Temporal arteritis presenting as an extrapyramidal disorder

A case of temporal arteritis presenting with extrapyramidal symptoms (tremor, rigidity and extrapyramidal hypertonus) unresponsive to conventional treatment is here described. The onset of headache and laboratory abnormalities suggestive of temporal arteritis prompted a temporal artery biopsy which confirmed the diagnosis; the administration of corticosteroids led to the resolution of all symptoms.

Stable angina pectoris and controlled ischemia: what causes the abnormalities in whole blood filterability?

The determinants of the altered whole blood filterability observed during coronary ischemia are still under discussion. Since no studies have been carried out to date on what exactly causes these alterations during the early stages of controlled ischemia in coronary heart disease, a model was set up using a bicycle ergometer test (with a 25 W increase every 2 minutes). Blood samples were taken from 48 stable angina pectoris patients and from a group of 28 matched controls before and immediately after exercise and 8 minutes later. Plasma viscosity, the filterability (through 5 microns diameter pore filters) of whole blood, erythrocytes, and polymorphonuclear and mononuclear leukocytes (separated by density gradient) were monitored. Alterations in whole blood filterability could be linked only to an impairment in polymorphonuclear cell filterability in those stable angina pectoris patients who reported chest pain and/or whose ST segment depression was greater than or equal to 2 mm.

The cytoskeleton as a target in quinone toxicity

The exposure of mammalian cells to toxic concentrations of redox cycling and alkylating quinones causes marked changes in cell surface structure known as plasma membrane blebbing. These alterations are associated with the redistribution of plasma membrane proteins and the disruption of the normal organization of the cytoskeletal microfilaments which appears to be due mainly to actin cross-linking and dissociation of alpha-actinin from the actin network. The major biochemical mechanisms responsible for these effects seem to involve the depletion of cytoskeletal protein sulfhydryl groups and the increase in cytosolic Ca2+ concentration following the alkylation/oxidation of free sulfhydryl groups in several Ca2+ transport systems. Depletion of intracellular ATP is also associated with quinone-induced plasma membrane blebbing. However, ATP depletion occurs well after the onset of the morphological changes, and thus it does not seem to be causatively related to their appearance. Thiol reductants, such as dithiothreitol, efficiently prevent the oxidation of cytoskeletal protein thiols, the increase in cytosolic free Ca2+ concentration and cell blebbing induced by redox cycling, but not alkylating, quinones. These results demonstrate that alkylating and redox cycling quinones cause similar structural and biochemical modifications of the cytoskeleton by means of different mechanisms, namely alkylation and oxidation of critical sulfhydryl groups.

Calcium-dependent DNA fragmentation in human synovial cells exposed to cold shock

Exposure of confluent human synovial McCoy's cells to near-freezing temperatures followed by rewarming at 37 degrees C resulted in endonuclease activation and cell death characteristic of a suicide process known as apoptosis. Both DNA fragmentation and cell killing were dependent on a sustained increase in the cytosolic Ca2+ concentration. Sensitivity to cold shock-induced endonuclease activation was critically dependent on the cell cycle (proliferative) status and limited to confluent cells, whereas cells in the logarithmic growth phase were completely resistant. However, DNA fragmentation was promoted in the proliferating McCoy's cells pretreated with H-7 or sphingosine, inhibitors of protein kinase C. In addition, phorbol ester, known to activate PKC, inhibited DNA fragmentation in the confluent cells. Our findings indicate that cold shock-induced DNA fragmentation in McCoy's cells is dependent on a sustained Ca2+ increase, and sensitivity to the process appears to be regulated by the status of protein kinase C.

Leucocyte rheology in controlled coronary ischaemia

Since no studies have been carried out on the exact origin of the alterations in white blood cell rheology during the early stages of controlled ischaemia in coronary arterial disease, a model was set up using a cycle ergometer test (with a 25 watts increase every 2 minutes). Blood samples were taken (before and after exercise and again 8 minutes later at recovery) from 18 patients with stable angina pectoris and a group of 22 matched controls. The filterability (through 5 micrometer diameter pore filters) of the polymorphonuclear leucocyte sub-population (separated by density gradient), the monocyte and lymphocyte sub-fractions (separated by adhesion to Petri dishes) as well as leucocyte activation (observed under a light microscope) were monitored. Our results showed that the total leucocyte count in patients and controls rose after exercise and was accompanied by a differential shift from the polymorphonuclear to the lymphocyte cells. The polymorphonuclear filterability rate increased significantly in patients when compared to their basal values at rest, and to the controls after exercise (+ 19.58%; P less than 0.002 vs basal values at rest; + 18.72%; P less than 0.002 vs controls). This increase persisted throughout the recovery period (+ 19.86%; P less than 0.002 vs basal values; and + 23.52% P less than 0.001 vs controls), indicating that a reduced polymorphonuclear leucocyte filterability can be associated with the first signs of ischaemia.

