Calcium channels in hepatocytes

Francisella noatunensis subsp. noatunensis triggers calcium metabolism gene modulation in Eleginops maclovinus

Francisella noatunensis subsp. noatunensis is the responsible agent of Francisellosis, a bacterial disease that affects an important amount of aquatic farmed species. Eleginops maclovinus is a fish that cohabits with salmonids cages in Chile and can also act as a vector of this bacterial disease. In the present study, we evaluated calcium metabolism in the liver of E. maclovinus injected intraperitoneally with different doses of F. noatunensis subsp. noatunensis (low 1.5 × 10¹, medium 1.5 × 10⁵ and high doses 1.5 × 10¹⁰ cells/μL). Fish were sampled at 1, 3, 7, 14, 21 and 28 days post injection (dpi). No mortalities nor clinical signs were observed. Plasma calcium levels were higher in the high doses group of F. noatunensis subsp. noatunensis at day 7 and 14 compared to the control group (fish injected with bacterial medium alone). Hypercalcemic factors increased at day 14 and 21 for the medium and low dose (parathyroid hormone-related protein precursor), while vitamin D₃ receptor increased its expression at times 1, 3 and 7 for the low dose. On the other hand, hypocalcemic factors such as calcitonin receptor and stanniocalcin increased its expression at time 7 and 14, respectively. Calmodulin involved in calcium storage decreased its expression during all experimental days in fish subjected to high bacterial dose. Proteins involved in calcium transport, such as L-type voltage-gated calcium channel and trpv5 increased their transcription at day 1 and 14, compared to calcium sensing-receptor and plasma membrane Ca^{2 +}- ATPase that showed peak expression at times 14 and 28. The results suggest a clear alteration of calcium metabolism, mainly in high bacterial doses. This study provides new knowledge about the calcium metabolism in fish infected with bacteria.

Toxicity of hepatotoxins: new insights into mechanisms and therapy

Liver injury caused by hepatotoxins, such as carbon tetrachloride (CCl4), ethanol, and acetaminophen (APAP), is characterised by varying degrees of hepatocyte degeneration and cell death via either apoptosis or necrosis. The generation of reactive intermediate metabolites from the metabolism of hepatotoxins, and the occurrence of reactive oxygen species (ROS) during the inflammatory reaction account for a variety of pathophysiologic pathways leading to cell death, such as covalent binding, disordered cytosolic calcium homeostasis, glutathione (GSH) depletion, onset of mitochondrial permeability transition (MPT) and associated lipid peroxidation. The metabolism of hepatotoxins by cytochrome P-450 enzyme subtypes is a key step of the intoxication; therefore, enzyme inhibitors are shown to minimise the hepatotoxin-associated liver damage. Understanding the function of transcription factors, such as nuclear factor kappaB (NF-kappaB) in acute liver injury, may provide some answers as to the molecular mechanisms of toxic insults. Moreover, substantial evidence exists that MPT is involved in ROS-associated hepatocellular injury and new findings offer a novel therapeutic approach to attenuate cell damage by blocking the onset of MPT. Thus, oxidant stress and lipid peroxidation are crucial elements leading to hepatotoxin-associated liver injury. In addition to specific treatment for a given hepatotoxin, the general strategy for prevention and treatment of the damage includes reducing the production of reactive metabolites of the hepatotoxins, using anti-oxidative agents, and selectively targeting therapeutics to Kupffer cells or hepatocytes for on-going processes, which play a role in mediating a second phase of the injury.

Lowering intracellular and extracellular calcium contents prevents cytotoxic effects of ethylene glycol-based vitrification solution in unfertilized mouse oocytes

We investigated the characteristics of the changes in intracellular calcium (Ca2+) concentration ([Ca2+](i)) and the viability of the unfertilized mouse oocytes exposed to various concentrations of ethylene glycol (EG)-containing solutions or vitrification solutions. Oocytes exposed to EG (1, 5, 10, 20, and 40% (v/v)) exhibited a rapid and dose-dependent increase in [Ca2+](i). The survival rate was 100% when oocytes were exposed to the EG concentration up to 5% through 5 min, while all oocytes were dead within 3 min when exposed to 10, 20, or 40% EG. When extracellular Ca2+ was removed, increase in [Ca2+](i) at 10 and 20% EG was less than that at the same concentrations of EG with extracellular Ca2+. The survival rates of the oocytes exposed to 10, 20, and 40% EG at 3 min were 100, 97, and 0%, respectively. In the presence of 20 microM 1,2-bis(o-aminopheoxy)ethane-N,N,N',N'-tetraacetic acid tetra acetoxymethyl ester (BAPTA-AM), a Ca2+ chelator, a small increase in [Ca2+](i) exposed to 10, 20, and 40% EG was observed until 4 min. Subsequently prolonged elevation of the [Ca2+](i) was observed in the oocytes exposed to 40% EG but not with 10 and 20% EG. The survival rate of the oocytes, in the presence of 20 microM BAPTA-AM, exposed to 10 and 20% EG was 100% throughout 5 min, while the oocytes exposed to 40% EG were alive only for 3 min. Treatment by the vitrification solution with various concentrations of EG (10, 20, and 40%) caused a smaller increase in [Ca2+](i), while the survival rates were higher compared to those without vitrification solution at the same concentrations of EG. These data suggested that the sustained [Ca2+](i) rises by EG in unfertilized mouse oocytes resulted in cell death. Therefore, the lowering of [Ca2+](i) in the oocytes exposed to the cryoprotectant may improve the viability of cryopreserved unfertilized oocytes.

