Energy-dependent calcium sequestration activity in rat liver microsomes

Effect of t-butyl hydroperoxide on liver microsomal membranes and microsomal calcium sequestration

In vitro exposure of hepatocytes or liver microsomes to t-butyl hydroperoxide resulted in a marked decrease of liver microsomal calcium pump activity. Decreased calcium pump activity was dependent upon both concentration and time. Liver microsomes could be protected from this effect by glutathione or dithiothreitol. In addition to decreased calcium pump activity, exposure of liver microsomes to t-butyl hydroperoxide produced a concentration-dependent aggregation of microsomal membrane protein as determined by polyacrylamide gel electrophoresis. Inhibition of microsomal calcium pump activity was observed when intact hepatocytes were incubated, in vitro, with t-butyl hydroperoxide. However, aggregation of microsomal membrane protein was not observed when hepatocytes were incubated with t-butyl hydroperoxide. The effects produced by exposure of liver microsomes to this compound do not appear to be a complete model of actions of the compound on intact cells.

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.

Calmodulin-dependent protein phosphorylation and calcium uptake in rat-liver microsomes

A 20-kDa protein becomes phosphorylated in a process stimulated by Ca2+ and calmodulin in the light microsomal fraction of rat liver homogenate. The uptake of Ca2+ in light microsomal fraction is also calmodulin-stimulated. The stimulation of Ca2+ transport is associated with the operation of a protein phosphorylation system dependent on Ca2+ and calmodulin. Transport is inhibited by a protein phosphatase which is stimulated by Ca2+ and calmodulin. It is proposed that the phosphorylation of the 20-kDa protein, which is stimulated by Ca2+ and calmodulin, plays a role in the regulation of the microsomal Ca2+, Mg2+-ATPase.

Inhibition of calcium sequestration activity of liver microsomes by 4-hydroxyalkenals originating from the peroxidation of liver microsomal lipids

Aldehydes released during peroxidation of liver microsomal lipids and identified as 4-hydroxyalkenals (4-hydroxynonenal being quantitatively the most significant) strongly inhibited the calcium sequestration activity of liver microsomes. The ID50 for 4-hydroxynonenal was 42 microM. The inhibition appeared to be correlated with the amount of the aldehyde bound to the microsomal protein. In rats intoxicated with BrCCl3 significant amounts of protein-bound aldehydes were formed at only 5 min after poisoning, a time at which the calcium sequestring capacity is markedly inhibited.

Endoplasmic reticulum whorls as a source of membranes for early cytaster formation in parthenogenetically stimulated sea urchin eggs

Sea urchin eggs exposed to a continuous hypertonic treatment rapidly form many concentric whorls of endoplasmic reticulum (ER) during the pre-activation period of the parthenogenetic development. These whorls, however, are only a temporary configurational alteration of ER which begin to break up just prior to egg activation. The conversion back to normal vesicles and lamellae occurs not only concurrently with the appearance of early cytastral areas, but also frequently in close association with the formation of these membranous areas. It is revealed here that membrane elements from disrupting whorls may become incorporated into adjacent, developing clear areas, early cytastral areas, and that this ER constitutes an initial major source of membranes for these early astral areas. Having previously suggested that the actual formation of ER whorls occurs in direct response to released intracellular calcium in hypertonic stressed eggs, the new findings, along with other related data and correlations, further suggest that whorl disruption and the formation of associated astral areas can be correlated with a corresponding decrease in the concentration of this released calcium in the cytoplasm.

Anaerobic metabolism of carbon tetrachloride and formation of catabolically resistant phospholipids

Anaerobic incubation of microsomes with 14C-carbon tetrachloride leads to trichloromethyl radicals which, apart from other detrimental reactions, bind covalently to phospholipids. These labelled lipids are extracted, phosphatidylcholine is isolated and incubated with phospholipase A2 from Naja naja snake venom and from porcine pancreas. Half of the covalently modified phosphatidylcholine is resistant toward phospholipase A2. Distribution of such catabolically resistant phospholipids from the endoplasmic reticulum to other cell organelles may play an important role in the pathway leading from the primary lesion to hepatocyte necrosis.