Liver cytosolic non-dialysable factor(s) can counteract GTP-dependent Ca2+ release in rat liver microsomal fractions

Readdition to rat liver microsomes of dialysed liver post-microsomal supernatant resulted in an almost complete inhibition of the Ca2+-releasing effect of GTP. Such inhibition was heat-labile, and was associated with non-ultrafiltrable supernatant components with a molecular weight higher than 30,000 D. A preliminary fractionation of liver supernatant showed that the inhibitory effect is recovered in the 40-50% ammonium sulfate-precipitated proteins, with an approx. 10-fold enrichment. The active ammonium sulfate fraction did not modify the GTP-induced Ca2+ increase of passive Ca2+ efflux from microsomes, nor did it affect microsomal GTP hydrolysis, which is likely required for its Ca2+ releasing effect. The active ammonium sulfate fraction appears to markedly favour the translocation of GTP-released Ca2+ into a microsomal GTP-insensitive pool. Separation of liver microsomes in smooth and rough fractions revealed that such GTP-insensitive Ca2+ pool is almost completely associated with smooth microsomes.

Blood pressure and functional aspects of the aging brain

This study was carried out to define the effects of both long-term hypertension and hypotension on the cerebral functioning of the elderly, comparing them to the effects of normotension. Ninety-eight subjects of both sexes, between 70 and 82 years of age, were divided into three groups on the basis of their mean blood pressure: 33 normotensives, 36 hypotensives and 29 hypertensives. They underwent a neuropsychological assessment, a haemorrheological evaluation and an EEG monitoring while awake. The results show that the three parameters were almost always within normal limits in the normotensives, while EEG alterations, lower neurophysiological scores and blood hyperviscosity were noted in both the hypertensives and hypotensives. Our results would seem to confirm that long-term hypertension may induce alterations in the cerebral electrogenesis and intellectual functions of the elderly and that also long-term hypotension might negatively interfere with some functional aspects of the aging brain.

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.

Non-invasive cardiovascular monitoring in Shy-Drager syndrome

The pattern of arterial blood pressure and heart rate were investigated in a patient with progressive autonomic dysfunction and nineteen healthy age-matched controls. A marked postprandial reduction of blood pressure was observed in the patient; this phenomenon was not noted in the control group; besides, the rhythmicity of blood pressure and heart rate were altered with peak values shifted approximately eight hours later with respect to the controls.

On the role of mitochondria in cell injury caused by vanadate-induced Ca2+ overload

Incubation of isolated rat hepatocytes with vanadate (0.25, 0.5 and 1 mM) resulted in progressive accumulation of Ca2+ in the intracellular compartments. Vanadate- induced Ca2+ accumulation was related to inhibition of the plasma membrane Ca2+-extruding system, but did not involve either enhanced plasma membrane permeability to Ca2+ or the enhanced operation of a putative Na+/Ca2+ exchanger. After an initial rise in the cytosolic free Ca2+ concentration, as revealed by phosphorylase activation, Ca2+ was sequestered predominantly by the mitochondria with little contribution from the endoplasmic reticulum. As the amount of Ca2+ in the mitochondria increased, a progressive decrease in mitochondrial membrane potential occurred, together with an impairment of the ability of these organelles to further sequester Ca2+. Associated with this, there was a decrease in intracellular ATP level, formation of surface blebs and cytotoxicity. Addition of an uncoupler to vanadate-treated hepatocytes dramatically accelerated the appearance of plasma membrane blebs and toxicity. Our results demonstrate that under conditions in which the plasma membrane Ca2+ pump is inhibited, mitochondria play an important role in protecting hepatocytes against damage induced by Ca2+ overload.

Role of Ca2+ in toxic cell killing

Recent work has shown that a sustained increase in cytosolic Ca2+ concentration is often linked to the onset of cytotoxicity. Sten Orrenius and colleagues describe several biochemical mechanisms that are stimulated by such a Ca2+ increase and can directly mediate cell death by causing disruption of the cytoskeleton, DNA fragmentation and extensive damage to other cell components.

Calcium chelator Quin 2 prevents hydrogen-peroxide-induced DNA breakage and cytotoxicity

Exposure of cultured Chinese hamster ovary (CHO) cells to hydrogen peroxide results in the production of extensive DNA breakage which can be prevented by the intracellular calcium chelator Quin 2. This effect occurs at Quin 2 AM concentrations as low as 0.1 microM and is maximal at 1 microM. Addition of the extracellular calcium chelator, EGTA, does not affect the level of DNA breakage generated by H2O2. Quin 2 also significantly reduces cellular toxicity caused by the oxidant. Experiments with spin-trapping techniques demonstrate that Quin 2 does not affect the formation of hydroxyl radicals generated by the action of Fe2+ on H2O2. Quin 2 at high concentrations, similar to those reached within the cell, actually enhanced generation of hydroxyl radical in the absence of other iron chelators under our experimental conditions. These results suggest that H2O2 or H2O2-derived radicals do not directly induce DNA strand breakage in intact mammalian cells; rather, these radicals may disturb intracellular Ca2+ homeostasis which results in secondary reactions ultimately leading to DNA strand breakage. In addition to strand breakage, membrane and protein oxidation probably contribute to the cytotoxic effect of H2O2.