Cold-induced calcium elevation triggers DNA fragmentation in immature pig oocytes

Fluo-4 loaded immature oocytes were cooled from 30 degrees C to various lower temperatures between 20 and 10 degrees C and changes in intracellular calcium (Ca(2+)) levels were measured. Pig oocytes cooled to 14 degrees C exhibited a clear biphasic Ca(2+) rise. Lower temperatures produced similar responses, while higher temperatures did not exert any effect. The Ca(2+) response appeared to rely on ryanodine dependent stores as removal of extracellular Ca(2+) and intracytoplasmic injection of heparin did not modify cold-induced Ca(2+) elevation, while procaine or ruthenium red virtually eliminated the response. Confocal analysis of subcellular Ca(2+) distribution during cooling revealed that the ion rises sharply within the nucleus. As Ca(2+) imbalance may activate nuclear endonucleases, DNA integrity of cooled pig oocytes was evaluated by TUNEL and comet assays. Most cooled oocytes showed clear signs of DNA fragmentation. Oocytes injected with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetracetic acid tetrapotassium salt (BAPTA), a Ca(2+) chelator, maintained their DNA integrity thus confirming that intracellular Ca(2+) is involved in triggering DNA fragmentation. The protective effect exerted by ruthenium red and/or procaine further confirmed this hypothesis. These data show that a moderate and transient cooling is sufficient to cause an intracellular Ca(2+) rise that leads to DNA damage. The addition of inhibitors of ryanodine dependent Ca(2+) stores may represent a valuable protective treatment to reduce chilling injuries.

Cold preservation-warm reoxygenation increases hepatocyte steady-state Ca(2+) and response to Ca(2+)-mobilizing agonist

Although the role of Ca(2+) in liver transplantation injury has been the object of several studies, direct evidence for alterations in intracellular Ca(2+) homeostasis after cold preservation-warm reoxygenation (CP/WR) has never been presented. We thus investigated the effects of CP/WR on steady-state Ca(2+) and responses to a Ca(2+)-mobilizing agonist. Isolated rat hepatocytes were suspended in University of Wisconsin solution, stored at 4 degrees C for 0, 24, and 48 h, and reoxygenated at 37 degrees C for 1 h. Cytosolic Ca(2+) was measured in single cells by digitized fluorescence videomicroscopy. CP/WR caused a significant increase in steady-state cytosolic Ca(2+), which was inversely proportional to cell viability. Pretreatment of hepatocytes with an agent that protects mitochondrial function attenuated the increase in steady-state cytosolic Ca(2+) and improved hepatocyte viability. Ca(2+) responses to the purinergic agonist ATP also increased significantly as a function of cold storage time. This increase was related to an increase in the size of inositol 1,4,5-trisphosphate-sensitive Ca(2+) stores and subsequent capacitative Ca(2+) entry. Thus CP/WR significantly perturbs steady-state hepatocellular Ca(2+) and responses to Ca(2+)-mobilizing agonists, which may contribute to hepatocyte metabolic dysfunction observed after CP/WR.

Starch-deferoxamine conjugate inhibits hepatocyte Ca2+ uptake during hemorrhagic shock and resuscitation

BACKGROUND

This study investigated whether hepatocyte Ca2+ dysregulation after hemorrhagic shock and resuscitation could be modulated by the iron chelator hydroxyethyl starch-conjugated deferoxamine (HES-DFO).

METHODS

In a randomized experimental study, anesthetized rats (n = 7) were bled for 60 minutes to maintain mean arterial blood pressure at 40 mm Hg. They were then resuscitated with 60% of shed blood and threefold the shed-blood volume as lactated Ringer's solution, 1 mL of pentastarch solution (hydroxyethyl starch 10%) per mL of shed blood, or 1 mL of HES-DFO solution (10%) per mL of shed blood. In isolated hepatocytes, the rate of Ca2+ influx (Ca2+ in), total Ca2+ uptake (Ca2+ up), and membrane Ca2+ flux (Ca2+ flux) were determined by 45Ca incubation. Reduced or oxidized glutathione and malondialdehyde concentrations were assessed fluorometrically.

RESULTS

Significant increases of hepatocellular Ca2+ in, Ca2+ up, and Ca2+ flux were observed in rats resuscitated with lactated Ringer's solution compared with control groups (p < 0.05). Although hydroxyethyl starch decreased Ca2+ in but not Ca2+ up, HES-DFO not only prevented the increase of Ca2+ in and Ca2+ up but also inhibited hepatocyte oxidative injury.

CONCLUSION

Iron-catalyzed oxyradical production and membrane peroxidation seem to alter hepatocyte Ca2+ homeostasis after hemorrhagic shock and resuscitation.

Changes in intracellular calcium induced by acute hypothermia in parenchymal, endothelial, and Kupffer cells of the rat liver

Disturbances in intracellular calcium have been implicated in liver graft damage after cold preservation and warm reperfusion. Despite improvements noted with the use of calcium channel blockers, such as nisoldipine, the exact nature and cellular basis of the presumed changes in intracellular calcium as well as the actual target of these blockers remain unclear. Isolated rat parenchymal, endothelial, and Kupffer cells were cultured and changes in intracellular calcium measured in vitro after acute hypothermia (5-8 degrees C) by fluorescence imaging using FURA-2. Between 50 and 80% of parenchymal, endothelial, and Kupffer cells exhibited significant increases in baseline calcium that were gradual and sustained for the duration of acute hypothermia. Removal of extracellular calcium completely abolished the positive response of hepatocytes and diminished the proportion of responding endothelial and Kupffer cells. The calcium channel blocker nisoldipine (1 microM) slightly diminished the proportion of positive responders in parenchymal but not in endothelial or Kupffer cells. However, nisoldipine did not modify the amplitude of the calcium rise in responding cells of all types. Acute hypothermia causes calcium influx into a majority of parenchymal, endothelial, and Kupffer cells. Nisoldipine does not effectively prevent these changes in intracellular calcium. Pathways of calcium entry resistant to the drug or other than voltage-dependent calcium channels may thus be involved.

