increased by CCl4 administration, indicating that liver injury is induced. The present study demonstrates that hepatic regucalcin is released into the serum of rats administered orally CCl4, suggesting that the estimation of serum regucalcin is a useful tool for diagnosis of liver injury.

Carbon tetrachloride-induced cell death in perfused livers from phenobarbital-pretreated rats under hypoxic conditions and various ionic milieu. Further evidence for calcium-dependent irreversible changes

The role of Ca2+ in the initiation of carbon tetrachloride (CCl4) hepatotoxicity was studied using perfused livers isolated from phenobarbital-pretreated rats in a single-pass system. Krebs-Henseleit bicarbonate buffer containing 1.3 mM CaCl2 (KHB) was the regular ionic milieu. In the liver perfused with fructose-supplemented regular KHB equilibrated with 95% N2-5% CO2, infusion of 0.5 mM CCl4 caused an early uptake of Ca2+ coupled with K+ leakage and Na+ uptake within the infusion time of 30 min, which was followed by a marked lactic dehydrogenase (LDH) leakage into the effluent perfusate and further Ca2+ uptake by the liver. With Ca(2+)-free medium, the prenecrotic K+ leakage and the successive LDH leakage were suppressed markedly. However, a perfusate exchange from regular to Ca(2+)-free KHB at the end of the prenecrotic stage did not protect against the LDH leakage, and the perfusate exchange conversely did not produce LDH leakage. Perfusion of the liver with high K+(Cl-) medium under 20% O2 markedly suppressed CCl4-induced LDH leakage even in the presence of Ca2+, whereas once CCl4 had acted under regular KHB perfusion, changing the medium to high K+ did not further prevent the LDH leakage. High K(+)-lactobionic acid medium containing Ca2+ and supplemented with fructose also suppressed LDH leakage under 95% N2 without the accompanying prenecrotic Ca2+ uptake. However, a change of the medium after CCl4 infusion to regular KHB containing Ca2+ caused LDH leakage and K+ leakage, with Ca2+ uptake. The prevention of LDH leakage in a different ionic milieu may not be due to suppression of CCl4 bioactivation, since the liver cytochrome P450 content decreased to a similar extent. These findings suggest that entry of extracellular Ca2+ into hepatocytes coupled with K+ leakage and Na+ entry is a prerequisite for CCl4-induced hepatocyte death and that association of Ca2+ with a CCl4-derived radical-mediated process may be necessary for early and irreversible plasma membrane damage.

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.

Do intracellular Ca2+ activity and hepatic glutathione play a role in the pathogenesis of lithocholic acid-induced cholestasis?

The possible relevance of alterations in intracellular Ca2+ and hepatic glutathione levels (GSH) in the pathogenesis of cholestasis induced by lithocholic acid (LCA) was examined by comparing effects of LCA and acetaminophen on these parameters and bile flow (BF) in rats. Intracellular Ca2+ activity was measured via glycogen phosphorylase a determination in rats given an intravenous bolus injection of either LCA (12 mumol/100 g body wt.), acetaminophen (60 mg/100 g body wt.), or a mixed solution of LCA and acetaminophen. BF was reduced immediately after LCA administration, with a maximum decrease occurring at 60 min followed by an increase to normal values at 210 min. On the other hand, glycogen phosphorylase a activity was elevated during all time periods after LCA treatment. Hepatic glutathione followed the BF curves being markedly depleted at the peak of cholestasis (60 min) and normal in the total recovery period (210 min). In contrast, acetaminophen had no effect on BF but significantly increased glycogen phosphorylase a activity and depleted hepatic glutathione levels. These results suggest that cholestatic effect of LCA is not due to changes in intracellular Ca2+ or hepatic glutathione levels.

Effects of 1-[(2-thiazolin-2-yl)amino]acetyl-4-(1,3-dithiol-2-ylidene)- 2,3,4,5-tetrahydro-1H-1-benzazepin-3,5-dione hydrochloride (KF-14363) on active oxygen production