Cytoskeletal alterations in human platelets exposed to oxidative stress are mediated by oxidative and Ca2+-dependent mechanisms

The metabolism of the redox-active quinone, menadione (2-methyl-1,4-naphthoquinone), in human platelets was associated with superoxide anion production, oxidation and depletion of intracellular glutathione, and modification of protein thiols. The cytoskeletal fraction extracted from menadione-treated platelets exhibited a dose-dependent increase in the amount of cytoskeleton-associated protein and a concomitant loss of protein thiols. These alterations were associated with oxidative modifications of actin, including beta-mercaptoethanol-sensitive crosslinking of actin to form dimers, trimers, and high-molecular-weight aggregates which also contained other cytoskeletal proteins, i.e., alpha-actinin and actin-binding protein. In addition, analysis of the cytoskeletal fraction from platelets treated with high concentrations (greater than or equal to 100 microM) of menadione by polyacrylamide gel electrophoresis under reducing conditions revealed a net decrease in the relative abundance of the individual cytoskeletal polypeptides. Under the same incubation conditions the platelets exhibited a sustained increase in cytosolic Ca2+ concentration. The presence of glucose, or the omission of Ca2+ from the incubation medium, prevented both the increase in cytosolic Ca2+ and the decrease in the relative amounts of cytoskeletal proteins. The latter effect was also largely prevented in platelets loaded with Quin-2 tetraacetoxymethyl ester to buffer the menadione-induced elevation of cytosolic Ca2+. Finally, the presence of a protease inhibitor, leupeptin, in the incubation medium prevented the menadione-induced decrease in the amount of actin-binding protein but not the decrease in the other cytoskeletal proteins. Our findings demonstrate that the multiple effects of oxidative stress on the platelet cytoskeleton are mediated by oxidative as well as by Ca2+-dependent mechanisms.

Glucocorticoids activate a suicide process in thymocytes through an elevation of cytosolic Ca2+ concentration

Glucocorticoid hormones kill immature thymocytes through the induction of a suicide process commonly referred to as "apoptosis." A characteristic marker for this process is the stimulation of endogenous endonuclease activity which results in the extensive cleavage of cell chromatin. In an attempt to characterize the biochemical events involved in this process, we studied the role of Ca2+ in glucocorticoid-induced DNA fragmentation and cell killing in thymocytes. Treatment of thymocytes from immature rats with the synthetic glucocorticoid methylprednisolone resulted in extensive DNA fragmentation which was preceded by an early, sustained increase in cytosolic Ca2+ concentration. This increase in Ca2+ level was blocked by cycloheximide and actinomycin D, inhibitors of de novo protein and mRNA synthesis, respectively. Prevention of the Ca2+ increase by buffering cytosolic Ca2+ with quin-2, or through incubation of the thymocytes in a "Ca2+-free" medium, prevented endonuclease activation and cell killing. Inhibitors of calmodulin also prevented DNA fragmentation without inhibiting the glucocorticoid-stimulated elevation of cytosolic Ca2+ concentration. The Ca2+ increase appeared to be due to the action of a heat-labile cytosolic factor, synthesized in response to glucocorticoids, which facilitated the influx of extracellular Ca2+. Our findings suggest that glucocorticoids induce thymocyte suicide through an elevation of cytosolic Ca2+ concentration resulting in endonuclease activation, DNA fragmentation, and cell death.

Correlation between cytosolic Ca2+ concentration and cytotoxicity in hepatocytes exposed to oxidative stress

To investigate the relationship between alterations of cytosolic Ca2+ concentration and development of cytotoxicity, isolated rat hepatocytes were loaded with the fluorescent indicator Quin-2 AM and then incubated with non-toxic or toxic levels of menadione (2-methyl-1,4-naphthoquinone) or tert-butyl hydroperoxide (t-BH). The resulting changes in cytosolic Ca2+ concentration were compared to those seen upon exposure of the hepatocytes to an alpha 1-adrenergic agonist, phenylephrine, as well as to those induced by menadione and t-BH in hepatocytes pretreated with agents that modify their toxicity. Exposure of hepatocytes to phenylephrine or non-toxic levels of menadione caused a moderate and transient increase in cytosolic Ca2+ (less than or equal to 0.7 microM), whereas a toxic concentration of menadione produced a marked, sustained increase in Ca2+ which fully saturated the binding capacity of Quin-2 (greater than 1.5 microM). Treatment of the hepatocytes with the protective agent, dithiothreitol, prevented both the increase in cytosolic Ca2+ and the cytotoxicity induced by menadione. On the other hand, pretreatment of cells with diethylmaleate to deplete intracellular glutathione made otherwise non-toxic concentrations of menadione cause both a sustained increase in cytosolic Ca2+ and cytotoxicity. Similarly, toxic concentrations of t-BH also caused a sustained increase in cytosolic Ca2+. The iron chelator, desferrioxamine, and dithiothreitol (DTT), which protected the cells from t-BH toxicity, also prevented the sustained elevation of cytosolic Ca2+. Our findings provide further support for the hypothesis that a perturbation of intracellular Ca2+ homeostasis is an early and critical event in the development of toxicity in hepatocytes exposed to oxidative stress.