Evidence for norepinephrine-activated Ca2+ permeable channels in guinea-pig hepatocytes using a patch clamp technique

To determine whether the hepatocyte plasma membrane possesses a Ca2+ channel. we applied a patch clamp technique to isolated guinea-pig hepatocytes. In a cell-attached configuration, using an internal pipette solution of 110 mM BaCl2 or CaCl2, we observed sporadic inward single channel currents (Po = 0.004 +/- 0.002, n = 6) at various membrane potentials. The unit amplitude was 0.60 +/- 0.15 pA (n = 6) at resting membrane potential. The single channel conductance was 20.4 +/- 4.6 pS (n = 6) and this channel showed no rectification and no voltage dependence. Bay K 8644, a dihydropyridine Ca2+ channel activator, did not affect this channel activity. Although norepinephrine in the pipette solution did not activate this channel, its external application increased channel activity. These observations suggest that guinea-pig hepatocytes possess Ca2+ permeable channels that differ from the voltage-operated Ca2+ channels found in excitable cells and that such channels are responsible for the agonist-stimulated Ca2+ entry in hepatocytes.

Permeable analogues of cGMP promote hepatic calcium inflow induced by the synergistic action of glucagon and vasopressin but inhibit that induced by vasopressin alone

Treatment of perfused rat liver with the nitric oxide-generating reagent molsidomine led to substantial increases in cGMP without itself affecting basal Ca2+ fluxes. Under these conditions the ability of glucagon plus vasopressin to induce Ca2+ influx was greatly enhanced. The permeable analogue of cGMP (8-bromo-cGMP) enhanced glucagon plus vasopressin-induced Ca2+ influx to a similar extent as that with molsidomine. This suggests that the effect of the latter is attributable to the generation of cGMP which itself enhances the ability of the two hormones to induce synergistic Ca2+ influx. While 8-bromo-cGMP (or molsidomine) did not influence Ca2+ fluxes induced by glucagon, these agents strongly inhibited Ca2+ influx induced by vasopressin alone. These data show that while 8-bromo-cGMP has no effect on basal Ca2+ fluxes, it is able to modify the Ca2+ influx induced by glucagon and vasopressin action in hepatic tissue.

Randomised controlled double-blind trial of the calcium channel antagonist amlodipine in the treatment of acute alcoholic hepatitis

BACKGROUND/AIMS

Calcium channel blockers have a hepatoprotective action in animal models of alcohol-induced liver injury but their effect in alcoholic liver disease in humans has not been previously investigated. We have conducted a randomised, placebo-controlled trial to investigate the possible benefit of the calcium channel blocker amlodipine in terms of 4-week survival in hospitalised patients with severe acute alcoholic hepatitis.

METHODS

Sixty-two patients with acute alcoholic hepatitis were randomised to receive 5-10 mg amlodipine each day for 1 year or an identical capsule containing placebo. In 36 (58%), acute alcoholic hepatitis was confirmed on biopsy and in the remainder on clinical and laboratory criteria. There were no statistically significant differences in clinical characteristics and disease severity in the treated and placebo groups.

RESULTS

Of the 32 patients receiving amlodipine, there were six deaths (19%) in the first 4 weeks compared with seven (23%) of the placebo patients (p=0.329). Causes of death were similar in the amlodipine and control groups, with liver failure predominant. Analysis by the Cox proportional hazards model after adjustment for other prognostic factors showed survival was not significantly influenced by active treatment (p=0.07). One patient in each group was withdrawn because of the development of hypotension, but this did not recur on reintroduction of the capsules.

CONCLUSIONS

This study shows that calcium channel blockers are well tolerated with few side effects in advanced alcoholic liver disease, but there is no conclusive evidence from this study that calcium channel blockers are helpful in the treatment of alcoholic hepatitis.

Hepatotoxicity of ethanol: protective effect of calcium channel blockers in isolated hepatocytes

This study examines the effects of three calcium channel blockers (verapamil, nifedipine and diltiazem) on isolated rat hepatocytes exposed to ethanol. In the first part of our study, hepatocytes were incubated with increasing concentrations of ethanol (100, 300, 500, 1000 mM) for varying times. Alanine aminotransferase (ALT), aspartate aminotransferase (AST) and lactate dehydrogenase (LDH) release were measured to evaluate the cytotoxic effects of ethanol. The concentration of 300 mM and time of incubation of 45 min were chosen for cytoprotection experiments in which calcium channel blockers, at two different concentrations, were added to the medium 30 min prior to the addition of ethanol. ALT, AST and LDH release as well as lipid peroxidation and cellular reduced glutathione (GSH) were measured. Nifedipine and verapamil (25 microM) reduced ALT, AST and LDH activities. The highest dose of diltiazem (50 microM) was more effective than the lowest one (25 microM). Ethanol caused a significant depletion of cellular GSH content as well as a moderate enhancement of lipid peroxidation. While none of the three calcium channel blockers was able to restore the decrease in GSH levels, diltiazem (25 microM) and nifedipine (50 microM) showed the greatest effect, significantly reducing lipid peroxidation.