The effects of KF-14363 on active oxygen production and membrane stabilization were studied. KF-14363 did not affect hypotonic hemolysis (10% and 70%) and did not inhibit lipid peroxide production induced by t-butyl hydroperoxide at concentrations of less than 100 microM. KF-14363 significantly inhibited active oxygen production in peritoneal exudate cells (PEEC) stimulated with arachidonic acid, A23187 and carbon tetrachloride (CCl4) at concentrations over 10 microM, 100 microM and 1 microM, respectively. It tended to inhibit formyl-methionyl-leucyl-phenylalanine-stimulated production of active oxygen in PEEC at concentrations over 10 microM, but there was no significant difference owing to large dispersion. Superoxide dismutase (SOD, 10(4) U/ml) significantly inhibited CCl4-stimulated production of active oxygen in PEEC. KF-14363 inhibited the radical production from CCl4 in the presence of a 9000 x g supernatant fraction of the rat liver which was administered with enzyme induction compounds (S9 mix). SOD (10(4) U/ml) was not effective in this system. In conclusion, KF-14363 inhibited active oxygen production in PEEC induced by various stimulants and also the radical formation from CCl4 in the presence of S9 mix solution.

Effects of oxygen deficiency and calcium omission on carbon tetrachloride hepatotoxicity in isolated perfused livers from phenobarbital-pretreated rats

The effect of oxygen concentration and Ca2+ omission on CCl4-induced hepatotoxicity was studied in a non-recirculating and hemoglobin-free liver perfusion system using phenobarbital-pretreated rats. With 95% O2-saturated perfusate, infusion of 0.5 mM CCl4 caused an instantaneous increase of thiobarbituric acid reactive substances (TBA-RS) in the effluent perfusate, accompanied by only a slight leakage of K+ and lactate dehydrogenase (LDH). CBrCl3 produced a far greater increase in the TBA-RS level, but again with slight K+ and LDH leakage. With 20% O2-saturated perfusate, CCl4 caused a marked LDH leakage, which was preceded by an early and considerable increase in K+ leakage coupled with Na+ uptake, Ca2+ uptake was initially slight, being enhanced concurrently with the LDH leakage. The TBA-RS level changed biphasically with an initial moderate and a succeeding greater increase coupled with LDH leakage. N,N"-Diphenyl-p-phenylenediamine and promethazine suppressed the TBA-RS production, but improved neither K+ nor LDH leakage. Omission of the Ca2+ from the perfusate reduced the initial K+ leakage as well as the later TBA-RS release, and markedly delayed the LDH leakage. In retrograde perfusion under low oxygen supply with Ca2+, CCl4 produced essentially the same toxic manifestations as those observed in the anterograde perfusion. Hepatocytes of the periportal and pericentral areas were not stained with trypan blue in the antero- and retrograde perfusion systems respectively. Thus, oxygen deficiency, rather than lipid peroxidation by itself, and the essential role of extracellular Ca2+ may be important for CCl4-induced hepatic cell necrosis, in which plasma membrane permeability change may be an early and critical event.

Modification of the inhibitory effects of CCl4 on phospholipid and protein biosynthesis by prostacyclin

CCl4 induced cellular injury and its modification by prostacyclin (PGI2) was studied in cultured rat hepatocytes. Biosynthesis of both intracellular and serum proteins and that of phospholipids decreased upon CCl4 treatments (IC50 7.0, 2.5 and 3.2 mM, respectively). After 1 hr exposure of the cells to CCl4, the reductions in the biosynthesis increased further with time. PGI2 treatments (10(-5)-10(-9) M) of the hepatocytes subsequent to CCl4 poisoning resulted in partial recovery from the cell injury evaluated at the fifth hour of the experiment. Optimal effects of PGI2 were found at a concentration of 10(-7)-10(-8) M, while higher and lower concentrations offered less protection. Upon the addition of CCl4 a higher catabolic rate of PIP2 and an increased formation of inositol phosphates were observed. This alteration was shown to precede the defects in the labelling of the major phospholipid components. Furthermore, these changes were circumvented in the presence of PGI2. Thus, PIP2 metabolism appears to be a critical process in the mechanism of this type of cellular injury and its protection by PGI2.

Mechanisms of creatine kinase release from isolated rat skeletal muscles damaged by propylene glycol and ethanol

The organic cosolvents propylene glycol and ethanol are found to cause skeletal muscle damage and creatine kinase release following intramuscular injection. The mechanisms of this organic cosolvent-induced enzyme release have not been elucidated. Cosolvent-induced creatine kinase release was enhanced by the addition of calcium to the incubation medium, and inhibited, albeit modestly, by dibucaine, a nonspecific phospholipase A2 inhibitor. The temporal pattern of creatine kinase release further suggested that cosolvent-induced enzyme release from skeletal muscles may be caused by an intracellular mechanism rather than by a direct solubilization of sarcolemma. This intracellular mechanism may involve the mobilization of calcium.