Alterations in hepatocyte cytoskeleton caused by redox cycling and alkylating quinones

Quinones may induce toxicity by a number of mechanisms, including alkylation and oxidative stress following redox cycling. The metabolism of quinones by isolated rat hepatocytes is associated with cytoskeletal alterations, plasma membrane blebbing, and subsequent cytotoxicity. The different mechanisms underlying the effects of alkylating (p-benzoquinone), redox cycling (2,3-dimethoxy-1,4-naphthoquinone), and mixed redox cycling/alkylating (2-methyl-1,4-naphthoquinone) quinones on hepatocyte cytoskeleton have been investigated in detail in this study. Analysis of the cytoskeletal fraction extracted from quinone-treated cells revealed a concentration-dependent increase in the amount of cytoskeletal protein and a concomitant loss of protein thiols, irrespective of the quinone employed. In the case of redox cycling quinones, these alterations were associated with an oxidation-dependent actin crosslinking (sensitive to the thiol reductant dithiothreitol). In contrast, with alkylating quinones an oxidation-independent cytoskeletal protein crosslinking (insensitive to thiol reductants) was observed. In addition to these changes, a dose-dependent increase in the relative abundance of F-actin was detected as a consequence of the metabolism of oxidizing quinones in hepatocytes. Addition of dithiothreitol solubilized a considerable amount of polypeptides from the cytoskeletal fraction isolated from hepatocytes exposed to redox cycling but not alkylating quinones. Our findings indicate that the hepatocyte cytoskeleton is an important target for the toxic effects of different quinones. However, the mechanisms underlying cytoskeletal damage differ depending on whether the quinone acts primarily by oxidative stress or alkylation.

Alterations of surface morphology caused by the metabolism of menadione in mammalian cells are associated with the oxidation of critical sulfhydryl groups in cytoskeletal proteins

Incubation of freshly-isolated (rat hepatocytes) or cultured (HeLa, GH3, and McCoy) mammalian cells with menadione (2-methyl-1,4-naphthoquinone) resulted in the appearance of numerous cell surface protrusions. The perturbation of surface structure was associated with an increase in the amount of cytoskeletal protein and the oxidation of sulfhydryl groups in actin, leading to the formation of high-molecular weight aggregates sensitive to treatment with thiol reductants. Our findings indicate that the oxidation of thiol groups in cytoskeletal proteins may be responsible for menadione-induced cell surface abnormalities in mammalian cells.

Menadione-induced bleb formation in hepatocytes is associated with the oxidation of thiol groups in actin

Incubation of isolated rat hepatocytes with menadione (2-methyl-1,4-naphthoquinone) or the thiol oxidant, diamide (azodicarboxylic acid bis(dimethylamide)), resulted in the appearance of numerous plasma membrane protrusions (blebs) preceding cell death. Analysis of the Triton X-100-insoluble fraction (cytoskeleton) extracted from treated cells revealed a dose- and time-dependent increase in the amount of cytoskeletal protein and a concomitant loss of protein thiols. These changes were associated with the disappearance of actin and formation of large-molecular-weight aggregates, when the cytoskeletal proteins were analyzed by polyacrylamide gel electrophoresis under nonreducing conditions. However, if the cytoskeletal proteins were treated with the thiol reductants, dithiothreitol or beta-mercaptoethanol, no changes in the relative abundance of actin or formation of large-molecular-weight aggregates were detected in the cytoskeletal preparations from treated cells. Moreover, addition of dithiothreitol to menadione- or diamide-treated hepatocytes protected the cells from both the appearance of surface blebs and the occurrence of alterations in cytoskeletal protein composition. Our findings show that oxidative stress induced by the metabolism of menadione in isolated hepatocytes causes cytoskeletal abnormalities, of which protein thiol oxidation seems to be intimately related to the appearance of surface blebs.

Meal-induced arterial blood pressure variations in the elderly

The effect of a standard meal on arterial blood pressure (ABP) was investigated in a group of 14 healthy elderly subjects (age greater than or equal to 65 years) and 11 controls (age less than or equal to 45 years) by means of automated noninvasive ABP monitoring. The magnitude of postprandial ABP reduction was significantly greater in the elderly subjects (systolic ABP: -22.3 +/- 4.9 vs. -7.5 +/- 2.2 mm Hg; p less than 0.05; diastolic ABP: -13.7 +/- 3.1 vs. -6.2 +/- 1.4 mm Hg; p less than 0.05) and ABP decrease was not compensated by heart rate acceleration.