Tight junction dynamics in the frog urinary bladder

In a previous study in frog skin (Castro et al., J. Memb. Biol. 134:15-29, 1993), it was shown that TJs experimentally disrupted by a selective deposition of BaSO4 could be resealed upon addition of Ca2+ to the apical solution; in the absence of apical Ca2+, the normal Ca2+ activity of the Na2SO4-Ringer's bathing the basolateral side was not able to induce TJ resealing. We now show that apical Ca2+ also activates the TJ sealing mechanism in frog urinary bladders. Three known procedures were utilized to increase TJ permeability, all in the absence of apical Ca2+: (i) exposure to high positive transepithelial clamping potentials; (ii) exposure of the apical surface to hypertonic solutions; and (iii) selective deposition of BaSO4 in the TJs. The resealing of the TJs was promoted by raising the concentration of Ca2+ in the apical solution. This effect of Ca2+ is not impaired by the presence of Ca2+ channel blockers (nifedipine, verapamil, Mn2+ or Cd2+) in the apical solution, indicating that junction resealing does not depend on Ca2+ entering the cells through the apical membrane. TJ resealing that occurs in response to raised apical Ca2+ most likely results from a direct effect of Ca2+, entering the disrupted TJs from the apical solution and reaching the zonula adhaerens Ca2+ receptors (E-cadherins). Protein kinase C (PKC) must play a significant role in the control of TJ assembly in this tight epithelia since the PKC inhibitor (H7) and the activator (diC8) markedly affect TJ recovery after disruption by apical hypertonicity. H7 treated tissues show marked recuperation of conductance even in the absence of apical Ca2+. In contrast, diC8 prevents tissue recuperation which normally occurs after addition of Ca2+ to the apical solution.

Nickel: an agent for investigating the relation between hormone-induced Ca2+ influx and bile flow in the perfused rat liver

Influx of Ca2+ induced by the synergistic action of glucagon plus vasopressin in the perfused rat liver was progressively inhibited by infusing increasing concentrations of Ni2+ to the perfusion medium. The onset of Ca2+ influx following vasopressin administration was delayed and inhibition occurred of both the initial rate of Ca2+ influx as well as the total amount of Ca2+ taken up by the liver. Inhibition of the Ca2+ influx rate was almost maximal at approximately 500 microM Ni2+; half-maximal inhibition occurred at less than 250 microM. Added Ni2+ also delayed the onset of the early transient bile flow peak. In addition, the duration of the transient peak in bile flow was prolonged by approximately 2 min by all concentrations of Ni2+ between 25-500 microM, the greatest amount of bile being released in the presence of 250 microM Ni2+. Concentrations of Ni2+ at 100 microM and above also inhibit the decrease in bile flow to below baseline levels. The data identify a multiple role for Ca2+ mobilisation in bile flow.

Protective effects of calcium channel blockers on acute bromobenzene toxicity to isolated rat hepatocytes. Inhibition of phenylephrine-induced calcium oscillations

BACKGROUND AND METHODS

Protective effects of verapamil, nifedipine, diltiazem, and ethylene glycol tetraacetic acid (EGTA) on acute bromobenzene (BB) toxicity to rat hepatocytes were evaluated, and cytosolic [Ca2+]i was monitored in single BB-exposed rat hepatocytes. Additionally, the effect of nifedipine on phenylephrine-stimulated calcium oscillations was investigated.

RESULTS

BB at 0.8-2.4 mM increased the lactate dehydrogenase (LDH) leakage rate dose-dependently. Pretreatment with verapamil (25-35 microM), nifedipine (35-45 microM), diltiazem (25 microM), or EGTA (1.5-5 mM) markedly attenuated the BB-induced (1.6 mM) LDH leakage rate during 2 h of incubations. BB did not cause any detectable acute change in [Ca2+]i. BB interfered with phenylephrine-stimulated calcium oscillations, by blocking the oscillations in 58% of the cells and reducing the oscillation frequency in the rest. Nifedipine (100 and 200 microM) blocked the phenylephrine-induced calcium oscillations completely in 55% and 88% of the cells, respectively.

CONCLUSIONS

The findings demonstrate that verapamil, nifedipine, diltiazem, and EGTA significantly protect rat hepatocytes against BB toxicity. BB interferes with phenylephrine-stimulated calcium oscillations. Nifedipine inhibits the oscillations at doses higher than those exerting a protective effect.

Protective effect of nifedipine against cytotoxicity and intracellular calcium alterations induced by acetaminophen in rat hepatocyte cultures

Alteration of calcium homeostasis has been proposed to play a major role in cell necrosis induced by a variety of chemical agents such as acetaminophen (APAP). In this study, a potential protective effect of the dihydropyridine calcium channel blocking agent, nifedipine, was investigated in vitro on acetaminophen-induced hepatocyte damage. Rat hepatocytes were exposed during 20 hours to various concentrations of APAP (0.50 to 4.00 mM). The following metabolic and functional parameters were investigated: -lactate dehydrogenase (LDH) release as an indicator of plasma membrane integrity, -cell viability evaluated by the colorimetric MTT assay, and intracellular calcium concentration as evaluated by two fluorimetric methods: a scanning laser cytometer using indo-1-AM as fluorescent probe and a fluorescence plate reader using fluo-3-AM as calcium indicator. Incubation of hepatocytes with APAP alone in the range 0.50 to 4.00mM resulted in a dose-response relationship with regard to LDH release (243% to 750% of control) and to the loss of cell viability (0 to 67% of control). Moreover these results were correlated with a significant increase in cytosolic calcium content (189 to 406 nM). Nifedipine treatment prior to APAP exposure, partially prevented LDH release, the plasma membrane blebbing, and thereby the loss of viability. In addition, intracellular calcium level progressively returned within the limits of the control values with increasing concentrations of nifedipine. It can be concluded that, in vitro conditions, nifedipine pretreatment exhibits a preventive effect against acetaminophen hepatocyte injury.

Cellular and subcellular calcium signaling in gastrointestinal epithelium

Ca2+ is a critical second messenger in virtually all cell types, including the various epithelial cell types within the digestive system. When measured in cell populations, Ca2+ signals usually appear as a single transient or prolonged elevation. In individual epithelial cells, signaling patterns often vary from cell to cell and may contain more complex features such as Ca2+ oscillations. Subcellular Ca2+ signals show a further level of complexity, such as Ca2+ waves, and may relate to the polarized structure and function of epithelial cells. The approaches to detect cytosolic Ca2+ signals, the patterns and mechanisms of Ca2+ signaling, and the role of such signals in regulating the function of polarized epithelium within the gastrointestinal tract, pancreas, and liver are reviewed in this report.