Kinetic properties of the ATP-dependent Ca2+ pump and the Na+/Ca2+ exchange system in basolateral membranes from rat kidney cortex

Basolateral plasma membranes from rat kidney cortex have been purified 40-fold by a combination of differential centrifugation, centrifugation in a discontinuous sucrose gradient followed by centrifugation in 8% percoll. The ratio of leaky membrane vesicles (L) versus right-side-out (RO) and inside-out (IO) resealed vesicles appeared to be L:RO:IO = 4:3:1. High-affinity Ca2+-ATPase, ATP-dependent Ca2+ transport and Na+/Ca2+ exchange have been studied with special emphasis on the relative transport capacities of the two Ca2+ transport systems. The kinetic parameters of Ca2+-ATPase activity in digitonin-treated membranes are: Km = 0.11 microM Ca2+ and Vmax = 81 +/- 4 nmol Pi/min X mg protein at 37 degrees C. ATP-dependent Ca2+ transport amounts to 4.3 +/- 0.2 and 7.4 +/- 0.3 nmol Ca2+/min X mg protein at 25 and 37 degrees C, respectively, with an affinity for Ca2+ of 0.13 and 0.07 microM at 25 and 37 degrees C. After correction for the percentage of IO-resealed vesicles involved in ATP-dependent Ca2+ transport, a stoichiometry of 0.7 mol Ca2+ transported per mol ATP is found for the Ca2+-ATPase. In the presence of 75 mM Na+ in the incubation medium ATP-dependent Ca2+ uptake is inhibited 22%. When Na+ is present at 5 mM an extra Ca2+ accumulation is observed which amounts to 15% of the ATP-dependent Ca2+ transport rate. This extra Ca2+ accumulation induced by low Na+ is fully inhibited by preincubation of the vesicles with 1 mM ouabain, which indicates that (Na+-K+)-ATPase generates a Na+ gradient favorable for Ca2+ accumulation via the Na+/Ca2+ exchanger. In the absence of ATP, a Na+ gradient-dependent Ca2+ uptake is measured which rate amounts to 5% of the ATP-dependent Ca2+ transport capacity. The Na+ gradient-dependent Ca2+ uptake is abolished by the ionophore monensin but not influenced by the presence of valinomycin. The affinity of the Na+/Ca2+ exchange system for Ca2+ is between 0.1 and 0.2 microM Ca2+, in the presence as well as in the absence of ATP. This affinity is surprisingly close to the affinity measured for the ATP-dependent Ca2+ pump. Based on these observations it is concluded that in isolated basolateral membranes from rat kidney cortex the Ca2+-ATPase system exceeds the capacity of the Na+/Ca2+ exchanger four- to fivefold and it is therefore unlikely that the latter system plays a primary role in the Ca2+ homeostasis of rat kidney cortex cells.

Characterization of calcium uptake into rough endoplasmic reticulum of rat pancreas

