Energy-dependent calcium sequestration activity in rat liver microsomes

Medicinal plants with hepatoprotective potentials against carbon tetrachloride-induced toxicity: a review

Background

Carbon tetrachloride (CCl4) is a well-characterized hepatotoxic agent. With rising cases of liver diseases, the identification, assessment, and development of hepatoprotective agents from plants source has become imperative.

Main body

With arrays of literature on plants with hepatoprotective potentials, this review sourced published literatures between 1998 and 2020 and systematically highlighted about 92 medicinal plants that have been reported to protect against CCl4-induced liver injury in animal models. The results show that herbal plants provide protection for the liver against CCl4 by downregulation of the liver marker enzymes and activation of antioxidant capacity of the liver cells with the restoration of liver architecture. We also provided the traditional and accompanying pharmacological uses of the plants. A variety of phytochemicals mostly flavonoids and polyphenols compounds were suggested to offer protection against liver injuries.

Conclusion

It can be concluded that there are a variety of phytochemicals in plant products with hepatoprotective activity against CCl4-induced toxicity in animal models.

Hepatic endoplasmic reticulum calcium fluxes: effect of free fatty acids and KATP channel involvement

As a common sequel to obesity, plasma and intracellular free fatty acid (FFA) concentrations are elevated and, as a consequence, manifold disturbances in metabolism may ensue. Biochemical processes in the cytosol and organelles, such as mitochondria and endoplasmic reticulum (ER), can be disturbed. In the ER, the maintenance of a high calcium gradient is indispensable for viability. In sarcoplasmic reticulum, selective FFA can induce ER stress by disrupting luminal calcium homeostasis; however, there are limited studies in hepatic microsomes. Our studies found that FFA has a noxious effect on rat hepatic microsomal calcium flux, and the extent of which depended on the number of double bonds and charge. Furthermore, insofar as the FFA had no effect on microsomal calcium efflux, their inhibitory action primarily involves calcium influx. Finally, other cationic channels have been found in hepatic ER, and evidence is presented of their interaction with the Ca²⁺ ATPase pump.

Pyridine nucleotide regulation of hepatic endoplasmic reticulum calcium uptake

Pyridine nucleotides serve an array of intracellular metabolic functions such as, to name a few, shuttling electrons in enzymatic reactions, safeguarding the redox state against reactive oxygen species, cytochrome P450 (CYP) enzyme detoxification pathways and, relevant to this study, the regulation of ion fluxes. In particular, the maintenance of a steep calcium gradient between the cytosol and endoplasmic reticulum (ER), without which apoptosis ensues, is achieved by an elaborate combination of energy–requiring ER membrane pumps and efflux channels. In liver microsomes, net calcium uptake was inhibited by physiological concentrations of NADP. In the presence of 1 mmol/L NADP, calcium uptake was attenuated by nearly 80%, additionally, this inhibitory effect was blunted by concomitant addition of NADPH. No other nicotinamide containing compounds ‐save a slight inhibition by NAADP‐hindered calcium uptake; thus, only oxidized pyridine nucleotides, or related compounds with a phosphate moiety, had an imposing effect. Moreover, the NADP inhibition was evident even after selectively blocking ER calcium efflux channels. Given the fundamental role of endoplasmic calcium homeostasis, it is plausible that changes in cytosolic NADP concentration, for example, during anabolic processes, could regulate net ER calcium uptake.

Immune privilege in corneal transplantation

Corneal transplantation is the most successful solid organ transplantation performed in humans. The extraordinary success of orthotopic corneal allografts, in both humans and experimental animals, is related to the phenomenon of "immune privilege". Inflammation is self-regulated to preserve ocular functions because the eye has immune privilege. At present, three major mechanisms are considered to provide immune privilege in corneal transplantation: 1) anatomical, cellular, and molecular barriers in the cornea; 2) tolerance related to anterior chamber-associated immune deviation and regulatory T cells; and 3) an immunosuppressive intraocular microenvironment. This review describes the mechanisms of immune privilege that have been elucidated from animal models of ocular inflammation, especially those involving corneal transplantation, and its relevance for the clinic. An update on molecular, cellular, and neural interactions in local and systemic immune regulation is provided. Therapeutic strategies for restoring immune privilege are also discussed.

Effects of chronic hypoxia on diaphragm function in deer mice native to high altitude

AIM

We examined the effects of chronic hypoxia on diaphragm function in high- and low-altitude populations of Peromyscus mice.

METHODS

Deer mice (P. maniculatus) native to high altitude and congeneric mice native to low altitude (P. leucopus) were born and raised in captivity to adulthood and were acclimated to normoxia or hypobaric hypoxia (12 or 9 kPa, simulating hypoxia at 4300 and 7000 m) for 6-8 weeks. We then measured indices of mitochondrial respiration capacity, force production, and fatigue resistance in the diaphragm.

RESULTS

Mitochondrial respiratory capacities (assessed using permeabilized fibres with single or multiple inputs to the electron transport system), citrate synthase activity (a marker of mitochondrial volume), twitch force production, and muscle fatigue resistance increased after exposure to chronic hypoxia in both populations. These changes were not well explained by variation in the fibre-type composition of the muscle. However, there were several differences in diaphragm function in high-altitude mice compared to low-altitude mice. Exposure to a deeper level of hypoxia (9 kPa vs 12 kPa) was needed to elicit increases in mitochondrial respiration rates in highlanders. Chronic hypoxia did not increase the emission of reactive oxygen species from permeabilized fibres in highlanders, in contrast to the pronounced increases that occurred in lowlanders. In general, the diaphragm of high-altitude mice had greater capillary length densities, produced less force in response to stimulation and had shorter relaxation times. The latter was associated with higher activity of sarcoplasmic reticulum Ca²⁺ -ATPase (SERCA) activity in the diaphragm of high-altitude mice.

CONCLUSION

Overall, our work suggests that exposure to chronic hypoxia increases the capacities for mitochondrial respiration, force production and fatigue resistance of the diaphragm. However, many of these effects are opposed by evolved changes in diaphragm function in high-altitude natives, such that highlanders in chronic hypoxia maintain similar diaphragm function to lowlanders in sea level conditions.

Alzheimer's disease pathology and the unfolded protein response: prospective pathways and therapeutic targets

Many vital interdependent cellular functions including proteostasis, lipogenesis and Ca homeostasis are executed by the endoplasmic reticulum (ER). Exogenous insults can impair ER performance: this must be rapidly corrected or cell death will ensue. Protective adaptations can boost the functional capacity of the ER and form the basis of the unfolded protein response (UPR). Activated in response to the accumulation of misfolded proteins, the UPR can halt protein translation while increasing protein-handling chaperones and the degradation of erroneous proteins through a conserved three-tier molecular cascade. However, prolonged activation of the UPR can result in the maladaptation of the system, resulting in the activation of inflammatory and apoptotic effectors. Recently, UPR and its involvement in neurodegenerative disease has attracted much interest and numerous potentially 'drugable' points of crosstalk are now emerging. Here, we summarize the functions of the ER and UPR, and highlight evidence for its potential role in the pathogenesis of Alzheimer's disease, before discussing several key targets with therapeutic potential.