Protective effect of various calcium antagonists against an experimentally induced calcium overload in isolated hepatocytes

The effect of the hepatotoxic substance diamidinothionaphthene (98/202) on cytosolic, mitochondrial and extra-mitochondrial calcium distribution was measured in isolated rat hepatocytes. The drastic disturbance of the intracellular calcium homeostasis caused by this substance (increase of the cytosolic and mitochondrial calcium contents and depletion of extra-mitochondrial calcium stores, which at last lead to cell death) gave rise to an investigation of the possible cytoprotective effect of calcium antagonists of various chemical classes: verapamil, diltiazem, and nifedipine on isolated hepatocytes. Our results show that all three calcium antagonists prevented cell death caused by 98/202. The 98/202-induced increase of cytosolic and mitochondrial calcium content was inhibited by all three calcium antagonists. However, only verapamil was able to inhibit the depletion of extra-mitochondrial calcium stores. Since 98/202-induced cell death occurs only in the presence of extracellular calcium, it is concluded that calcium antagonists are also able to inhibit the influx of extracellular calcium in liver cells, which leads to a calcium overload of the cytosol and mitochondria. The various ways of interfering with the calcium homeostasis of liver cells qualifies the hepatotoxic substance 98/202 as a suitable in vitro hepatotoxicity model for testing the hepatoprotective effect of different calcium antagonists.

Calcium: its modulation in liver by cross-talk between the actions of glucagon and calcium-mobilizing agonists

Images

[Calcium and liver]

Cells expand energy to lower the concentration of free calcium in the cytosol ([Ca2+]i) to a very low level. Extracellular Ca2+ entering via channels situated in the plasma membrane is expelled into the extracellular medium by a Ca(2+)-Mg(2+)-ATPase or by Na(+)-Ca2+ exchangers. The Ca2+ that enters the cell is sequestered, once inside the cytosol, by a Ca(2+)-Mg(2+)-ATPase, which concentrates Ca2+ in specialized domains of the endoplasmic reticulum. The nucleus and the mitochondria also concentrate Ca2+, but less efficiently. The stimulation of numerous receptors by hormones, growth factors and neurotransmitters coupled to GTP-binding proteins provokes a rapid increase in [Ca2+]i by mobilizing Ca2+ from intra- and extracellular compartments. Membrane coupling is ensured by the activation of a phospholipase C-beta, which hydrolyses a doubly phosphorylated phosphoinositide, phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2). The inositol (1,4,5)-trisphosphate (InsP3) consequently formed binds to a receptor consisting in 4 homologous of 250 kDa each. The InsP3 receptor has been localized to a specialized region, rich in Ca2+, of the endoplasmic reticulum. The receptor has been purified and its sequence obtained. Reincorporated into planar bilayers, it displays the properties of a channel. In the cell, opening of the InsP3 receptor-channel provokes the release of the Ca2+ accumulated within the endoplasmic reticulum. Analyzing the kinetics of channel opening by the methods of rapid mixing, rapid filtration or flash photolysis of caged InsP3 has revealed that InsP3 opens the channel within a very short time, probably less than 30 msec. The InsP3 receptor-channel is autoregenerative. With the sustained stimulation of a Ca2+ influx the release of Ca2+ leads to an augmentation of [Ca2+]i, which is responsible for triggering cellular responses. The complexity of Ca2+ signals produced by stimulated cells has been revealed by studies in which highly effective techniques have been used to detect Ca2+ ions in the cytosol, such as bioluminescent proteins, fluorescent indicators or ionic currents sensitive to Ca2+. It appears that variations in [Ca2+]i induced by stimulation consist of oscillations of which the frequency, but not the amplitude, depends on the concentration of the hormone. Moreover, by summing the images picked up with a video recorder, it has been possible to demonstrate the changes in [Ca2+]i at the subcellular level and the waves of Ca2+ in stimulated cells.

Ischemic liver cell damage and calcium accumulation in rats

Regions of calcium accumulation were studied by 45Ca-autoradiography and microdensitometry during liver ischemia and reperfusion in rats. In autoradiographic studies 45Ca accumulated in all zones of the liver lobuli after 1-2 h of liver ischemia and 1, 3 and 6 h of reperfusion, but did not accumulate in non-ischemic liver lobes. Significant 45Ca accumulation occurred only with reperfusion after 1 or 2 h of liver ischemia. In the presence of reperfusion, 45Ca accumulation correlated with the length of ischemia. In histopathologic studies, liver necrosis was absent in lobuli after 1-h reperfusion. Some liver necrosis was observed in the group reperfused for 3 h and widespread liver necrosis appeared after 6-h reperfusion. Regions of 45Ca accumulation coincided with sites of microscopic liver cell damage and necrosis. These results suggested that calcium accumulation might be responsible for the liver cell damage induced by ischemia with reperfusion, that the intensity of calcium accumulation in 45Ca-autoradiograms indicates the degree of ischemia damage, and that the primary event leading to hepatocellular necrosis might be calcium accumulation and subsequent liver cell death.

Evidence that stimulation of plasma-membrane Ca2+ inflow is an early action of glucagon and dibutyryl cyclic AMP in rat hepatocytes

The ability of glucagon (1 nM) and of dibutyryl cyclic AMP (50 microM) to increase cytosolic free Ca2+ concentration ([Ca2+]i) in Fura-loaded rat hepatocytes was examined in a system wherein Ca2+ inflow was induced by the re-admission of excess Ca2+ to a nominally Ca(2+)-free medium. An increase in [Ca2+]i did not occur in the absence of either agonist, but did so after co-addition of either agonist with Ca2+. Increasing the time between addition of dibutyryl cyclic AMP (or of glucagon) and Ca2+ led to increases in [Ca2+]i; half-maximal and maximal increases were observed at 0 s (i.e. at co-addition) and 5-7 s respectively. Dibutyryl cyclic AMP and Ca2+ each exhibited a concentration-dependence when their respective concentrations were changed for a fixed time interval between additions. Half-maximal and maximal effects were obtained with 30 microM and 50 microM dibutyryl cyclic AMP and with 0.5 mM and approx. 1 mM Ca2+ respectively. The data demonstrate an early action of glucagon and dibutyryl cyclic AMP on [Ca2+]i. It is argued that the agonist-induced rise in [Ca2+]i results from an increase in plasma-membrane Ca2+ inflow, an effect that appears to occur much earlier than that on mobilization of internal stores of Ca2+.