ATP-dependent Ca2+ uptake into isolated pancreatic acinar cells with permeabilized plasma membranes, as well as into isolated endoplasmic reticulum prepared from these cells, was measured using a Ca2+ -specific electrode and 45Ca2+. Endoplasmic reticulum was purified on an isopycnic Percoll gradient and characterized by marker enzyme distribution. When compared to the total homogenate, the typical marker for the rough endoplasmic reticulum RNA was enriched threefold and the typical marker for the plasma membrane Na+,K+(Mg2+)ATPase was decreased 20-fold. When different fractions of the Percoll gradient were compared, 45Ca2+ uptake correlated with the RNA content and not with the Na+,K+(Mg2+)ATPase activity. The characteristics of nonmitochondrial Ca2+ uptake into leaky isolated cells and 45Ca2+ uptake into isolated endoplasmic reticulum were very similar: Calcium uptake was maximal at 0.3 and 0.2 mmol/liter free Mg2+, at 1 and 1 mmol/liter ATP, at pH 6.0 and 6.5, and free Ca2+ concentration of 2 and 2 mumol/liter, respectively. Calcium uptake decreased at higher free Ca2+ concentration. 45Ca2+ uptake was dependent on monovalent cations (Rb+ greater than K+ greater than Na+ greater than Li+ greater than choline+) and different anions (Cl- greater than Br- greater than SO4(2-) greater than NO3- greater than I- greater than cyclamate- greater than SCN-) in both preparations. Twenty mmol/liter oxalate enhanced 45Ca2+ uptake in permeabilized cells 10-fold and in vesicles of endoplasmic reticulum, fivefold. Calcium oxalate precipitates in the endoplasmic reticulum of both preparations could be demonstrated by electron microscopy. The nonmitochondrial Ca2+ pool in permeabilized cells characterized in this study has been previously shown to regulate the cytosolic free Ca2+ concentration to 0.4 mumol/liter. Our results provide firm evidence that the endoplasmic reticulum plays an important role in the regulation of the cytosolic free Ca2+ concentration in pancreatic acinar cells.

Effect of divalent cations and chelators on metaphase to telophase progression and nuclear envelope formation in Chinese hamster cells

Chinese hamster DON cells in log phase were treated with Colcemid in the G2 period with or without divalent cation chelating agents. The metaphase cells were isolated and incubated in two ways: 1) without Colcemid but with chelating agents or La3+ and observed for metaphase to telophase progression, and 2) with Colcemid, with or without chelating agents and the rate of micronuclei formation in the absence of anaphase monitored. The effect of the chelating agents on cellular 45Ca2+ during metaphase to telophase progression was also studied. The results indicate that Ca2+ and possibly Mg2+ ions are involved in the regulation of certain segments of mitosis. The reduction of environmental and plasma membrane associated Ca2+ with the chelators and La3+ promoted the metaphase to telophase progression as well as nuclear envelope and micronuclei formation.

A new direction in the study of carbon tetrachloride hepatotoxicity

An unknown chain of causality links the early events of CCl4 metabolism to emergence of the classical spectrum of pathological changes elicited by this hepatotoxin. Recent developments suggest that an early disturbance in hepatocellular Ca2+ homeostasis may be involved. The possibility that generation of toxic products of lipid peroxidation may act as toxicological "second messengers" linking events near cytochrome P-450 to distant parts of the cell seems unlikely. This mini-review attempts to highlight major recent developments underlying the current situation.

The phosphoprotein intermediate of a Ca2+ transport ATPase in rat liver endoplasmic reticulum

Smooth endoplasmic reticulum vesicles from rat liver display an ATP-supported Ca2+ transport which is mediated by a (Ca2+ + Mg2+)-ATPase. During the catalytic cycle the terminal phosphate from ATP is incorporated to form an acid-precipitable reaction product(118 000-Mr in SDS-gel electrophoresis) with stability characteristics of an acylphosphate. Comparative studies with sarcoplasmic reticulum vesicles from fast-twitch skeletal muscle suggest that the 118 000-Mr phosphopeptide may be identified with the phosphorylated reaction intermediate of a Ca2+ transport ATPase in endoplasmic reticulum, similar to that in sarcoplasmic reticulum of muscle.

Calcium transport by bull spermatozoa plasma membranes

Plasma membrane isolated from frozen ejaculated bull spermatozoa were found to contain calcium transport activity. Thin-section electronmicrography of these membranes revealed relatively homogeneous vesicular membranes with sizes ranging from 2000 to 6000 A in diameter. Membrane vesicles that were exposed to oxalate as a calcium-trapping agent accumulated Ca2+ in the presence of Mg2+ and ATP. One microM of the calcium-ionophore A23187, added initially, completely inhibited net Ca2+ uptake and, if added later, caused the release of Ca2+ accumulated previously. An Arrhenius plot for the rate of Ca2+ uptake revealed a break at 32--33 degrees C, and Ea of 4.4 kcal/mol above the break and 32.2 kcal/mol below. The Ca+ uptake was inhibited by low concentrations of quercetin, which is known to be an inhibitor of (Ca2+ + Mg2+)-ATPase in many systems.