Dissociation between myosin phosphorylation and shortening velocity in canine trachea

The relationship between glycogen phosphorylase activity (an index of cytosolic Ca2+ content), myosin light-chain phosphorylation, isotonic shortening velocity, and isometric tension was examined in canine trachealis. Responses were measured in tracheal strips contracted with various concentrations of methacholine or K+. Both agonists produced prolonged and concentration-dependent increases in isometric tension that reached 90% of the plateau level within 1 (methacholine) to 5 (K+) min and remained stable over 60 min. In contrast to the monotonic increase in isometric tension, shortening velocity reached a maximum almost immediately (12-48 s) after the addition of either methacholine or K+ and then declined over time to a steady-state level that was 25-40% of the peak. Phosphorylase activity also increased transiently, reaching a maximum 1-2 min after the addition of either agonist before declining to near-basal levels over the 60-min observation period. Unlike the increases in shortening velocity and phosphorylase activity, agonist-induced myosin phosphorylation was not markedly transient. Moreover, regardless of the contractile agonist used, no correlation was found between myosin phosphorylation and shortening velocity when these parameters were compared at corresponding time points. This suggests that myosin phosphorylation is not the sole determinant of shortening velocity in canine trachealis.

Activation of glycogen phosphorylase in rat pheochromocytoma PC12 cells and isolated hepatocytes by organophosphates

Several organophosphates including diisopropylfluorophosphonate (DPF) and a variety of compounds used as chemical warfare agents produced dose- and time-dependent increases in phosphorylase-a, the phosphorylated form of glycogen phosphorylase in rat pheochromocytoma cells, PC12, and isolated hepatocytes. Increases in phosphorylase-a did not occur in cells exposed to the carbamates, physostigmine or pyridostigmine, or to O-ethyl S-2-diisopropylaminoethylmethyl-phosphonathiolate (VX), an organophosphate which is protonated at physiological pH. When extracellular pH was increased to pH 8, VX acted like the other organophosphates and increased phosphorylase-a activity. The possibility that organophosphates increase phosphorylase-a in intact cells by releasing Ca2+ from intracellular binding sites is supported by the following findings: organophosphate-induced increases in phosphorylase-a did not correlate with changes in cyclic AMP in the two cell types studied; in PC12 cells, increases in this activity occurred in the absence of extracellular calcium and were not inhibited by the calcium channel blocker, verapamil; fluorescence of the calcium sensitive dye, Quin-2, in PC12 cells preloaded with the acetoxymethyl ester of the dye was increased by soman; finally, addition of the calcium ionophore, A23187, to PC12 cells maintained in calcium-free medium caused sarin-stimulated phosphorylase-a activity to return rapidly to basal levels. Collectively, these data argue strongly that organophosphates increase phosphorylase-a activity in intact cells via a novel mechanism involving release of calcium from intracellular binding sites.

Effects of carbon tetrachloride on calcium homeostasis. A critical reconsideration

The incubation of isolated rat hepatocytes with 0.172 mM carbon tetrachloride caused a rapid decrease in the calcium content of both mitochondrial and extramitochondrial compartments. However, the release of Ca2+ from the intracellular stores was not associated with an increase in the cytosolic Ca2+ levels as measured by activation of phosphorylase alpha or by Quin-2 fluorescence. A rapid rise in hepatocyte free calcium was only observed with concentrations of CCl4 higher than 0.172 mM. The lack of activation of phosphorylase alpha was not due to the inhibition of the enzyme by CCl4, since in CCl4-treated hepatocytes the phosphorylase activity could be stimulated by glucagon, butyryl--cAMP or by the increase of cell calcium induced by the addition of A23187. Ca2+-dependent ATPase of plasma membranes was only slightly affected in the early phases of poisoning with CCl4 when both mitochondrial and extramitochondrial calcium pools were already lowered. This led to the conclusion that calcium released from intracellular organelles could be extruded from the cells in sufficient amounts to prevent the increase of the cytosolic levels. A rise in hepatocyte free calcium was observed during the second hour of incubation with CCl4, concomitantly with the appearance of both LDH leakage and plasma membrane blebbing. The addition of EGTA to the medium prevented both the increase in cytosolic Ca2+ and the blebbing suggesting that they were a consequence of an influx of calcium into the cells. However, neither EGTA nor the addition of inhibitors of calcium-dependent phospholipase A2 or non-lysosomal proteases were able to protect against cell death. These latter results suggested that the alterations of calcium distribution induced by CCl4 in isolated hepatocytes were not a primary cause of the toxic effects, although they did not exclude that a sustained rise in cytosolic Ca2+ could contribute in the progression of cell injury.