Association between corneal endothelial cell densities and elevated cytokine levels in the aqueous humor

Annual reduction rate of corneal endothelial cell density (ECD) varies among etiologies, however, the cause of chronic endothelial cell loss is still unknown. We recently reported the elevation of inflammatory cytokines in the aqueous humor (AqH) in eyes with bullous keratopathy and low ECD. To evaluate the association between ECD and aqueous cytokine levels, we collected a total of 157 AqH samples prospectively. The AqH levels of cytokines were measured and multivariate analyses were conducted to find the correlation between ECD, aqueous cytokine levels and clinical factors, such as number of previous intraocular surgeries and protein concentration in AqH. As a result, ECD was negatively correlated with specific cytokine levels, including IL-1α, IL-4, IL-13, MIP-1β, TNF-α and E-selectin (all P < 0.05). The aqueous cytokine levels showed different correlations with these clinical factors; the number of previous intraocular surgeries was associated with all cytokines except MIP-1α. The AqH protein concentration and the status of intraocular lens showed similar patterns of elevation of IL-1α, IL-4, IL-6, IL-8, IL-10, IL-13, IL-17A, MIP-1β, MCP-1, E-selectin, P-selectin and sICAM-1. In conclusion, elevation of AqH cytokine levels was associated with reduced ECDs. AqH cytokine levels showed significant correlations with clinical factors associated with low ECDs.

Targeting Cellular Calcium Homeostasis to Prevent Cytokine-Mediated Beta Cell Death

Pro-inflammatory cytokines are important mediators of islet inflammation, leading to beta cell death in type 1 diabetes. Although alterations in both endoplasmic reticulum (ER) and cytosolic free calcium levels are known to play a role in cytokine-mediated beta cell death, there are currently no treatments targeting cellular calcium homeostasis to combat type 1 diabetes. Here we show that modulation of cellular calcium homeostasis can mitigate cytokine- and ER stress-mediated beta cell death. The calcium modulating compounds, dantrolene and sitagliptin, both prevent cytokine and ER stress-induced activation of the pro-apoptotic calcium-dependent enzyme, calpain, and partly suppress beta cell death in INS1E cells and human primary islets. These agents are also able to restore cytokine-mediated suppression of functional ER calcium release. In addition, sitagliptin preserves function of the ER calcium pump, sarco-endoplasmic reticulum Ca²⁺-ATPase (SERCA), and decreases levels of the pro-apoptotic protein thioredoxin-interacting protein (TXNIP). Supporting the role of TXNIP in cytokine-mediated cell death, knock down of TXNIP in INS1-E cells prevents cytokine-mediated beta cell death. Our findings demonstrate that modulation of dynamic cellular calcium homeostasis and TXNIP suppression present viable pharmacologic targets to prevent cytokine-mediated beta cell loss in diabetes.

Differences in long chain polyunsaturates composition and metabolism in male and female rats

Human studies and some animal work have shown more docosahexaenoic acid (DHA) and arachidonic acid (ARA) was accumulated or converted from precursors in females compared to males. This study explored in-depth the effect of gender on fatty acid composition and polyunsaturated fatty acid metabolism in rats fed one of two well-defined diets containing 10% total fat. One diet contained 15% of linoleic acid (LA) and 3% of α-linolenic acid (ALA) of the total fatty acids (LA+ALA diet), while the other diet contained 15% LA and 0.05% ALA (LA diet). At the age of 20 weeks, all animals were orally administered a single dose of a mixture of deuterium-labeled LA and ALA. Caudal venous blood was then drawn at 0, 2, 4, 8, 12, 24, 48, 96 and 168h. The concentrations of the deuterated precursors and their metabolites in plasma total lipids were quantified by GC/MS negative chemical ionization. Endogenous fatty acids were quantified by GC/FID analysis. When expressed as the percentage of oral dosage, female rats accumulated more precursors and more products, deuterated DHA and deuterated n-6 docosapentaenoic acid (²H₅-DPAn-6), in plasma than did male rats in both the LA+ALA diet and the LA diet. For the endogenous non-labeled PUFA, greater concentrations of DHA and DPAn-6 were similarly observed in female rats compared to males within each diet. A lower concentration of non-labeled ARA was observed only in female rats fed the LA+ALA diet. In summary, greater endogenous and exogenous DHA and DPAn-6 was observed in female rat plasma and this was independent of dietary ALA status.

The role of ER stress in lipid metabolism and lipotoxicity

The endoplasmic reticulum (ER) is a cellular organelle important for regulating calcium homeostasis, lipid metabolism, protein synthesis, and posttranslational modification and trafficking. Numerous environmental, physiological, and pathological insults disturb ER homeostasis, referred to as ER stress, in which a collection of conserved intracellular signaling pathways, termed the unfolded protein response (UPR), are activated to maintain ER function for cell survival. However, excessive and/or prolonged UPR activation leads to initiation of self-destruction through apoptosis. Excessive accumulation of lipids and their intermediate products causes metabolic abnormalities and cell death, called lipotoxicity, in peripheral organs, including the pancreatic islets, liver, muscle, and heart. Because accumulating evidence links chronic ER stress and defects in UPR signaling to lipotoxicity in peripheral tissues, understanding the role of ER stress in cell physiology is a topic under intense investigation. In this review, we highlight recent findings that link ER stress and UPR signaling to the pathogenesis of peripheral organs due to lipotoxicity.

Skeletal Muscle Phospholipid Metabolism Regulates Insulin Sensitivity and Contractile Function

Skeletal muscle insulin resistance is an early defect in the development of type 2 diabetes. Lipid overload induces insulin resistance in muscle and alters the composition of the sarcoplasmic reticulum (SR). To test the hypothesis that skeletal muscle phospholipid metabolism regulates systemic glucose metabolism, we perturbed choline/ethanolamine phosphotransferase 1 (CEPT1), the terminal enzyme in the Kennedy pathway of phospholipid synthesis. In C2C12 cells, CEPT1 knockdown altered SR phospholipid composition and calcium flux. In mice, diet-induced obesity, which decreases insulin sensitivity, increased muscle CEPT1 expression. In high-fat diet-fed mice with skeletal muscle-specific knockout of CEPT1, systemic and muscle-based approaches demonstrated increased muscle insulin sensitivity. In CEPT1-deficient muscles, an altered SR phospholipid milieu decreased sarco/endoplasmic reticulum Ca(2+) ATPase-dependent calcium uptake, activating calcium-signaling pathways known to improve insulin sensitivity. Altered muscle SR calcium handling also rendered these mice exercise intolerant. In obese humans, surgery-induced weight loss increased insulin sensitivity and decreased skeletal muscle CEPT1 protein. In obese humans spanning a spectrum of metabolic health, muscle CEPT1 mRNA was inversely correlated with insulin sensitivity. These results suggest that high-fat feeding and obesity induce CEPT1, which remodels the SR to preserve contractile function at the expense of insulin sensitivity.

Lipogenesis mitigates dysregulated sarcoplasmic reticulum calcium uptake in muscular dystrophy

Muscular dystrophy is accompanied by a reduction in activity of sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA) that contributes to abnormal Ca(2+) homeostasis in sarco/endoplasmic reticulum (SR/ER). Recent findings suggest that skeletal muscle fatty acid synthase (FAS) modulates SERCA activity and muscle function via its effects on SR membrane phospholipids. In this study, we examined muscle's lipid metabolism in mdx mice, a mouse model for Duchenne muscular dystrophy (DMD). De novo lipogenesis was ~50% reduced in mdx muscles compared to wildtype (WT) muscles. Gene expressions of lipogenic and other ER lipid-modifying enzymes were found to be differentially expressed between wildtype (WT) and mdx muscles. A comprehensive examination of muscles' SR phospholipidome revealed elevated phosphatidylcholine (PC) and PC/phosphatidylethanolamine (PE) ratio in mdx compared to WT mice. Studies in primary myocytes suggested that defects in key lipogenic enzymes including FAS, stearoyl-CoA desaturase-1 (SCD1), and Lipin1 are likely contributing to reduced SERCA activity in mdx mice. Triple transgenic expression of FAS, SCD1, and Lipin1 (3TG) in mdx myocytes partly rescued SERCA activity, which coincided with an increase in SR PE that normalized PC/PE ratio. These findings implicate a defect in lipogenesis to be a contributing factor for SERCA dysfunction in muscular dystrophy. Restoration of muscle's lipogenic pathway appears to mitigate SERCA function through its effects on SR membrane composition.