Ca2+ antagonists do not protect isolated perfused rat hepatocytes from anoxic injury

Ca2+ antagonists were studied during anoxia in perfused isolated rat hepatocytes. Cytosolic free calcium (Ca2+i) was measured with aequorin. Anoxia was induced for 2 h by saturating the perfusate with 95% N2/5+ CO2. Anoxia increased Ca2+i in two distinct phases reaching a maximum of 1.5 microM. The increase in Ca2+i was caused by Ca2+ influx from the extracellular fluids because the main Ca2+i surge was totally abolished in Ca(2+)-free media. LDH release increased 6-fold during the second hour of anoxia, but when Ca2+ was removed from the perfusate during the anoxic period, LDH rose only 2.7-fold. Ca2+ antagonists (10(-7) to 10(-5) M) did not prevent the increase in Ca2+i and the rise in LDH release. On the contrary, high concentrations (10(-6) and 10(-5) M) of the blockers nifedipine and diltiazem significantly increased anoxic cell injury. The observation that the increase in LDH and the rise in Ca2+i were not suppressed by Ca2+ antagonists suggests that (i) Ca2+ antagonists protect the whole liver from anoxic injury by acting on cells other than parenchymal cells; (ii) the influx of Ca2+ responsible for the massive increase in hepatocyte Ca2+i evoked by anoxia did not take place through voltage-sensitive Ca2+ channels but must have occurred via the Na(+)-Ca2+ antiporter operating in the reverse mode (Ca2+ influx vs. Na+ efflux), and (iii) high concentrations of Ca2+ antagonists may be deleterious to the parenchymal cells of the liver.

Calcium antagonists can be classified using in vitro toxicity and potency indices

The toxicity of eleven calcium antagonists from different chemical families was determined in rat hepatocyte primary cultures. The calcium antagonist potency of the same compounds was also determined in isolated rabbit aortic rings contracted with high K+. The hepatocytotoxicity of the calcium antagonists was not directly linked to blockade of voltage-operated calcium channels, since there was no correlation between the rank order of hepatotoxicity and that for calcium antagonist potency. The toxicity and calcium antagonist potency of each calcium antagonist examined were used to calculate an in vitro therapeutic index value for each compound. It was observed that therapeutic indices fell into three distinct groups and we therefore propose that the in vitro therapeutic index can be used to subclassify the calcium antagonist group of drugs. The proposed classification corresponds very closely with one already suggested by Spedding on pharmacological grounds. In conclusion, the in vitro therapeutic index may provide a useful tool in the characterization and subclassification of novel calcium antagonist compounds.

Acute effects of cholestatic and choleretic bile salts on vasopressin- and glucagon-induced hepato-biliary calcium fluxes in the perfused rat liver

The effects were investigated of the choleretic bile salt glycoursodeoxycholate (G-UDCA) and of the cholestatic bile salt taurochenodeoxycholate (T-CDCA) on changes in perfusate Ca2+, glucose and oxygen and in bile calcium and bile flow induced by the administration of (a) vasopressin, (b) glucagon and (c) glucagon plus vasopressin together to the perfused rat liver [Hamada, Karjalainen, Setchell, Millard & Bygrave (1992) Biochem. J. 281, 387-392]. G-UDCA itself increased the secretion of calcium in the bile several-fold, but its principal effect was to augment each of the above-mentioned metabolic events except glucose and oxygen output; particularly noteworthy was its ability to augment the 'transients' in bile calcium and bile flow seen immediately after the administration of vasopressin with or without glucagon. T-CDCA, by contrast, produced opposite effects and attenuated all of the parameters measured, and in particular the transients in bile calcium and bile flow. The data provide evidence of a strong correlation between calcium fluxes occurring on both the sinusoidal and the bile-canalicular membranes and that all are modifiable by glucagon, Ca(2+)-mobilizing hormones and bile salts.

Cadmium and mercury accumulation in rat hepatocytes: interactions with other metal ions

The uptake of essential metals, such as calcium, copper (Cu), iron (Fe), and zinc (Zn), occurs through processes that include energy-independent carrier mechanisms as well as ion channels. Since cadmium (Cd) and mercury (Hg) inhibit the uptake of these metals, this study examined whether the essential metals in turn affect Cd and Hg accumulation. The uptake of 3 microM Cd was inhibited by Cu, Fe, Zn, and Hg, with 100 microM Zn having the greatest effect. Kinetic analysis indicated that these metals inhibited Cd accumulation in a competitive manner. In comparison, neither the essential metals nor Cd had any significant effect on Hg accumulation. At 4 degrees C the accumulation of Cd was reduced to 20% of that at 37 degrees C, while Hg uptake remained unaffected. The efflux of Cd from the hepatocytes was biphasic, energy-independent, and not affected by Zn, Cu, or Fe. Thus the essential metals decreased Cd accumulation by inhibiting its uptake. On the other hand, Hg decreased Cd accumulation by both inhibiting its uptake and enhancing its efflux. As determined by the organic SH blockers, nearly two-thirds of the Cd entered the hepatocytes through processes involving the SH ligands. The uptake of Hg, however, was not affected by the SH blockers. Furthermore, the fraction of membrane-bound Hg at 3 microM concentration was 2.5 times greater than Cd, indicating that Hg is associated with additional binding sites not utilized by Cd. These results suggest that in hepatocytes Cd uptake occurs mainly through the SH-containing transport processes associated with the uptake of Zn and, to a smaller extent, Cu and Fe. Hg can inhibit Cd uptake by binding to these sites; however, its own uptake occurs via other processes that remain to be elucidated.