Interaction of hypoxia and carbon tetrachloride toxicity in hepatocyte monolayers

The toxicity of carbon tetrachloride (CCl4) in monolayer cultures of primary hepatocytes was investigated at oxygen concentrations that prevail in the liver under conditions that range from normoxia to hypoxia: 0.5, 1, 2, and 20% O2. CCl4 was administered in the vapor phase at concentrations that produce aqueous concentrations at 37 degrees C of 0.4, 2.0, and 4.0 mM. Damage was assayed by leakage of aspartate transaminase and the inclusion of Trypan Blue immediately after the 2-hr incubation and after an additional 6-hr incubation in 20% O2. Only in the case of 0.5% O2 and 4 mM CCl4 were the monolayers damaged (18%) immediately after the 2-hr exposure; all other exposed cells were undamaged at that time point and the dose response of cell death as a function of CCl4 and oxygen concentration was not evident until the 6-hr time point. The monolayers exposed to 4 mM CCl4 and 1, 2, or 20% O2 exhibited little immediate damage but were all 100% dead 6 hr later. The monolayers exposed to 2 mM CCl4 and 0.5, 1, 2, or 20% O2 were 53, 48, 40, and 22 +/- 2% dead after 6 hr, respectively. These results suggest that effects of CCl4 exposure, for example alterations in the function or synthesis of essential proteins, require several hours to affect cell viability.

Halocarbon hepatotoxicity is not initiated by Ca2+-stimulated endonuclease activation

Previous studies from this laboratory have demonstrated that cytosolic Ca2+ rapidly rises to supraphysiologic levels in liver cells exposed to the hepatotoxins carbon tetrachloride (CCl4) and 1,1-dichloroethylene (DCE) in vivo and in vitro. The present study examines whether this increase in intracellular Ca2+ activates endonucleases that could initiate or contribute to the ensuing hepatotoxic events. Initial experiments demonstrated that there was no generalized breakdown of hepatic DNA in intact rats exposed to CCl4 and DCE, as assessed by the appearance of nucleosomal fragments in liver nuclear DNA separated on agarose gels. Nor was generalized fragmentation observed in DNA isolated from primary hepatocyte cultures exposed to halocarbons, except at very late times following loss of plasma membrane integrity. Endonuclease activation was further examined at a more sensitive level by specifically monitoring hypersensitive sites (HSS) in serum albumin gene. Actively transcribed genes, such as albumin in liver tissue, are extremely sensitive to attack by exogenous nucleolytic enzymes at discrete sites. We speculated that subtle halocarbon-induced endonuclease activation would first become evident at these sites. To locate HSS, DNA was digested with restriction enzymes Eco R1 or Hind III, electrophoresed on agarose gels, blotted onto nitrocellulose, and hybridized to a 32P-labeled 1400 bp rat albumin genomic clone. No cleavage at hypersensitive sites was detected in DNA isolated from rat liver or hepatocyte DNA at early times when elevations of Ca2+ were developing. Thus, these data indicate that endonuclease activation by intracellular Ca2+ and resultant nucleolytic destruction of DNA is not an early event in the hepatotoxicity produced by halocarbons.