Reduced efficiency of sarcolipin-dependent respiration in myocytes from humans with severe obesity

OBJECTIVE

Sarcolipin (SLN) regulates muscle energy expenditure through its action on sarco/endoplasmic reticulum Ca(2+) -ATPase (SERCA) pump. It is unknown whether SLN-dependent respiration has relevance to human obesity, but whole-transcriptome gene expression profiling revealed that SLN was more highly expressed in myocytes from individuals with severe obesity (OB) than in lean controls (LN). The purpose of this study was to examine SLN-dependent cellular respiratory rates in LN and OB human muscles.

METHODS

Primary myocytes were isolated from muscle biopsy from seven LN and OB Caucasian females. Cellular respiration was assessed with and without lentivirus-mediated SLN knockdown in LN and OB myocytes.

RESULTS

SLN mRNA and protein abundance was greater in OB compared to LN cells. Despite elevated SLN levels in wild-type OB cells, respiratory rates among SLN-deficient cells were higher in OB compared to LN. Obesity-induced reduction in efficiency of SLN-dependent respiration was associated with altered sarcoplasmic reticulum phospholipidome.

CONCLUSIONS

SLN-dependent respiration is reduced in muscles from humans with severe obesity compared to lean controls. Identification of the molecular mechanism that affects SLN efficiency might lead to interventions that promote an increase in skeletal muscle energy expenditure.

Tadalafil inhibits the cAMP stimulated glucose output in the rat liver

The purpose of the present work was to verify if tadalafil affects hepatic glucose output, one of the primary targets of cAMP, in the isolated perfused rat liver. No effects on glycogen catabolism and oxygen uptake were found under basal conditions for tadalafil concentrations in the range between 0.25 and 10 μM. However, tadalafil had a clear and time-dependent inhibitory effect on the cAMP- and glucagon-stimulated glucose release. Constant infusion of tadalafil in the range between 0.25 and 10 μM eventually abolished 100% of the stimulatory action of those effectors. The tadalafil concentrations producing half-maximal rates of inhibition of the cAMP and glucagon stimulated glycogenolysis were 0.46±0.04 and 1.07±0.16 μM, respectively. These concentrations are close to the plasma peak concentrations in patients after ingestion of 20 mg tadalafil. The drug also diminished the activity of glycogen phosphorylase a and increased the activities of glucose 6-phosphatase, glucokinase, pyruvate kinase and glucose 6-phosphate dehydrogenase. These actions occurred only in the cellular environment. Tadalafil did not affect binding of cAMP to protein kinase A. Diminution of cAMP-stimulated glucose output is the opposite of what can be expected from a phosphodiesterase inhibition, the most common effect attributed to tadalafil. Diminution of glucose output by tadalafil can be attributed (a) to an interference with glycogen phosphorylase stimulation and (b) to an increased futile cycling of glucose 6-phosphate and glucose with a concomitant increased flow of hexose units into cellular metabolic pathways. The effects described in the present work may prove to represent important side effects of tadalafil.

Muscle lipogenesis balances insulin sensitivity and strength through calcium signaling

Exogenous dietary fat can induce obesity and promote diabetes, but endogenous fat production is not thought to affect skeletal muscle insulin resistance, an antecedent of metabolic disease. Unexpectedly, the lipogenic enzyme fatty acid synthase (FAS) was increased in the skeletal muscle of mice with diet-induced obesity and insulin resistance. Skeletal muscle-specific inactivation of FAS protected mice from insulin resistance without altering adiposity, specific inflammatory mediators of insulin signaling, or skeletal muscle levels of diacylglycerol or ceramide. Increased insulin sensitivity despite high-fat feeding was driven by activation of AMPK without affecting AMP content or the AMP/ATP ratio in resting skeletal muscle. AMPK was induced by elevated cytosolic calcium caused by impaired sarco/endoplasmic reticulum calcium ATPase (SERCA) activity due to altered phospholipid composition of the sarcoplasmic reticulum (SR), but came at the expense of decreased muscle strength. Thus, inhibition of skeletal muscle FAS prevents obesity-associated diabetes in mice, but also causes muscle weakness, which suggests that mammals have retained the capacity for lipogenesis in muscle to preserve physical performance in the setting of disrupted metabolic homeostasis.

The role of endoplasmic reticulum in hepatic lipid homeostasis and stress signaling

The endoplasmic reticulum (ER) is a critical site of protein, lipid, and glucose metabolism, lipoprotein secretion, and calcium homeostasis. Many of the sensing, metabolizing, and signaling mechanisms for these pathways exist within or on the ER membrane domain. Here, we review the cellular functions of ER, how perturbation of ER homeostasis contributes to metabolic dysregulation and potential causative mechanisms of ER stress in obesity, with a particular focus on lipids, metabolic adaptations of ER, and the maintenance of its membrane homeostasis. We also suggest a conceptual framework of metabolic roundabout to integrate key mechanisms of insulin resistance and metabolic diseases.

Changes in calcium fluxes in mitochondria, microsomes, and plasma membrane vesicles of livers from monosodium L-glutamate-obese rats

The purpose of this work was to evaluate if the fat liver accumulation interferes with intracellular calcium fluxes and the liver glycogenolytic response to a calcium-mobilizing α(1)-adrenergic agonist, phenylephrine. The animal model of monosodium L-glutamate (MSG)-induced obesity was used. The adult rats develop obesity and steatosis. Calcium fluxes were evaluated through measuring the (45)Ca(2+) uptake by liver microsomes, inside-out plasma membrane, and mitochondria. In the liver, assessments were performed on the calcium-dependent glycogenolytic response to phenylephrine and the glycogen contents. The Ca(2+) uptake by microsomes and plasma membrane vesicles was reduced in livers from obese rats as a result of reduction in the Ca(2+)-ATPase activities. In addition, the plasma membrane Na(+)/K(+)-ATPase was reduced. All these matched effects could contribute to elevated resting intracellular calcium levels in the hepatocytes. Livers from obese rats, albeit smaller and with similar glycogen contents to those of control rats, released higher amounts of glucose in response to phenylephrine infusion, which corroborates these observations. Mitochondria from obese rats exhibited a higher capacity of retaining calcium, a phenomenon that could be attributed to a minor susceptibility of the mitochondrial permeability transition pore opening.

Aberrant lipid metabolism disrupts calcium homeostasis causing liver endoplasmic reticulum stress in obesity

The endoplasmic reticulum (ER) is the main site of protein and lipid synthesis, membrane biogenesis, xenobiotic detoxification and cellular calcium storage, and perturbation of ER homeostasis leads to stress and the activation of the unfolded protein response. Chronic activation of ER stress has been shown to have an important role in the development of insulin resistance and diabetes in obesity. However, the mechanisms that lead to chronic ER stress in a metabolic context in general, and in obesity in particular, are not understood. Here we comparatively examined the proteomic and lipidomic landscape of hepatic ER purified from lean and obese mice to explore the mechanisms of chronic ER stress in obesity. We found suppression of protein but stimulation of lipid synthesis in the obese ER without significant alterations in chaperone content. Alterations in ER fatty acid and lipid composition result in the inhibition of sarco/endoplasmic reticulum calcium ATPase (SERCA) activity and ER stress. Correcting the obesity-induced alteration of ER phospholipid composition or hepatic Serca overexpression in vivo both reduced chronic ER stress and improved glucose homeostasis. Hence, we established that abnormal lipid and calcium metabolism are important contributors to hepatic ER stress in obesity.