Concomitant stimulation by vasopressin of biliary and perfusate calcium fluxes in the perfused rat liver

Changes in perfusate Ca2+ (measured with a Ca(2+)-selective electrode) and changes in bile calcium (measured by atomic absorption spectroscopy) were continuously and simultaneously monitored after infusion of (a) vasopressin, (b) glucagon and (c) both vasopressin and glucagon together to the perfused rat liver. Also monitored were perfusate glucose and oxygen concentrations and bile flow. Vasopressin induces a sharp, transient, pulse of increased bile flow and increased bile calcium within 1 min of infusion, concomitant with rapid changes in perfusate Ca2+ fluxes, glucose output and oxygen uptake. This is immediately followed by a decrease in both bile flow and bile calcium for as long as the hormone is administered. Changes induced by glucagon are a relatively slow onset of perfusate Ca2+ efflux and oxygen uptake, but rapid glucose output, and a small but significant and transient decrease in bile flow and bile calcium which, despite the continued infusion of the hormone, spontaneously and rapidly returns to normality. However, the greatest responses are observed after co-administration of both hormones. Coincident with the augmented perfusate Ca2+ fluxes (influx) seen in earlier work, there occurs within 1 min of vasopressin infusion a sharp increase in bile secretion and bile calcium greater in magnitude than that produced by vasopressin alone. Immediately thereafter bile secretion and bile calcium decline below basal values and remain there for as long as the hormones are administered. Glucagon and vasopressin therefore each have opposing effects on bile flow and bile calcium. However, the action of vasopressin is enhanced by the prior administration of glucagon. The data thus reveal features about the actions of glucagon and Ca(2+)-mobilizing hormones on bile flow and bile calcium not previously recorded and provide a novel framework around which the whole issue of hepato-biliary Ca2+ homoeostasis can be assessed in normal and diseased liver.

Coordination of hormone-induced calcium signals in isolated rat hepatocyte couplets: demonstration with confocal microscopy

Excitable cells often display rapid coordination of hormone-induced intracellular calcium signals. Calcium elevations that begin in a single epithelial cell also may spread to adjacent cells, but coordination of hormone-induced signals among epithelial cells has not been described. We report the use of confocal microscopy to determine the inter- and intracellular distribution of cytosolic calcium in isolated rat hepatocyte couplets, an isolated epithelial cell system in which functional polarity is maintained. Both vasopressin and phenylephrine evoked sequential coordinated calcium signals in the couplets, even during cytosolic calcium oscillations. The coupling was abolished by closure of intercellular gap junction channels by treatment with octanol. These observations demonstrate that hormone-induced intracellular calcium signals are coordinated among hepatocytes and suggest that gap junction channels mediate this intercellular integration of tissue responsiveness.

Effects of Ca2+ agonists on cytosolic Ca2+ in isolated hepatocytes and on bile secretion in the isolated perfused rat liver

The effects of increases in cytosolic Ca2+ on hepatocyte bile secretion are unknown. A number of agents that alter levels of cytosolic Ca2+ in the hepatocyte also produce hepatic vasoconstriction and activate protein kinase C, which complicates interpretations of their effects on bile secretion. To better understand the role of cytosolic Ca2+ in bile secretion, we examined the effect of the Ca2+ ionophore A23187 (0.1 mumol/L), the Ca2+ agonist vasopressin (10 nmol/L) and the Ca(2+)-mobilizing agent, 2,5-di(tert-butyl)-1,4-benzohydroquinone (25 mumol/L) on cytosolic Ca2+ in isolated hepatocytes and on bile flow in the isolated perfused rat liver, using vasodilators and inhibitors of protein kinase C and Ca2+ influx. Single-pass perfused livers were used, and cytosolic Ca2+ was measured by luminescent photometry in isolated hepatocytes loaded with the Ca(2+)-sensitive photoprotein aequorin. After A23187 perfusion, a sustained 74% +/- 10% (mean +/- S.D.) decrease in bile flow and a sustained 271% +/- 50% increase in perfusion pressure was observed. Simultaneous pretreatment with the vasodilator papaverine (25 mumol/L) and the protein kinase C inhibitor H-7 (50 mumol/L) abolished the pressure increase but not the decrease in bile flow, whereas pretreatment with Ni2+ (25 mumol/L) to block the influx of extracellular Ca2+ markedly reduced both the pressure increase and the decrease in bile flow. Vasopressin produced a transient (mean = 6 min) 75% +/- 4% decrease in bile flow and a sustained 7% +/- 4% increase in perfusion pressure. Pretreatment with H-7 alone corrected the vasopressin-induced pressure increase but also failed to eliminate the decrease in bile flow, whereas pretreatment with Ni2+ decreased the magnitude of the decrease by two-thirds without affecting the increase in perfusion pressure, 2,5'-di(tert-butyl)-1,4-benzohydroquinone produced a transient 65% +/- 20% decrease in bile flow and a transient 56% +/- 15% increase in perfusion pressure. In isolated hepatocytes, bromo-A23187, the nonfluorescent form of the ionophore, produced a sustained 56% +/- 32% increase in the cytosolic Ca2+ signal, whereas vasopressin resulted in a transient 241% +/- 75% increase and 2,5-di(tert-butyl)-1,4-benzohydroquinone resulted in a sustained 149% +/- 66% increase. The ionophore-induced increase in Ca2+ was abolished completely by pretreatment of the hepatocytes with Ni2+, whereas the vasopressin-induced increase was reduced by 38%.(ABSTRACT TRUNCATED AT 400 WORDS)