Prolonged fructose feeding and aldose reductase inhibition: effect on the polyol pathway in kidneys of normal rats

The effects of diets with differing carbohydrate composition on the kidney polyol pathway were investigated. The diets employed were F = fructose rich, G = glucose rich, S = cornstarch rich, and were fed for 30 days to six groups of 12 normal male Sprague-Dawley rats with and without addition of the aldose reductase inhibitor tolrestat (T). Fructose feeding resulted in higher kidney sorbitol levels (F = 0.847 +/- 0.152, G = 0.354 +/- 0.087, S = 0.207 +/- 0.041 microM/g wet wt, P less than 0.05). This was not observed in the tolrestat-treated animals (F + T = 0.182 +/- 0.024, G + T = 0.149 +/- 0.021, S + T = 0.152 +/- 0.020 microM/g wet wt). Aldose reductase activity was reduced with tolrestat administration (F = 0.0208 +/- 0.0023, F + T = 0.0048 +/- 0.0005; G = 0.0210 +/- 0.0002, G + T = 0.0059 +/- 0.0008; S = 0.0227 +/- 0.0022, S + T = 0.0062 +/- 0.0007 microU). Myoinositol levels did not differ among groups (F = 1.973 +/- 0.182, G = 2.291 +/- 0.307, S = 2.066 +/- 0.155 microM/g wet wt), but tended to increase with aldose reductase inhibition (F + T = 2.253 +/- 0.186, G + T = 2.713 +/- 0.166, S + T = 2.618 +/- 0.221 microM/g wet wt). Plasma glucose was higher in the fructose-fed rats (F = 10.78 +/- 0.55, G = 9.09 +/- 0.058, S = 9.03 +/- 0.52, F + T = 9.75 +/- 0.61, G + T = 8.42 +/- 0.64, S + T = 8.81 +/- 0.49 mM/liter). It is concluded that prolonged fructose feeding results in the accumulation of sorbitol in the kidney, caused by increased flux of glucose through the polyol pathway. This can be prevented by aldose reductase inhibition.

Formation and reduction of glutathione-protein mixed disulfides during oxidative stress. A study with isolated hepatocytes and menadione (2-methyl-1,4-naphthoquinone)

Incubation of isolated rat hepatocytes with menadione (2-methyl-1,4-naphthoquinone) resulted in a dose-dependent depletion of intracellular reduced glutathione (GSH), most of which was oxidized to glutathione disulfide (GSSG). Menadione metabolism was also associated with a dose- and time-dependent inhibition of glutathione reductase, impairing the regeneration of GSH from GSSG produced during menadione-induced oxidative stress. Inhibition of glutathione reductase by pretreatment of hepatocytes with 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) greatly potentiated both GSH depletion and GSSG formation during the metabolism of low concentrations of menadione. Concomitant with GSH oxidation, mixed disulfides between glutathione and protein thiols were formed. The amount of mixed disulfides produced and the kinetics of their formation were dependent on both the intracellular GSH/GSSG ratio and the activity of glutathione reductase. The mixed disulfides were mainly recovered in the cytosolic fraction and, to a lesser extent, in the microsomal and mitochondrial fractions. The removal of glutathione from protein mixed disulfides formed in hepatocytes exposed to oxidative stress was dependent on GSH and/or cysteine and appeared to occur predominantly via a thiol-disulfide exchange mechanism. However, incubation of the microsomal fraction from menadione-treated hepatocytes with purified glutathione reductase in the presence of NADPH also resulted in the reduction of a significant portion of the glutathione-protein mixed disulfides present in this fraction. Our results suggest that the formation of glutathione-protein mixed disulfides occurs as a result of increased GSSG formation and inhibition of glutathione reductase activity during menadione metabolism in hepatocytes.

Alterations in inositol phosphate production during oxidative stress in isolated hepatocytes

The level of inositol phosphates was measured in rat hepatocytes treated with 2-methyl-1,4-naphthoquinone (menadione) or tert-butyl hydroperoxide, which cause Ca2+ mobilization from intracellular stores and an increase in cytosolic free Ca2+ concentration. Although neither agent produced any apparent changes in the resting level of inositol phosphates, pretreatment of hepatocytes with either menadione or tert-butyl hydroperoxide, as well as with several sulfhydryl reagents, markedly inhibited the increase in inositol phosphates induced by both hormonal and nonhormonal stimuli. Addition of dithiothreitol to menadione- or tert-butyl hydroperoxide-treated hepatocytes reversed this inhibition and reestablished responsiveness to extracellular stimuli. Our findings suggest that the inhibition of the inositol phosphate response by menadione and tert-butyl hydroperoxide occurs through the modification of critical sulfhydryl group(s) and that the alterations in intracellular Ca2+ homeostasis occurring during the metabolism of menadione and tert-butyl hydroperoxide in hepatocytes are not mediated by inositol phosphates.