Three models of free radical-induced cell injury

Three models of free radical-induced cell injury are presented in this review. Each model is described by the mechanism of action of few prototype toxic molecules. Carbon tetrachloride and monobromotrichloromethane were selected as model molecules for alkylating agents that do not induce GSH depletion. Bromobenzene and allyl alcohol were selected as prototypes of GSH depleting agents. Paraquat and menadione were presented as prototypes of redox cycling compounds. All these groups of toxins are converted, during their intracellular metabolism, to active species which can be radical species or electrophilic intermediates. In most cases the activation is catalyzed by the microsomal mixed function oxidase system, while in other cases (e.g. allyl alcohol) cytosolic enzymes are responsible for the activation. Radical species can bind covalently to cellular macromolecules and can promote lipid peroxidation in cellular membranes. Of course both phenomena produce cell damage as in the case of CCl4 or BrCCl3 intoxication. However, the covalent binding is likely to produce damage at the molecular site where it occurs; lipid peroxidation, on the other hand, besides causing loss of membrane structure, also gives rise to toxic products such as 4-hydroxyalkenals and other aldehydes which in principle can move from the site of origin and produce effects at distant sites. Electrophilic intermediates readily reacts with cellular nucleophiles, primarily with GSH. The result is a severe GSH depletion as in the case of bromobenzene or allyl alcohol intoxication. When the depletion reaches some threshold values lipid peroxidation develops abruptly and in an extensive way. This event is accompanied by cellular death. The reason for which lipid peroxidation develops in a cell severely depleted of GSH remains to be clarified. Probably the loss of the defense systems against a constitutive oxidative stress is not compatible with cellular life. Some free radicals generated by one-electron reduction can react with oxygen to give superoxide anions which can be converted to other more dangerous reactive oxygen species. This is the case of paraquat and menadione. Damage to cellular macromolecules is due to the direct action of these oxygen radicals and, at least in the menadione-induced cytotoxicity, lipid peroxidation is not involved. All these initial events affect the protein sulfhydryl groups in the membranes. Since some protein thiols are essential components of the molecular arrangement responsible for the Ca2+ transport across cellular membranes, loss of such thiols can affect the calcium sequestration activity of subcellular compartments, that is the capacity of mitochondria and microsomes to regulate the cytosolic calcium level.(ABSTRACT TRUNCATED AT 400 WORDS)

Evaluation of the calcium mobilizing action of acetaminophen and bromobenzene in rat hepatocyte cultures

Acetaminophen (APAP) and bromobenzene (BrB) are reported to selectively inhibit plasma membrane (PM) but not endoplasmic reticulum (ER) Ca2+ transport in rat liver (1). The ability of these hepatotoxicants to increase cytoplasmic Ca2+ levels as a result of disrupted Ca2+ pumping was determined in cultured rat hepatocytes by monitoring the activity of glycogen phosphorylase alpha, a Ca2+ -sensitive (via phosphorylase kinase) enzyme. Following exposure to 2.5 to 10 mM APAP for five minutes, dose-dependent increases in phosphorylase alpha activity were observed (58 to 190 U/g protein). Endoplasmic reticulum Ca2+ pump activity was not inhibited after any dose of APAP (56 nmol Ca2+ per milligram protein per 30 minutes). Phosphorylase alpha activity remained elevated for 60 minutes after exposure to APAP (124 microliters/g protein). Following exposure to 0.5 to 2 mM BrB for five minutes, phosphorylase alpha activity also increased (58 to 229 U/g protein) in a dose-related manner. Endoplasmic reticulum Ca2+ pump activity was inhibited after BrB exposure (from 58 to 16 nmol Ca2+ per milligram protein per 30 minutes). Phosphorylase alpha activity remained elevated for 60 minutes after exposure to BrB (147 U/g protein). Evidence of elevated cytoplasmic Ca2+ is consistent with the inhibition of Ca2+ -extruding/sequestering mechanisms at hepatocyte PM and/or ER. Prolonged elevation of cytosolic Ca2+ levels could overstimulate Ca2+ -sensitive processes within liver cells and thus initiate or contribute to hepatotoxic injury.

Liver cell calcium homeostasis in carbon tetrachloride liver cell injury: new data with fura2

The calcium fluorescent probe fura2 was used to measure concentration of free calcium in the cytosol of isolated rat hepatocytes in suspension. The resting level in untreated hepatocytes was 121 nM. On addition of CCl4 at a concentration of 0.5 mM, cytosolic free calcium rose sharply and reached a statistically significant (P less than 0.05) steady plateau level of about 190 nM within five minutes. With a concentration of 1.0 mM CCl4, cytosolic free calcium rose within ten minutes to a plateau level of about 200 nM. Use of fura2, along with the capacity of Mn2+ ions to effectively quench fura2 fluorescence, provided the basis for a simple and decisive method to determine whether the added CCl4 was permeabilizing the hepatocyte plasma membrane by direct solvent action. It was found that up to a concentration of 1.0 mM, CCl4 did not permeabilize the plasma membrane, but direct attack on the plasma membrane was unequivocally demonstrated for concentrations of 2 mM CCl4 and above. Finally, an hypothesis is presented for resolution of the puzzling dilemma that emerged from the observation, reported from two laboratories, that CCl4 can rapidly mobilize liver mitochondrial calcium despite the well-known relative resistance of these organelles to the damaging effects of this toxic agent.