Effects of newly reported arachidonic acid metabolites on microsomal Ca++ binding, uptake and release

The 5,6-; 8,9-; 11,12- and 14,15-epoxyeicosatrienoic acids and their respective hydration products, the vic-diols, recently reported as metabolites of arachidonic acid in rat liver microsomes, were examined for effect on release of 45Ca from canine aortic smooth muscle microsomes. At 10(-6) M, the diols had no effect, but the 5,6-; 11,12- and 14,15-epoxyacids increased the loss of 45Ca. Further studies with the 14,15-epoxyacid demonstrated a dose-dependent decrease of Ca++ uptake (ATP present) in canine aortic microsomes in 0.03 mM Ca++, whereas Ca++ binding (ATP absent) was not affected. Ca++ uptake, binding and release in rat liver microsomes was similarly affected by the 14,15-epoxyacid, the major epoxyeicosatrienoic acid derivative produced by rat liver microsomal incubations. It is suggested that alterations in Ca++ metabolism might be a possible mechanism of action for these derivatives of arachidonic acid.

45Ca2+ uptake and phospholipid methylation in isolated rat liver microsomes

The effects of glucagon, epinephrine and insulin on hepatic phospholipid methylation were studied. Glucagon, either injected into rats or added to perfused livers, stimulated methylation in subsequently isolated microsomes. Epinephrine also increased phospholipid methylation. Insulin by itself did not influence the rate of the reaction, but, when administered prior to glucagon, it blocked the effect of the latter. The possibility that the observed stimulation of phospholipid methylation might be causally linked to the reported stimulation by glucagon of 45Ca2+ uptake in subsequently isolated liver microsomes was examined. Both the substrate and the competitive inhibitor of the methylation reaction, S-adenosylmethionine and S-adenosylhomocysteine, had profound effect on the rate of phospholipid methylation, without having comparable effects on Ca2+ uptake. S-adenosylmethionine in increasing concentration stimulated methylation four-fold, while no significant changes in 45Ca2+ uptake were seen. S-adenosylhomocysteine did not inhibit 45Ca2+ uptake even at levels causing more than 95% decrease in methylation. In conclusion, while both phospholipid methylation and 45Ca2+ uptake seem to be hormonally controlled, the correlation between these two processes was not sufficient to support the notion that the changes in 45Ca2+ uptake are caused by the changes in phospholipid methylation.

Compartmentation of calcium in digitonin-disrupted guinea pig pancreatic acinar cells

The treatment of guinea pig pancreatic acinar cells with digitonin leads to disruption of the plasma membrane, as judged by the liberation of cytosolic enzymes, without significant alteration of the mitochondrial membrane. The transport of calcium by the particulate residue was studied, and two different pools could be distinguished. One was supported by ATP or ADP, succinate providing the respiratory substrate, and was sensitive to the inhibitors, Ruthenium red and azide. The other pool needed the presence of ATP, ADP being ineffective, and also was unaffected by Ruthenium red or by azide, but was stimulated several-fold by oxalate. The Ruthenium red-sensitive calcium pool has characteristics resembling those of the transport of calcium by a mitochondrial fraction prepared from digitonin-treated acinar cells. In contrast, the Ruthenium red-insensitive calcium transport has characteristics resembling those of a microsomal fraction obtained from guinea pig pancreas. When the transport of calcium in digitonized cells was assayed at a calcium concentration range of 10(-8)-10(-4) M, preferential Ruthenium red-insensitive calcium transport could be observed at submicromolar calcium concentrations.