Influence of tamoxifen on gluconeogenesis and glycolysis in the perfused rat liver

The actions of tamoxifen, a selective estrogen receptor modulator used in chemotherapy and chemo-prevention of breast cancer, on glycolysis and gluconeogenesis were investigated in the isolated perfused rat liver. Tamoxifen inhibited gluconeogenesis from both lactate and fructose at very low concentrations (e.g., 5μM). The opposite, i.e., stimulation, was found for glycolysis from both endogenous glycogen and fructose. Oxygen uptake was unaffected, inhibited or stimulated, depending on the conditions. Stimulation occurred in both microsomes and mitochondria. Tamoxifen did not affect the most important key-enzymes of gluconeogenesis, namely, phosphoenolpyruvate carboxykinase, pyruvate carboxylase, fructose 1,6-bisphosphatase and glucose 6-phosphatase. Confirming previous observations, however, tamoxifen inhibited very strongly NADH- and succinate-oxidase of freeze-thawing disrupted mitochondria. Tamoxifen promoted the release of both lactate dehydrogenase (mainly cytosolic) and fumarase (mainly mitochondrial) into the perfusate. Tamoxifen (200μM) clearly diminished the ATP content and increased the ADP content of livers in the presence of lactate with a diminution of the ATP/ADP ratio from 1.67 to 0.79. The main causes for gluconeogenesis inhibition are probably: (a) inhibition of energy metabolism; (b) deviation of intermediates (malate and glucose 6-phosphate) for the production of NADPH required in hydroxylation and demethylation reactions; (c) deviation of glucosyl units toward glucuronidation reactions; (d) secondary inhibitory action of nitric oxide, whose production is stimulated by tamoxifen; (e) impairment of the cellular structure, especially the membrane structure. Stimulation of glycolysis is probably a compensatory phenomenon for the diminished mitochondrial ATP production. The multiple actions of tamoxifen at relatively low concentrations can represent a continuous burden to the overall hepatic functions during long treatment periods.

Lack of appreciable species differences in nonspecific microsomal binding

Species differences in microsomal binding were evaluated for 43 drug molecules in human, monkey, dog and rat liver microsomes, using a fixed concentration of microsomal protein. The dataset included 32 named drugs and 11 proprietary compounds encompassing a broad spectrum of physicochemical properties (11 acids, 24 bases, 8 neutral, c log D -1 to 7, MW 200 to 700 and free fraction <0.001 to 1). Free fractions (f(u,mic)) in monkey, dog, rat and human microsomes were highly correlated, with linear regression correlation coefficients greater than 0.97. The average fold-difference in f(u,mic) between monkey, dog, or rat, and human was 1.6-, 1.3-, and 1.5-fold, respectively. Species differences in f(u,mic) were also assessed for a range of microsomal protein concentrations (0.2-2 mg/mL) for midazolam, clomipramine, astemizole, and tamoxifen, drugs with low to high microsomal binding. The mean fold species-difference in f(u,mic) for midazolam, clomipramine, astemizole, and tamoxifen was 1.1-, 1.2-, 1.3-, and 2.0-fold, respectively, and was independent of normalized microsomal protein concentration. For a fixed concentration of microsomal protein, greater than 76% and 90% of drugs examined in this study had preclinical species f(u,mic) within 1.5- and 2-fold, respectively, of experimentally measured human values.

Calcium bridge triggers capsid disassembly in the cell entry process of simian virus 40

The calcium bridge between the pentamers of polyoma viruses maintains capsid metastability. It has been shown that viral infection is profoundly inhibited by the substitution of lysine for glutamate in one calcium-binding residue of the SV40 capsid protein, VP1. However, it is unclear how the calcium bridge affects SV40 infectivity. In this in vitro study, we analyzed the influence of host cell components on SV40 capsid stability. We used an SV40 mutant capsid (E330K) in which lysine had been substituted for glutamate 330 in protein VP1. The mutant capsid retained the ability to interact with the SV40 cellular receptor GM1, and the internalized mutant capsid accumulated in caveolin-1-mediated endocytic vesicles and was then translocated to the endoplasmic reticulum (ER) region. However, when placed in ER-rich microsome, the mutant capsid retained its spherical structure in contrast to the wild type, which disassembled. Structural analysis of the mutant capsid with cryo-electron microscopy and image reconstruction revealed altered pentamer coordination, possibly as a result of electrostatic interaction, although its overall structure resembled that of the wild type. These results indicate that the calcium ion serves as a trigger at the pentamer interface, which switches on capsid disassembly, and that the failure of the E330K mutant capsid to disassemble is attributable to an inadequate triggering system. Our data also indicate that calcium depletion-induced SV40 capsid disassembly may occur in the ER region and that this is essential for successful SV40 infection.

Transport, transformation and distribution space of propofol in the rat liver studied by means of the indicator-dilution technique

1. The transport, transformation and distribution space of the endovenous anaesthetic propofol in the isolated perfused rat liver were investigated by using the multiple-indicator dilution technique with constant infusion (step input). 2. The behaviour of propofol in the liver was described by a space-distributed variable transit-time model. The drug permeated the cell membrane at very high rates and its distribution into the cellular space was flow-limited. The apparent distribution space of propofol varied between 284 and 125 times the water space, and was inversely related to the tested portal concentrations (33-250 microM). 3. The corresponding ratios of intra- to extracellular concentration varied between 319 and 187, revealing a very high affinity of the liver for propofol. They most probably reflect binding to several cellular structures, including membranes and proteins. 3. The single-pass rate coefficients for biotransformation decreased with increases in the portal concentration of propofol. The liver released significant amounts of 4-hydroxypropofol, reaching 41.7 % of the total single pass of 67.2 microM propofol biotransformation. These results disprove previous notions that hydroxylation is rate limiting for conjugation and suggest that the liver might function as a 4-hydroxypropofol source for conjugation to glucuronic acid or sulfate in other tissues.

In vitro effects of environmentally relevant polybrominated diphenyl ether (PBDE) congeners on calcium buffering mechanisms in rat brain

Polybrominated diphenyl ethers (PBDEs) are widely used as additive flame-retardants and have been detected in human blood, adipose tissue, and breast milk. Developmental and long-term exposures to these chemicals may pose a human health risk, especially to children. We have previously demonstrated that polychlorinated biphenyls (PCBs), which are structurally similar to PBDEs and cause neurotoxicity, perturb intracellular signaling events including calcium homeostasis and protein kinase C translocation, which are critical for neuronal function and development of the nervous system. The objective of the present study was to test whether environmentally relevant PBDE congeners 47 and 99 are also capable of disrupting Ca(2+) homeostasis. Calcium buffering was determined by measuring (45)Ca(2+)-uptake by microsomes and mitochondria, isolated from adult male rat brain (frontal cortex, cerebellum, hippocampus, and hypothalamus). Results show that PBDEs 47 and 99 inhibit both microsomal and mitochondrial (45)Ca(2+)-uptake in a concentration-dependent manner. The effect of these congeners on (45)Ca(2+)-uptake is similar in all four brain regions though the hypothalamus seems to be slightly more sensitive. Among the two preparations, the congeners inhibited (45)Ca(2+)-uptake in mitochondria to a greater extent than in microsomes. These results indicate that PBDE 47 and PBDE 99 congeners perturb calcium signaling in rat brain in a manner similar to PCB congeners, suggesting a common mode of action of these persistent organic pollutants.