Cystamine induces toxicity in hepatocytes through the elevation of cytosolic Ca2+ and the stimulation of a nonlysosomal proteolytic system

Infusion of cystamine into the isolated, perfused rat liver resulted in tissue damage preceded by the formation of cystamine-protein mixed disulfides which were mainly detected in the plasma membrane fraction. Hepatotoxicity was prevented when dithiothreitol was infused after cystamine or when the calcium antagonist, verapamil, was co-infused with the disulfide. In isolated hepatocytes, the formation of cystamine-protein mixed disulfides was associated with an inhibition of plasma membrane Ca2+-ATPase activity and a decreased rate of Ca2+ efflux from the cells. This resulted in intracellular Ca2+ accumulation which was followed by a stimulation of both phospholipid hydrolysis and proteolysis, as indicated by enhanced rates of release of radioactivity from hepatocytes prelabeled with [14C]arachidonate and [14C]valine, respectively. Preincubation of hepatocytes with the calmodulin inhibitor, calmidazolium, or with the phospholipase inhibitors, chlorpromazine and dibucaine, inhibited the stimulation of [14C]arachidonate release by cystamine. However, none of these agents prevented the onset of cystamine toxicity in hepatocytes. In contrast, pretreatment of the cells with antipain or leupeptin, two inhibitors of Ca2+-activated proteases, abolished the stimulation of proteolysis by cystamine and also protected the cells from cystamine toxicity. Our results suggest that the perturbation of intracellular Ca2+ homeostasis by cystamine is caused by the inhibition of Ca2+ efflux associated with the formation of cystamine-protein mixed disulfides in the plasma membrane and that subsequent cytotoxicity results from Ca2+-activation of a nonlysosomal proteolytic system.

tert-Butylhydroperoxide-induced toxicity in isolated hepatocytes: contribution of thiol oxidation and lipid peroxidation

Incubation of isolated rat hepatocytes with tert-butylhydroperoxide resulted in marked cytotoxicity preceded by intracellular glutathione depletion and extensive lipid peroxidation. Addition of antioxidants delayed, but did not prevent, this toxicity. A significant decrease in protein-free sulfhydryl groups also occurred in the presence of tert-butylhydroperoxide; direct oxidation of protein thiols and mixed disulfide formation with glutathione were responsible for this decrease. The involvement of protein thiol depletion in tert-butylhydroperoxide-induced cytotoxicity is suggested by our observation that administration of dithiothreitol, which caused re-reduction of the oxidized sulfhydryl groups and mixed disulfides, efficiently protected the cells from toxicity. Moreover, depletion of intracellular glutathione by pretreatment of the hepatocytes with diethyl maleate accelerated and enhanced the depletion of protein thiols induced by tert-butylhydroperoxide and potentiated cell toxicity even in the absence of lipid peroxidation.

Cytotoxicity of phenazine methosulfate in isolated rat hepatocytes is associated with superoxide anion production, thiol oxidation and alterations in intracellular calcium ion homeostasis

The metabolism of phenazine methosulfate (PMS) by isolated rat hepatocytes is associated with superoxide anion production, and with a substantial decrease in intracellular levels of reduced glutathione, most of which is oxidized to GSSG. A marked loss of protein-free sulfhydryl groups also occurs when intracellular glutathione is depleted, and cytotoxicity follows. These effects are associated with the inhibition of the plasma membrane Ca2+-ATPase and with intracellular accumulation of calcium ion which is preferentially sequestered in mitochondria. Maintenance of protein sulfhydryl groups in the reduced state by dithiothreitol (DTT) prevents the alterations in intracellular calcium homeostasis and protects against toxicity.

Ca2+ homeostasis and cytotoxicity in isolated hepatocytes: studies with extracellular adenosine 5'-triphosphate

The incubation of isolated rat hepatocytes with extracellular adenosine 5'-triphosphate (ATP) resulted in an inhibition of Ca2+ efflux. The ATP-induced Ca2+ accumulation as determined by the increase in phosphorylase a activity and the Ca2+-sensitive fluorescent indicator (2-[(2-bis-[carboxymethyl]-amino-5-methylphenoxy)-methyl]-6-methoxy-8- bis-[carboxymethyl]aminoquinoline-tetrakis-[acetoxymethyl]ester) (Quin 2-AM) was associated with both the hydrolysis of ATP and the phosphorylation of a 110 kDa protein. No significant alteration in the intracellular ATP level was observed. The appearance of surface blebs and cytotoxicity followed the rise in cytosolic Ca2+, suggesting that the increased free Ca2+ may be responsible for the loss of viability. When a calmodulin inhibitor, 1-[bis(4-chlorophenyl)methyl]-3-[ 2-(2,4-dichlorophenyl)-2-[(2,4-dichlorophenyl)methoxy] ethyl]-1H- imidazolium chloride (calmidazolium), was included in the medium prior to ATP addition, bleb formation was reduced and the loss of viability was completely prevented, indicating that a Ca2+-calmodulin process may be involved in the initiation of cytotoxicity.

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.