Localization of ATP-dependent calcium transport activity in mouse pancreatic microsomes

Electron-dense deposits representing calcium oxalate crystals which result from ATP-dependent calcium uptake have been localized within vesicles of of a heavy microsomal fraction prepared from mouse pancreatic acini. In the absence of either ATP or oxalate, no electron-dense deposits could be observed. By subfractionation of microsomes on discontinuous sucrose gradients, it could be shown that the highest energy-dependent calcium transport activity was associated with the rough endoplasmic reticulum. In rough microsomes, the 45Ca2+-uptake measured was 7 times greater than that of smooth microsomes in the presence of ATP and oxalate and about 3 times greater in he presence of ATP alone. When ribosomes were released from the rough endoplasmic reticulum vesicles by treatment with KCl in the presence of puromycin, the stripped microsomes showed a 40% increase in the specific 45Ca2+-uptake activity measured in he presence of ATP and oxalate and an increase of 80 to 90% in the presence of ATP alone. From these results it can be concluded that the calcium transport activity of microsomes prepared from mouse pancreatic acini is located predominantly in the rough endoplasmic reticulum membrane.

Carbon tetrachloride and bromotrichloromethane toxicity. Dual role of covalent binding of metabolic cleavage products and lipid peroxidation in depression of microsomal calcium sequestration

We have investigated the importance of covalent binding and lipid peroxidation on the depression of microsomal calcium sequestration associated with in vitro metabolism of 14CCl4. Studies with CBrCl3 are also reported. In aerobic systems, promethazine was used to block lipid peroxidation, measured as malondialdehyde (MDA) generation. Effects of low levels of lipid peroxidation were tested in Fe2+-supplemented systems free of halogenated hydrocarbons. The results indicate that microsomal calcium sequestration can be depressed significantly by metabolism of either CCl4 or CBrCl3 in the absence of MDA generation, or by lipid peroxidation occurring in the absence of halogenated hydrocarbons.

Enhanced hepatotoxicity and inhibition of liver endoplasmic reticulum calcium pump by CCl4 in rats fed a thiamine deficient diet

Male Sprague-Dawley rats were fed a thiamine deficient diet for three weeks, then treated with a range of CCl4 doses (0.01-1-ml/kg). Rats fed the deficient diet grew more slowly (body weight 65 percent of control) and had elevated liver glutathione (GSH) (220 percent of control). CCl4 hepatotoxicity, assessed by serum glutamicpyruvic transaminase (SGPT) activity and histological examination 24 hours after the hepatotoxin, was augmented in the group fed the thiamine deficient diet. Likewise, CCl4 inhibition of liver endoplasmic reticulum (ER) function (glucose-6-phosphatase (G6Pase) and calcium pump activities one hour after CCl4) was enhanced in rats fed the deficient diet. These results suggest that thiamine deficiency enhances CCl4 damage to membranes of the ER and enhances CCl4 hepatotoxicity.

ATP-dependent CA(2+) transport in endoplasmic reticulum isolated from roots ofLepidium sativum L

Endoplasmic reticulum membranes were isolated from roots of garden cress (Lepidium sativum L. cv Krause) using differential and discontinuous sucrose gradient centrifugation. The endoplasmic reticulum fraction was 80% rough endoplasmic reticulum oriented with the cytoplasmic surface directed outward and contaminated with 12% unidentified smooth membranes and 8% mitochondria. Marker enzyme analysis showed that the activity for endoplasmic reticulum was enriched 2.4-fold over total membrane activity while no other organelle activity showed an enrichment. All evidence indicated that the fraction was composed of highly enriched endoplasmic reticulum membranes. Ca(2+) uptake activity was measured using the filter technique described by Gross and Marmé (1978). The results of these experiments showed an ATP-dependent, oxalate-stimulated Ca(2+) uptake into vesicles of the endoplasmic reticulum fraction. The majority of the transport activity was microsomal since specific inhibitors of mitochondrial Ca(2+) transport (ruthenium red, LaCl3 and oligomycin) inhibited the activity by only 25%. Sodium azide showed no inhibition. The transport was likely directly coupled to ATP hydrolysis since there was no inhibition with carbonylcyanidem-chlorophenylhydrazone. The transport activity was specific for ATP showing only 36% and 29% of the activity with inosine diphosphate and guanosine 5'-triphosphate, respectively. The results indicate a Ca(2+) transport function located on the endoplasmic reciculum of garden cress roots.