Both translocon and a cation channel are involved in the passive Ca2+ leak from the endoplasmic reticulum: A mechanistic study on rat liver microsomes

Steady-state levels of calcium ions in endoplasmic reticulum reflect a balance between active inward transport, mediated by MgATP-dependent Ca(2+) pumps, and passive backflux of the ions, through putative "leak channels". We have investigated the efflux of Ca(2+) from rat liver microsomal vesicles, passively pre-equilibrated in the presence radiolabelled Ca(2+). Similarly, we have also evaluated the efflux of a low-Mwt uncharged compound, i.e., sucrose. The results show that two major passive Ca(2+) efflux pathways exist. One appeared to involve the translocon pore, since it was stimulated by the translocon opener puromycin, and also allowed the passage of sucrose. Putative channels likely mediated the other one, since it required counter ion influx and was inhibited by Gd(3+) and La(3+). The latter pathway did not appear to involve inactive Ca(2+) pumps, Bcl2 proteins, or known channels, such as the InsP3 and ryanodine receptors. While sucrose efflux was highly represented in a rough microsomal subfraction--enriched in the translocon component Sec61alpha--the efflux of Ca(2+) was represented both in smooth and in rough microsomes. We conclude that the passive efflux of Ca(2+) from the (liver) ER could be mediated by both the translocon pore and putative Ca(2+) leak channels. However, the relative role of these Ca(2+) efflux pathways in the intact cell as well as the molecular nature of the Ca(2+) leak channel(s) remain to be clarified.

Effects of methotrexate on calcium flux in rat liver mitochondria, microsomes and plasma membrane vesicles

The metabolic effects of methotrexate in perfused livers are similar to those exerted by hormones acting through Ca(2+)-dependent mechanisms. The aim of the present study was to determine whether the effects of methotrexate are mediated by a direct action on cellular Ca(2+) fluxes. Methotrexate did not affect the ATP-dependent (45)Ca(2+) uptake by mitochondria, microsomes and inside-out plasma membrane vesicles and Ca(2+) efflux from plasma membrane vesicles. However, methotrexate was able to stimulate (45)Ca(2+) release from preloaded microsomes. The amount of Ca(2+) released by methotrexate was similar to that induced by IP(3). Methotrexate could be acting through the capacitative calcium entry mechanism.

Regulation of Mg2+-independent Ca2+-ATPase by a low molecular mass protein purified from bovine brain

The goat sperm microsomal membranes have been found to contain an Mg2+-independent Ca2+-ATPase, a low affinity but highly active enzyme sharing similarities with the SERCA family of ATPases. The present study reports the identification and characterization of a 14 kilodalton cytosolic protein from bovine brain which can act as an endogenous stimulator of the enzyme with an S50 (concentration producing 50% stimulation) of 0.8 mu molar. Kinetic analysis suggests that the stimulation is noncompetitive with respect to the substrate, and the binding site(s) of the stimulator and substrate are distinct. Binding of the stimulator to the enzyme is reversible. The stimulator increases the affinity of the enzyme for calcium as evident from a decrease in K0.5 of the enzyme for calcium in presence of the stimulator. Radioactive labeling of the enzyme with [gamma-32P]-ATP suggests that the stimulator enhances the rate of dephosphorylation of the phosphoenzyme intermediate without altering the phosphorylation reaction step. The stimulatory effect of the protein has been observed only for the Mg2+-independent form of the enzyme, the Mg2+-dependent form being unaffected.

Differential Effects of Commercial Polybrominated Diphenyl Ether and Polychlorinated Biphenyl Mixtures on Intracellular Signaling in Rat Brain in Vitro

Polybrominated diphenyl ethers (PBDEs) are widely used as flame retardants and have been detected in human blood, adipose tissue, and breast milk. Developmental and long-term exposures to these contaminants may pose a human health risk, especially to children. Previously, we demonstrated that polychlorinated biphenyls (PCBs), which are neurotoxic and structurally similar to PBDEs, perturbed intracellular signaling events, including calcium homeostasis and subsequent events such as protein kinase C (PKC), which are critical for the normal function and development of the nervous system. The objective of the present study was to test whether commercial PBDE mixtures (DE-71, a pentabrominated dipheyl ether mixture, and DE-79, a mostly octabromodiphenyl ether mixture) affected intracellular signaling mechanisms in a similar way to that of PCBs and other organohalogens, as an attempt to understand the common mode of action for these persistent chemicals. PKC translocation was studied by determining (3)H-phorbol ester ((3)H-PDBu) binding in rat cerebellar granule cells, and calcium buffering was determined by measuring (45)Ca(2+) uptake by microsomes and mitochondria isolated from adult male rat brain (frontal cortex, cerebellum, and hippocampus). As seen with PCBs, DE-71 increased PKC translocation and inhibited (45)Ca(2+) uptake by both microsomes and mitochondria in a concentration-dependent manner. The effect of DE-71 on (45)Ca(2+) uptake seems to be similar in all three brain regions. Between the two organelles, DE-71 inhibited mitochondrial (45)Ca(2+) uptake to a greater extent than microsomal (45)Ca(2+) uptake. DE-79 had no effects on either neurochemical event even at 30 mug/ml. Aroclor 1254 altered both events to a greater extent compared to DE-71 on a weight basis. When the results were compared on a molar basis, Aroclor 1254 altered PKC translocation and microsomal (45)CaP(2+) uptake to a greater extent than DE-71, however, Aroclor 1254 and DE-71 equally affected mitochondrial (45)Ca(2+) uptake. These results indicate that PBDEs perturbed intracellular signaling mechanisms in rat brain as do other organohalogen compounds and the efficacy between the commercial PCB and PBDE mixtures seem to vary with different endpoints.

Effects of PCB 84 enantiomers on [3H]-phorbol ester binding in rat cerebellar granule cells and 45Ca2+-uptake in rat cerebellum

There is evidence that polychlorinated biphenyl (PCB) congeners with ortho chlorine substituents have potential to cause neurotoxicity. Many PCB congeners implicated in these neurotoxic effects are chiral. It is currently unknown if the enantiomers of chiral PCB congeners have different neurotoxic effects. We herein report the effect of racemic 2,2',3,3',6-pentachlorobiphenyl (PCB 84) and its enantiomers on two neurochemical measures, protein kinase C (PKC) translocation as determined by [3H]-phorobol ester binding in cerebellar granule cells and Ca2+-sequestration as determined by 45Ca2+-uptake by microsomes isolated from adult rat cerebellum. Both (+)- and (-)-PCB 84 increased [3H]-phorobol ester binding in a concentration-dependent manner with (-)-PCB 84 being slightly more potent. Racemic PCB 84 was significantly more potent and efficacious than the pure enantiomers alone. (-)- and (+)-PCB 84 each inhibited microsomal 45Ca2+-uptake to a similar extent, whereas racemic PCB 84 was more potent and efficacious. These results indicate that PCB 84 enantiomers alone can have different potencies, and these may differ from that of the racemic mixture, observations that may have important implications for understanding the mechanisms of neurotoxicity of chiral PCB congeners.