On the role of thiol groups in the inhibition of liver microsomal Ca2+ sequestration by toxic agents

ATP-dependent Ca2+ sequestration by rat liver microsomes was assayed using three different methods, and characterized with regard to the effect of various inhibitors. When glucose and hexokinase were added in combination to deplete ATP in the incubation, Ca2+ uptake was followed by rapid release of Ca2+ from the microsomes. Ca2+ sequestration was inhibited by reagents that cause alkylation (e.g. p-chloromercuribenzoate) or oxidation (e.g. diamide) of protein sulfhydryl groups. Moreover, pretreatment of the microsomes with cystamine, which causes formation of mixed disulfides with protein thiols, also resulted in the inhibition of Ca2+ sequestration. It is concluded that microsomal Ca2+ sequestration is critically dependent on protein sulfhydryl groups, and that modification of protein thiols may be an important mechanism for the inhibition of microsomal Ca2+ sequestration by a variety of toxic agents.

Glutathione S-conjugates stimulate ATP hydrolysis in the plasma membrane fraction of rat hepatocytes

Incubation of a rat hepatocyte plasma membrane fraction with micromolar concentrations of either glutathione disulfide or various glutathione S-conjugates resulted in a several-fold increase in the rate of ATP hydrolysis. This stimulation was further enhanced when the plasma membrane fraction had been pretreated with agents that arylate or oxidize sulfhydryl groups, suggesting that this ATPase activity is modulated by the protein thiol status of the plasma membrane. It is proposed that this newly discovered ATPase may function in the cellular extrusion of both glutathione disulfide and glutathione S-conjugates.

Demonstration and partial characterization of glutathione disulfide-stimulated ATPase activity in the plasma membrane fraction from rat hepatocytes

A highly purified plasma membrane fraction isolated from rat hepatocytes was found to catalyze the hydrolysis of ATP in response to micromolar concentrations of glutathione disulfide (GSSG). This process exhibited distinct kinetic parameters suggesting the existence of both a high and low affinity component. The apparent Km values (GSSG) for ATP hydrolysis were 140 microM and 1 mM for the high affinity and low affinity components, respectively. Disulfides other than GSSG were also found to stimulate ATP hydrolysis. The similarity between the kinetic properties of the GSSG-stimulated ATPase and those reported for GSSG transport in erythrocytes (Kondo, T., Dale, G. L., and Beutler, E. (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 6359-6362) suggests that the ATPase may function in the active extrusion of intracellular GSSG.

Inhibition of hepatocyte plasma membrane Ca2+-ATPase activity by menadione metabolism and its restoration by thiols

Incubation of isolated rat hepatocytes with cytotoxic concentrations of menadione resulted in inhibition of plasma membrane Ca2+-ATPase activity. This could be restored by subsequent treatment with either dithiothreitol or reduced glutathione, suggesting that the inhibition by menadione was due to oxidation of sulfhydryl groups critical for Ca2+-ATPase activity.

Insulin degradation in human erythrocyte: effects of cations

Insulin degradation by human erythrocyte fractions was studied using the TCA-precipitation method. Hemolysate exhibited an insulin degrading activity higher than membranes. Triton X-100 treatment of membranes led to the appraisal of Triton-soluble degrading activity and of a more efficient Triton-not-soluble degrading activity. Monovalent cations (Na+, K+, Li+) did not modify the insulin degradation by any of the erythrocyte fractions. Divalent cations, Ca++ and Zn++ selectively enhanced insulin degradation by the membranous fractions, and Cu++ and Zn++ strongly inhibited insulin degradation by all the erythrocyte fractions. The results supported the hypothesis of the existence of at least two different degrading systems in human erythrocytes: soluble (cytosolic) Ca++ and Mg++ insensitive system(s) and membrane associated Ca++ and Mg++ sensitive system(s).

Alterations in intracellular thiol homeostasis during the metabolism of menadione by isolated rat hepatocytes

The effects of menadione (2-methyl-1,4-naphthoquinone) metabolism on intracellular soluble and protein-bound thiols were investigated in freshly isolated rat hepatocytes. Menadione was found to cause a dose-dependent decrease in intracellular glutathione (GSH) level by three different mechanisms: (a) Oxidation of GSH to glutathione disulfide (GSSG) accounted for 75% of the total GSH loss; (b) About 15% of the cellular GSH reacted directly with menadione to produce a GSH-menadione conjugate which, once formed, was excreted by the cells into the medium; (c) A small amount of GSH (approximately 10%) was recovered by reductive treatment of cell protein with NaBH4, indicating that GSH-protein mixed disulfides were also formed as a result of menadione metabolism. Incubation of hepatocytes with high concentrations of menadione (greater than 200 microM) also induced a marked decrease in protein sulfhydryl groups; this was due to arylation as well as oxidation. Binding of menadione represented, however, a relatively small fraction of the total loss of cellular sulfhydryl groups, since it was possible to recover about 80% of the protein thiols by reductive treatments which did not affect protein binding. This suggests that the loss of protein sulfhydryl groups, like that of GSH, was mainly a result of oxidative processes occurring within the cell during the metabolism of menadione.