Calcium uptake in isolated hepatic plasma-membrane vesicles

A liver plasma-membrane fraction capable of Ca2+ uptake was isolated. The fraction exhibited high Na+, K+-ATPase, low glucose-6 phosphatase activity, and transported alanine in a Na+-dependent fashion. The uptake of Ca2+ was ATP-dependent; UTP, GTP, or CTP did not substitute for ATP. The presence of oxalate did not significantly alter the rate of uptake. The pH optimum of the reaction was basic (no uptake was visible at pH 6.8). These properties are at variance with those of the endoplasmic reticulum Ca2+ uptake system, which is oxalate-dependent, and has an acid pH optimum. The ATP-dependent Ca2+ uptake has a Km(Ca2+) of 1.4 X 10(-8) M and a Vmax of transport of 30 nmol X mg protein-1 X min-1. No conclusive results were obtained on the calmodulin-sensitivity of the process: addition of calmodulin to the vesicles did not stimulate uptake, and the anti-calmodulin drug trifluoperazine had no inhibitory effect. However, another anti-calmodulin drug (R24571) had a limited, but statistically significant, inhibitory action. A partial release of the accumulated Ca2+ from the vesicles could be induced by the addition of Na+, and incubation of the vesicles in a high Na+ medium (as compared to high K+ medium) resulted in lower (about 25%) calcium uptake. Partial release of the accumulated Ca2+ could be induced also by the addition of H+. The releasing effect of H+, taken together with the absence of Ca2+ uptake at acid pH, suggests the possibility of a H+/Ca2+ exchange.

No direct correlation between binding of sex hormones and calcium pump activity by rat liver microsomal preparations

In the previous paper (J. steroid Biochem. 16 (1982) 437-446. [5]), we demonstrated that in vitro liver microsomal preparations of adult male rats possessed binding sites specific for progesterone (Prog) of high affinity (KD approximately 25.2 nM) and high capacity (*Nmax approximately 6.43pmol/mg of microsomal protein), using 1.30 mM NaCl-based incubation buffer. To explore the biological roles of liver microsomal Prog binding, we investigated the effects of such binding on liver microsomal Ca2+ pump activity. Firstly, we obtained results similar to those previously obtained concerning the characteristics of microsomal Prog binding using 100 mM KCl-based incubation buffer, usually used for experiments on microsomal Ca2+ pump activity. For microsomal 45Ca2+ uptake we also obtained results similar to those already demonstrated be several investigators. That is to say liver microsomal 45Ca2+ uptake was markedly increased by the addition of 30 mM oxalate and 5 mM ATP, and was not inhibited by the addition of 5 mM NaN3 into the incubation buffer. However, the addition of 1.0 microM Prog, as well as 17 beta-hydroxy-5 alpha-androstan-3-one (5 alpha-DHT) and estradiol-17 beta (E2 --17 beta), which should be a sufficiently saturable concentration for liver microsomal binding capacity for Prog was 5-10 times higher than that for 5 alpha-DHT and E2-17 beta. In addition, Prog (1.0 microM) had little effect on 45Ca2+ release from prelabeled microsomes. In conclusion, we suggest, therefore, that there is no direct correlation between binding of sex hormones and Ca2+ pump activity by rat liver microsomal preparations.

Ca2+ transporting activity of membrane fractions isolated from the post-mitochondrial supernatant of rat liver

The post-mitochondrial supernatant of rat liver contains two vesicular fractions which transport Ca2+ actively. The heavier fraction, sedimenting at 17.500 xg, 20 min, is enriched in plasma membrane markers and apparently contains both a Ca2+ pumping ATPase and a Na+/Ca2+ exchanger. These activities have been attributed to the plasma membrane vesicles. The lighter fraction, sedimenting at 100.000 xg, 60 min, is enriched in endoplasmic reticulum markers, and contains only a Ca2+ pumping ATPase, which can be differentiated from that of the heavier fraction on the basis of the sensitivity to vanadate. The Ca2+ pumping activity of endoplasmic reticulum appears to be regulated by both a cAMP-dependent, and a calmodulin-dependent system. The former system involves a heat-stable protein fraction from the cytosol. The regulation by the cAMP and the calmodulin-dependent systems involves the phosphorylation of several proteins in the endoplasmic reticulum membrane.