Naproxen affects Ca2+ fluxes in mitochondria, microsomes and plasma membrane vesicles

There is substantial evidence that nonsteroidal anti-inflammatory drugs (NSAIDs) affect cellular processes regulated by Ca(2+) ions, including the metabolic responses of the liver to Ca(2+)-dependent hormones. The aim of the present study was to determine whether the effects of naproxen are mediated by a direct action on cellular Ca(2+) fluxes. The effects of naproxen on 45Ca(2+) fluxes in mitochondria, microsomes and inside-out plasma membrane vesicles were examined. Naproxen strongly impaired the mitochondrial capacity to retain 45Ca(2+) and inhibited also ATP-dependent 45Ca(2+) uptake by microsomes. Naproxen did not modify 45Ca(2+) uptake by inside-out plasma membrane vesicles, but it inhibited the hexokinase/glucose-induced Ca(2+) efflux from preloaded vesicles. Additional assays performed in isolated mitochondria revealed that naproxen causes mitochondrial uncoupling and swelling in the presence of Ca(2+) ions. These effects were prevented by EGTA, ruthenium red and cyclosporin A, indicating that naproxen acts synergistically with Ca(2+) ions by promoting the mitochondrial permeability transition. The experimental results suggest that naproxen may impair the metabolic responses to Ca(2+)-dependent hormones acting by at least two mechanisms: (1) by interfering with the supply of external Ca(2+) through a direct action on the plasma membrane Ca(2+) influx, and (2) by affecting the refilling of the agonist-sensitive internal stores, including endoplasmic reticulum and mitochondria.

Kinetic properties of the glucose 6-phosphatase of the liver from arthritic rats

According to previous reports, adjuvant-induced arthritic rats present reduced activities of the hepatic glucose 6-phosphatase. A kinetic study was done in order to characterize this phenomenon. Microsomes were isolated from livers of arthritic and control rats (Holtzman strain) and the glucose 6-phosphatase was measured at various temperatures (13-37 degrees C) and glucose 6-phosphate concentrations. Irrespective of the temperature, the enzyme from arthritic rats presented a reduction of both V(max) and K(M). Detergent treatment of liver microsomes from control rats increased the activity, but no increase was found when microsomes from arthritic rats were treated in the same way. The mannose 6-phosphatase activity of detergent-treated microsomes from arthritic rats was only 25% of the activity found with detergent-treated microsomes from control rats. Without detergent treatment, the mannose 6-phosphatase activities of both control and arthritic rats were minimal. The activation energy, derived from V(max), was not changed by arthritis. In vivo arthritic rats presented higher hepatic glucose 6-phosphate concentrations, a phenomenon that is consistent with a reduced activity of glucose 6-phosphatase. It was concluded that in arthritic rats, the hydrolase is probably reduced, without a similar change in the translocase activity.

Increased [3H]phorbol ester binding in rat cerebellar granule cells and inhibition of 45Ca(2+) buffering in rat cerebellum by hydroxylated polychlorinated biphenyls

Our previous structure-activity relationship (SAR) studies indicated that the effects of polychlorinated biphenyls (PCBs) on neuronal Ca(2+) homeostasis and protein kinase C (PKC) translocation were associated with the extent of coplanarity. Chlorine substitutions at ortho position on the biphenyl, which increase the non-coplanarity, are characteristic of the most active congeners in vitro. In the present study, we investigated the effects of selected hydroxylated PCBs, which are major PCB metabolites identified in mammals, on the same measures where PCBs had differential effects based on structural configuration. These measures include PKC translocation as determined by [3H]phorbol ester ([3H]PDBu) binding in cerebellar granule cells, and Ca(2+) sequestration as determined by 45Ca(2+) uptake by microsomes isolated from adult rat cerebellum. All the selected hydroxy-PCBs with ortho-chlorine substitutions increased [3H]PDBu binding in a concentration-dependent manner and the order of potency as determined by E(50) (concentration that increases control activity by 50%) is 2',4',6'-trichloro-4-biphenylol (32 +/- 4 microM), 2',5'-dichloro-4-biphenylol (70 +/- 9 microM), 2,2',4',5,5'-pentachloro-4-biphenylol (80 +/- 7 microM) and 2,2',5'-trichloro-4-biphenylol (93 +/- 14 microM). All the selected hydroxy-PCBs inhibited microsomal 45Ca(2+) uptake to a different extent. Among the hydroxy-PCBs selected, 2',4',6'-trichloro-4-biphenylol is the most active in increasing [3H]PDBu binding as well as inhibiting microsomal 45Ca(2+) uptake. 3,5-Dichloro-4-biphenylol and 3,4',5-trichloro-4-biphenylol did not increase [3H]PDBu binding, but inhibited microsomal 45Ca(2+) uptake. This effect was not related to ionization of these two hydroxy-PCBs. Hydroxylated PCBs seemed to be as active as parent PCBs in vitro. These studies indicate that PCB metabolites such as hydroxy-PCBs might contribute significantly to the neurotoxic responses of PCBs.

Neurochemical effects of environmental chemicals: in vitro and in vivo correlations on second messenger pathways

Polychlorinated biphenyls (PCBs) are persistent, bioaccumulative, toxic, and widely distributed environmental chemicals. There is now both epidemiological and experimental evidence that PCBs cause cognitive deficits; however, the underlying cellular or molecular mechanism(s) is not known. We have hypothesized that altered signal transduction/second messenger homeostasis by PCBs may be associated with these effects since second messengers in signal transduction pathways, such as calcium, inositol phosphates (IP), and protein kinase C (PKC), play key roles in neuronal development and their function. In vitro studies using cerebellar granule neurons and isolated organelle preparations indicate that ortho-PCBs increase intracellular free Ca2+ levels by inhibiting microsomal and mitochondrial Ca2+ buffering and the Ca2+ extrusion process. Ortho-PCBs also increase agonist-stimulated IP accumulation and cause PKC translocation at low micromolar concentrations where no cytotoxicity is observed. On the other hand, non-ortho-PCBs are not effective in altering these events. Further SAR studies indicate that congeners with chlorine substitutions favoring non-coplanarity are active in vitro, while congeners favoring coplanarity are relatively inactive. Subsequent in vivo studies have shown that repeated exposure to a PCB mixture, Aroclor 1254, increases PKC translocation and decreases Ca2+ buffering in the brain, similar to in vitro studies. These changes in vivo are associated with elevated levels of non-coplanar ortho-PCB congeners at levels equivalent to 40-50 microM in brain, the concentrations that significantly inhibited second messenger systems in neuronal cultures in vitro. Current research is focusing on PCB-induced alterations in second messenger systems following developmental exposure.

Extracellular calcium is required for the polychlorinated biphenyl-induced increase of intracellular free calcium levels in cerebellar granule cell culture

Recent studies from the laboratory indicate that polychlorinated biphenyl (PCB) congeners can alter signal transduction and calcium homeostasis in neuronal preparations. These effects were more pronounced for the ortho-substituted, non-coplanar congeners, although the mechanisms underlying these effects are not clear. In the present study the time-course and concentration-dependent effects of coplanar and non-coplanar PCBs on intracellular free calcium concentration ([Ca2+]i) in cerebellar granule cell cultures were compared using the fluorescent probe fura-2. The ortho-substituted congeners 2,2'-dichlorobiphenyl (DCB) and 2,2',4,6,6'-pentachlorobiphenyl (PeCB) caused a gradual increase of [Ca2+]i while the non-ortho-substituted congeners 4,4'-DCB and 3,3',4,4',5-PeCB had no effect. The increase of [Ca2+]i produced by 2,2'-DCB was time- and concentration-dependent. Further studies examined possible mechanisms for this rise in [Ca2+]i. In contrast to the muscarinic agonist carbachol, the effects of 2,2'-DCB on [Ca2+]i were not blocked by thapsigargin and required the presence of extracellular calcium. The effects of ortho-substituted PCBs may depend on their ability to inhibit calcium sequestration as 2,2'-DCB significantly inhibited 45Ca2+-uptake by microsomes and mitochondria while 3,3',4,4',5-PeCB had no effect. In addition, 2,2'-DCB significantly increased the binding of [3H]inositol 1,4,5-trisphosphate to receptors on cerebellar microsomes, suggesting another possible mechanism by which ortho-substituted PCBs can mobilize [Ca2+]i. These results show that PCBs increase [Ca2+]i in vitro via a mechanism that requires extracelluar calcium, and support previous structure-activity studies indicating that ortho-substituted PCBs are more potent than non-ortho-substituted PCBs.