Menadione-induced cytotoxicity is associated with protein thiol oxidation and alteration in intracellular Ca2+ homeostasis

The toxicological implications of alterations in intracellular thiol homeostasis during menadione metabolism have been investigated using freshly isolated rat hepatocytes. A strict correlation between depletion of protein sulfhydryl groups and loss of cell viability was observed. Loss of protein thiols preceded cell death, and occurred more rapidly in cells with decreased levels of reduced glutathione. Depletion of protein thiols was also associated with inhibition of Ca2+ efflux from the cells and perturbation of intracellular Ca2+ homeostasis. It is proposed that the oxidative stress induced by menadione metabolism in isolated hepatocytes results in the depletion of both soluble and protein thiols, and that the latter effect is critically associated with a perturbation of Ca2+ homeostasis and loss of cell viability.

Alterations in intracellular calcium compartmentation following inhibition of calcium efflux from isolated hepatocytes

Addition of ATP to the incubation medium of freshly isolated rat hepatocytes causes a marked inhibition of the efflux of Ca2+ from the cells, and its accumulation in intracellular compartments. After an initial rise in cytosolic free Ca2+ concentration, as indicated by the activation of phosphorylase, Ca2+ is preferentially sequestered in the mitochondria, without any apparent contribution by the endoplasmic reticulum. Impairment of mitochondrial Ca2+ homeostasis by pyridine nucleotide oxidation associated with tert-butyl hydroperoxide metabolism, prevents the ATP-dependent cellular Ca2+ accumulation and causes a release of Ca2+ from the hepatocytes into the medium. Conversely, maintenance of the mitochondrial pyridine nucleotides in a more reduced state, e. g. in presence of 3-hydroxybutyrate in the medium, prevents this hydroperoxide-induced release of intracellular Ca2+. Under conditions of impaired mitochondrial Ca2+ sequestration, there appears to be a redistribution of a minor fraction of the intracellular Ca2+ from the mitochondria to the endoplasmic reticulum. Our results provide additional evidence for the critical involvement of the plasma membrane Ca2+-extruding system in the physiological regulation of the cytosolic free Ca2+ concentration in hepatocytes, and suggest that the mitochondria play a more important role than the endoplasmic reticulum in the regulation of the cytosolic free Ca2+ level when the plasma membrane Ca2+ pump is inhibited.

Increased whole body hematocrit: venous hematocrit ratio in diabetes mellitus, evidence of microcirculatory hemoconcentration

Plasma volume and red cell volume were measured in 24 diabetic outpatients. From the blood volume measurements, the whole body hematocrit was derived, and the ratio between whole body hematocrit:venous hematocrit (WBH:VH ratio) was considered to represent an index of the difference in red cell distribution between small and large blood vessels. The WBH:VH ratio was increased in 9 out of 13 males and in 1 out of 11 females, being inversely correlated to the plasma volume (r = -0.51, p less than 0.001). Although the significance of these findings is far from clear, the occurrence of small vessel hemoconcentration in male patients with diabetes mellitus may be relevant to the pathophysiology of vascular complications of diabetes.

Pyridine-nucleotide oxidation, Ca2+ cycling and membrane damage during tert-butyl hydroperoxide metabolism by rat-liver mitochondria

As tert-butyl hydroperoxide is metabolized by the glutatione peroxidase--glutathione reductase enzyme system present in liver mitochondria, rapid and extensive oxidation of NADH and slow NADPH oxidation are observed. This NAD(P)H oxidation can be prevented, or reversed, more effectively by 2-hydroxybutyrate than by isocitrate, indicating an important role of mitochondrial NAD(P)+ transhydrogenase activity in maintaining a high NADPH/NADP+ ratio for glutathione reductase. In Ca2+-loaded mitochondria tert-butyl hydroperoxide-induced NAD(P)H oxidation is followed by Ca2+ release from the mitochondria. If either 2-hydroxybutyrate or isocitrate is present, no Ca2+ release can be induced by the hydroperoxide. Following Ca2+ efflux the NAD(P)H oxidation process becomes irreversible and membrane damage occurs. These late effects do not take place if ruthenium red is added to prevent re-uptake of released Ca2+ by the mitochondria. Thus, we conclude that the metabolism of tert-butyl hydroperoxide leads to a release of mitochondrial Ca2+ via oxidation of pyridine nucleotides, and that subsequent membrane damage is not directly associated with this Ca2+ efflux but results from continued cycling of released Ca2+.

Increase in cytosolic Ca2+ concentration during t-butyl hydroperoxide metabolism by isolated hepatocytes involves NADPH oxidation and mobilization of intracellular Ca2+ stores

Activation of phosphorylase a in hepatocytes incubated with t-butyl hydroperoxide indicates that hydroperoxide metabolism is associated with an increase in cytosolic free Ca2+ concentration which appears to be mediated by NADPH oxidation and to involve mobilization of intracellular Ca2+ stores.