Subcellular localization of calcium in the coronet cells and tanycytes of the saccus vasculosus of the rainbow trout, Salmo gairdneri Richardson

The intracellular localization of calcium in the saccus vasculosus of the rainbow trout, Salmo gairdneri Richardson, was studied by means of ultracytochemical and X-ray microanalytical techniques. Using a variant of the glutaraldehyde/potassium pyroantimonate-osmium tetroxide method, Ca was detected in mitochondria, smooth endoplasmic reticulum and primary vesicles of coronet cells, and in mitochondria and smooth endoplasmic reticulum of tanycytes. Mitochondria and smooth endoplasmic reticulum in both cell types are considered as general Ca-stores. The primary vesicles in the ciliary globules of coronet cells are viewed as additional Ca-reservoirs. Possible roles of these Ca-stores in the regulation of transport activities of coronet cells in the homeostasis of the CSF are discussed.

The lipid composition of human liver microsomes

The lipid composition of human liver microsomes isolated from liver biopsy samples obtained at abdominal surgery has been determined. Human liver microsomal phospholipid is composed of 49% phosphatidylcholine, 31% phosphatidylethanolamine, 14% phosphatidylserine + phosphatidylinositol and 6% sphingomyelin, very similar to the phospholipid composition of rat liver microsomes. The fatty acid composition of human liver microsomes is remarkable only for its content of polyunsaturated fatty acids, with 20% of the fatty acids consisting of arachidonic, docosatetraenoic, docosapentaenoic and docosahexaenoic acids. This value contrasts with 33% in rats and 9% in rabbits. The molar cholesterol/phospholipid ratio in human liver microsomes is 0.069, similar to the ratio in rat and rabbit microsomes.

Carbon disulfide hepatoxicity and inhibition of liver microsome calcium pump

This work has shown that CS2 promptly inhibits the liver ER calcium pump only in those animals that subsequently develop hepatic necrosis. In this respect, inhibition of the ER calcium pump by CS2 resembles the actions of chlorinated hydrocarbon hepatotoxins. This lends further support to the suggestion that disruption of calcium homeostasis is an important early step in the action of at least some hepatotoxins [1-5]. CS2 appears to be the first example of a hepatotoxin other than chlorinated hydrocarbons that inhibit the liver ER calcium pump early in the course of intoxication. Finally, studies by others [17,18] suggest a mechanism by which CS2 can interact with and inhibit the liver ER calcium pump.

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

In order to recognize the significance of elevated liver calcium level in the early phase of carbon tetrachloride intoxication, the subcellular distribution of calcium in liver was investigated one hour after administration of CCl4 intraperitoneally. During preparation of subcellular fractions attempts were made to prevent redistribution of calcium by adding Ruthenium Red and EGTA or LaCl3 to homogenization medium, and by shortening of differential centrifugation to a minimum. The latter caused a loss of purity in sub cellular fractions which was overcome by correction of calcium values from atom absorption spectrometry by means of marker enzyme activity. The calcium levels in normal rat liver were found to be 70 nmoles/g liver wet wt in cytosol, 310 nmoles/g liver in microsomes and about 500 nmoles/g liver in mitochondria. A minor part of the latter fraction may belong to nuclei and plasma membranes. One hour after CCl4 administration calcium levels in cytoplasma were not altered, in microsomes were decreased to 200 nmoles/g liver and in mitochondria were elevated to 2.5 mumoles/g liver. In rats pretreated with vitamin D the whole additional calcium, after carbon tetrachloride application in the range of 9 mumoles/g liver, was sequestrated in mitochondria. In the early phase of carbon tetrachloride intoxication all border membranes of liver cells have to participate in bringing about the reversible increase of liver cell calcium.