Neurotoxicity of environmental chemicals and their mechanism of action

Despite a ban on their manufacture in 1977, polychlorinated biphenyls (PCBs) are still found in significant quantities in the environment. Developmental exposure to PCBs and related compounds has been reported to be neurotoxic in human and animals. Research in our laboratory has focused on the possible site(s) and mechanism(s) of PCB-induced developmental neurotoxicity. Recent experiments with rats found that developmental exposure to Aroclor-1254 (ARC) affects the acquisition of a lever press response and produces long-term changes in calcium buffering and protein kinase C (PKC) activity in the brain. In vitro studies in our laboratory have found that ARC increases [3H]phorbol ester binding, an indirect measure of PKC translocation, and inhibits calcium buffering in microsomes and mitochondria. Other experiments indicate that PCB congeners with chlorine substitutions at ortho- or low lateral substitutions are active in vitro, while non-ortho-substituted congeners are less active or inactive. Other research suggests that the lack of coplanarity of the PCB molecule is related to in vitro activity of PCB congeners. These studies indicate that in vivo developmental exposure to PCBs alters behavior and second messenger systems during adulthood, while in vitro experiments indicate that nervous system activity is related to ortho-substituted congeners that tend to be non-coplanar in configuration. Our results are consistent with the hypothesis that developmental neurotoxicity of ARC is due, in part, to the presence of ortho-substituted PCB congeners.

Repeated exposure of adult rats to Aroclor 1254 causes brain region-specific changes in intracellular Ca2+ buffering and protein kinase C activity in the absence of changes in tyrosine hydroxylase

Polychlorinated biphenyls (PCBs) are ubiquitous environmental contaminants, some of which may be neurotoxic. In vitro studies from this laboratory indicated that noncoplanar PCBs perturbed intracellular signal transduction mechanisms including Ca2+ homeostasis, receptor-mediated inositol phosphate production, and translocation of protein kinase C (PKC). In the present study, we examined the effects of PCBs in vivo by dosing adult male Long-Evans rats orally with Aroclor 1254 (0, 10, or 30 mg/kg/day; 5 days/week for 4 weeks) in corn oil. At 24 h after the last dose, rats were tested for motor activity in a photocell device for 30 min. Immediately, the rats were euthanized, blood was collected for thyroid hormone analysis, and brains were removed, dissected into regions (cerebellum, frontal cortex, and striatum), and subcellular fractions were obtained for neurochemical analysis. Following Aroclor 1254 treatment, body weight gain in the high-dose group was significantly lower than the control and low-dose groups. Horizontal motor activity was significantly lower in rats dosed with 30 mg/kg Aroclor 1254. Ca2+ buffering by microsomes was significantly lower in all three brain regions from the 30 mg/kg group. In the same dose group, mitochondrial Ca2+ buffering was affected in cerebellum but not in cortex or striatum. Similarly, total cerebellar PKC activity was decreased significantly while membrane-bound PKC activity was significantly elevated at 10 and 30 mg/kg. PKC activity was not altered either in cortex or the striatum. Neurotransmitter levels in striatum or cortex were slightly altered in PCB-exposed rats compared to controls. Furthermore, repeated oral administration of Aroclor 1254 to rats did not significantly alter forebrain tyrosine hydroxylase immunoreactivity or enzymatic activity. Circulating T4 (total and free) concentrations were severely depressed at both doses in Aroclor 1254-exposed rats compared to control rats, suggesting a severe hypothyroid state. These results indicate that (1) in vivo exposure to a PCB mixture can produce changes in second messenger systems that are similar to those observed after in vitro exposure of neuronal cell cultures; (2) second messenger systems seem to be more sensitive than alterations in neurotransmitter levels or tyrosine hydroxylase involved in dopamine synthesis during repeated exposure to PCBs; and (3) the observed motor activity changes were independent of changes in striatal dopamine levels.

Aluminum alters intracellular calcium homeostasis in vitro

The present paper reports data regarding the influence of aluminum, at micromolar concentrations, on intracellular calcium homeostasis. Al3+ modifies Ca2+ uptake in the endoplasmic reticulum (ER), accelerates Ca2+ release from mitochondria and strongly inhibits Ca2+-ATPase activity with a consequent high-level calcium accumulation inside the cell. These results suggest that Al3+ neurotoxicity may be related to an alteration of the intracellular calcium regulatory system.

Characterization of the Golgi complex cleared of proteins in transit and examination of calcium uptake activities

To characterize endogenous molecules and activities of the Golgi complex, proteins in transit were > 99% cleared from rat hepatocytes by using cycloheximide (CHX) treatment. The loss of proteins in transit resulted in condensation of the Golgi cisternae and stacks. Isolation of a stacked Golgi fraction is equally efficient with or without proteins in transit [control (CTL SGF1) and cycloheximide (CHX SGF1)]. Electron microscopy and morphometric analysis showed that > 90% of the elements could be positively identified as Golgi stacks or cisternae. Biochemical analysis showed that the cis-, medial-, trans-, and TGN Golgi markers were enriched over the postnuclear supernatant 200- to 400-fold with and 400- to 700-fold without proteins in transit. To provide information on a mechanism for import of calcium required at the later stages of the secretory pathway, calcium uptake into CTL SGF1 and CHX SGF1 was examined. All calcium uptake into CTL SGF1 was dependent on a thapsigargin-resistant pump not resident to the Golgi complex and a thapsigargin-sensitive pump resident to the Golgi. Experiments using CHX SGF1 showed that the thapsigargin-resistant activity was a plasma membrane calcium ATPase isoform in transit to the plasma membrane and the thapsigargin-sensitive pump was a sarcoplasmic/endoplasmic reticulum calcium ATPase isoform. In vivo both of these calcium ATPases function to maintain millimolar levels of calcium within the Golgi lumen.

Changes of adenosine triphosphate-dependent calcium uptake in microsomal fractions of rat liver during sepsis

BACKGROUND

Intracellular calcium concentration is an important regulator of cellular metabolism. Endoplasmic reticulum membranes play an important role in the regulation of cytoplasmic calcium in the mammalian liver. The characterization of the changes of calcium uptake in endoplasmic reticulum may contribute to the potential intracellular mechanisms for cellular dysfunction during sepsis.

METHODS

The effects of sepsis on the calcium uptake in rough endoplasmic reticulum of rat liver were studied. Sepsis was induced by means of cecal ligation and puncture (CLP). The control rats underwent sham operation. Microsomal fractions were isolated from the liver with differential centrifugation.

RESULTS

The calcium uptake by liver endoplasmic reticulum was decreased by 30% to 35% (p < 0.05) during early sepsis (9 hours after CLP) and by 38% to 43% (p < 0.05) during late sepsis (18 hours after CLP), respectively. The maximum velocity values for adenosine triphosphate (ATP) and for Ca2+ were also decreased by 25% to 37% (p < 0.05) during early sepsis and by 35% to 42% (p < 0.05) during late sepsis. The Michaelis-Menten constant for ATP and Ca2+ transport had no difference among three groups. The magnesium stimulation and vanadate inhibitory activity were also decreased by 17% to 38% (p < 0.05) during early sepsis and by 34% to 50% (p < 0.05) during late sepsis.

CONCLUSIONS

These data demonstrate that ATP-dependent calcium uptake in rough endoplasmic reticulum of rat liver was impaired during early and late sepsis. Because the low intracellular calcium concentration plays an important role in the regulation of cellular function, an impairment in the ATP-dependent calcium uptake by endoplasmic reticulum during early and late sepsis may have a pathophysiologic significance in contributing to the development of altered hepatic metabolism during sepsis.

Photodynamic effects induced by meso-tetrakis[4-(carboxymethyleneoxy)phenyl]porphyrin using rat hepatic microsomes as model membranes

Porphyrins, in combination with light, offer an alternate approach to the treatment of cancer, in the form of photodynamic therapy (PDT). With a view to locate new porphyrins for use in PDT, we evaluated the ability of a novel water-soluble porphyrin, meso-tetrakis[4-(carboxymethyleneoxy)phenyl]porphyrin (T4CPP) to induce photodamage in membranes, using rat hepatic microsomes as a model system. Hepatic microsomes treated with T4CPP and exposed to visible light showed significant lipid peroxidation, as assessed by the formation of conjugated dienes, lipid hydroperoxides, and thiobarbituric acid-reactive substances. The peroxidation induced was both time- and concentration-dependent. T4CPP plus light also resulted in the destruction of the microsomal enzymes adenosine triphosphatase and glucose-6-phosphatase. Analysis of the products of peroxidation and selective inhibition by specific inhibitors showed that the oxidative damage induced was mainly due to singlet oxygen and partly due to hydroxyl radical. The porphyrin T4CPP was efficiently labeled with 99mTc. When this 99mTc-labeled porphyrin was injected into a mammary-tumor-bearing rat, it accumulated in the tumor. Our studies suggest that T4CPP, due to its potential to localize in tumors and to induce membrane damage as exemplified by alteration in rat liver microsomes, may have possible applications in this new modality of cancer treatment.

Reduced anesthetic requirements in aged rats: association with altered brain synaptic plasma membrane Ca(2+)-ATPase pump and phospholipid methyltransferase I activities

Aging is associated with a decrease in anesthetic requirements. Animal models of aging manifest alteration of brain Ca2+ homeostasis and increased methyltransferase I (PLMTI) activity. In this study we evaluated concurrently anesthetic requirements and brain plasma membrane Ca(2+)-ATPase (PMCA) and PLMTI activities in young and aged rats. Halothane, desflurane, isoflurane and xenon MEDs (lowest partial pressures that suppress a pain response) were measured in 2 and 25 month old, male Fisher-344 rats. Halothane MED was also measured in 2 and 30 month old F344/BNF1 rats, a strain that undergoes aging with less debilitation. PMCA pumping and PLMTI activities were measured in synaptic plasma membranes (SPM) prepared from the cortex and diencephalon-mesencephalon (DM). For aged Fisher-344 rats, MEDs for halothane, desflurane, isoflurane and xenon were reduced to 81%, 82%, 67% and 86%, respectively, of young controls; PMCA activity was diminished to 91% in cortical SPM and 82% in DM SPM; and cortical and DM PLMTI activities were increased to 131% and 114% of young control. For F344/BNF1 rats, MED for halothane was reduced to 87%, PMCA activity was diminished to 90% in cortical SPM and 72% DM SPM, and PLMTI activity was increased to 133% in cortical SPM and 112% in DM SPM. The strong association between age and reduced anesthetic requirements for inhalational agents on the one hand and altered PMCA and PLMTI activity on the other lends support to the underlying hypothesis that PMCA and PLMTI may be involved in the production of the anesthetic state.

An ATP-dependent iron transport system in isolated rat liver nuclei

A concerted translational control is responsible for maintaining an iron level in the cytosol that is both adequate for the synthesis of iron-containing proteins and does not represent a danger to the cell. However, little is known about how iron level is controlled in the nucleus. Nuclei of rat liver take up iron from ferric citrate by a process that is dependent on ATP. This system shares several properties with known P-type ATPases, suggesting that a P-type ATPase in the nuclear membrane is responsible for iron transport. (i) Adenosine 5'-(beta,gamma-iminodiphosphate), a non-hydrolyzable ATP analogue, does not support iron uptake; (ii) the uptake is strongly inhibited by vanadate; (iii) there is an absolute requirement for Mg2+; and (iv) reagents that oxidize SH groups inhibit uptake, and this inhibition can be prevented by dithiothreitol. The energy of activation for the uptake (11.5 kcal/mol) and the Km for ATP (0.4 mM) are similar to values for other known cation transport ATPases. Inhibitors of Na+,K+-ATPase, sarcoplasmic reticulum Ca2+-ATPase, proton V-ATPase, and nuclear Ca2+-ATPase have no effect on uptake. Ferric citrate can be replaced by Fe-ATP as a source of iron for the transport system; however, two other stronger iron chelators, Tiron and desferrioxamine, completely inhibit the uptake. Taken together, these data strongly suggest that an Fe-ATPase, distinct from other known P-type ATPases, is responsible for iron transport in the nucleus.

Inhibition of microsomal and mitochondrial Ca2+-sequestration in rat cerebellum by polychlorinated biphenyl mixtures and congeners. Structure-activity relationships

Recent studies from our laboratory indicate that polychlorinated biphenyl (PCB) congeners in vitro perturbed signal transduction mechanisms including cellular Ca2+-homeostasis and protein kinase C translocation. We have now investigated the structure-activity relationship (SAR) of three PCB mixtures, 24 PCB congeners and one dibenzofuran for their effects on microsomal and mitochondrial Ca2+-sequestration in rat cerebellum. Ca2+-sequestration by these intracellular organelles was determined using radioactive 45CaCl2. All three mixtures studied, Aroclor 1016, Aroclor 1254 and Aroclor 1260, were equally potent in inhibiting microsomal and mitochondrial Ca2+-sequestration with IC50 values of 6-8 microM. 1,2,3,7,8-Pentachlorodibenzofuran had no effect on Ca2+-sequestration by these organelles. The SAR among the congeners revealed: (1) congeners with ortho-/meta- or ortho-, para-chlorine substitutions were the most potent in inhibiting microsomal and mitochondrial Ca2+-sequestration (IC50 = 2.4-22.3 microM); (2) congeners with only para- but without ortho-substitutions were not effective in inhibiting Ca2+-sequestration by microsomes and mitochondria; (3) increased chlorination was not related to the effectiveness of these congeners. The present SAR studies indicate that the effects of most PCB congeners in vitro may be related to an interaction at specific sites having preference for low lateral substitution or lateral content (meta- or para) in the presence of ortho-substitution.

Nitrous oxide and xenon enhance phospholipid-N-methylation in rat brain synaptic plasma membranes

Halothane and isoflurane increase the rate of phospholipid methylation (PLM) in rat brain synaptosomal membranes, a process linked to the coupling of neuronal excitation to neurotransmitter release. In contrast, synaptic plasma membrane (SPM) Ca2+ ATPase (PMCA) pumping is reduced by exposure to halothane, isoflurane, xenon and nitrous oxide (N2O). To examine further the relationship between PLM, PMCA and anesthetic action, we investigated the effect of clinically relevant concentrations of two less potent anesthetic gases, N2O and xenon, on PLM in SPM. Biochemical assays were performed on SPM exposed to 1.3 MAC of N2O (2 atm), 1.3 MAC of xenon (1.23 atm) or an equivalent pressure of helium for control. N2O or xenon exposure increased PLM to 115% or 113%, respectively, of helium control (p < 0.02). Similar exposures to N2O or xenon depressed PMCA activity to 78% and 85% of control (p < 0.05). Observations that PLM and PMCA are both altered by a wide variety of inhalation anesthetic agents at clinically relevant partial pressures lend support to a possible involvement and interaction of these processes in anesthetic action.