Differences in cadmium and mercury uptakes by hepatocytes: role of calcium channels

Calcium uptake in cells occurs through specific membrane channels. Since cadmium and mercury inhibit calcium uptake, this study examined whether the calcium channels may also be involved in the uptake of these metals. Primary cultures of rat hepatocytes were incubated with 3 microM CdCl2 or HgCl2 in the absence or presence of four different organic calcium channel blockers or a calcium agonist. The calcium channel blockers had no significant effect on mercury accumulation. In comparison, the uptake of cadmium was inhibited by diltiazem and verapmil (50-250 microM) as well as by nifedipine and nitrendipine (25-100 microM), with a maximum inhibition of 31% after 30 min incubation with 250 microM verapamil. The calcium agonist vasopressin (20 nM) increased cadmium accumulation by 15%. This effect was blocked by 250 microM verapamil. Kinetic analysis showed that verapamil decreased the Vmax of cadmium uptake, without altering the Km, indicating a noncompetitive inhibition. The calcium channel blockers were ineffective at 4 degrees C. These data suggest that about a third of the cadmium enters hepatocytes through the calcium channels. The mechanism of mercury uptake, on the other hand, is very different as it does not appear to involve the calcium channels.

Cytoprotective effects of calcium channel blockers. Mechanisms and potential applications in hepatocellular injury

Deregulation of calcium homeostasis is strongly implicated in the development of cellular injury, including in hepatocytes, and is thought to be the limiting step in transition to an irreversible stage. Calcium channel blockers appear to exert their cytoprotective effects through several mechanisms. These may involve blockade of L-(long-lasting)-type calcium channels, reduction of oxidative stress, antagonism at inflammatory mediator receptor sites and interaction at other intracellular sites. Studies relating to the liver are few but suggest that calcium channel blockers may have a role to play in limiting hepatocellular damage, especially those arising from exposure to a variety of toxic agents.

Spatial and temporal organization of calcium signalling in hepatocytes

Treatment of hepatocytes with agonists which act via the second messenger inositol 1,4,5-trisphosphate (Ins(1,4,5)P3), results in increases of cytosolic free Ca2+ [( Ca2+]i) which are manifest as a series of discrete [Ca2+]i transients or oscillations. With increasing agonist dose [Ca2+]i oscillation frequency increases and the initial latent period decreases, but the amplitude of the [Ca2+]i oscillations remains constant. Studies of these [Ca2+]i oscillations at the subcellular level have indicated that the [Ca2+]i changes do not occur synchronously throughout the cell, but initiate at a specific subcellular domain, adjacent to a region of the plasma membrane, and then propagate through the cell as a [Ca2+]i wave. For a given ceil, the locus of [Ca2+]i wave initiation is constant for every oscillation in a series and is also identical when the cell is sequentially stimulated with different agonists or when the phospholipase C-linked G protein is activated directly using AIF4-. The kinetics of the [Ca2+]i waves indicate that a Ca(2+)-activated mechanism is involved in propagating the oscillatory [Ca2+]i increases throughout the cell, and the data appear to be most consistent with a process of Ca(2+)-induced Ca2+ release. It is proposed that the ability to propagate [Ca2+]i oscillations into regions of the cell distal to the region in which the signal transduction apparatus is localized could serve an important function in allowing all parts of the cell to respond to the stimulus.

The nature and mechanism of activation of the hepatocyte receptor-activated Ca2+ inflow system

Progress in elucidation of the properties of the hepatocyte receptor-activated Ca2+ inflow system (RACIS) has been hampered by difficulties in measuring rates of Ca2+ inflow to hepatocytes. These difficulties have led, for example, to different conclusions about the relationship between the extracellular Ca2+ concentration and the movement of Ca2+ through the RACIS. The hepatocyte RACIS admits Mn2+ and a number of other divalent cations as well as Ca2+. Many of these cations also inhibit the movement of Ca2+ through this system. While the RACIS is inhibited by high concentrations of verapamil and by some other Ca2+ antagonists, it is relatively insensitive to inhibition by organic compounds which inhibit other Ca2+ channels and Ca2+ transporters. There is circumstantial evidence which suggests that the hepatocyte RACIS is an exchange system, possibly one which catalyses Ca(2+)-H+ exchange or the co-transport of Ca2+ and OH-. Other circumstantial evidence suggests that the RACIS is a channel, with some similarities to voltage-operated Ca2+ channels in excitable cells. However, experiments using the patch-clamp technique have not yet detected agonist-stimulated Ca2+ movement across the hepatocyte plasma membrane. The molecular components of the RACIS probably differ from those which facilitate the large inflow of Ca2+ to hepatocytes which occurs in the absence of an agonist. The mechanism by which agonists activate the RACIS has not been elucidated.(ABSTRACT TRUNCATED AT 250 WORDS)

Balancing of energy-consuming processes of rat hepatocytes

A method for the quantification of energy consuming processes described by Siems et al. for reticulocytes and by Müller et al. for ascites tumour cells was applied to balance the ATP-consumption of isolated rat hepatocytes. On the basis of decreased coupled respiration rates following the specific inhibition of energy-requiring reactions, the energy demands of protein turnover, nucleic acid synthesis, Na+/K(+)-ATPase and Ca2(+)-transport of hepatocytes in different incubation media were assessed. These processes together with urea synthesis account for about 60 per cent of the total energy consumption in a glucose and amino acid-enriched Eagle/Borsook medium. The metabolic flux rates of total ATP-consumption and ATP-consumption of single energy-requiring processes in hepatocytes are compared with those in reticulocytes and different tumour cell types.

Bile acids mobilise internal Ca2+ independently of external Ca2+ in rat hepatocytes

In the present study, we investigated the possible role of external Ca2+ in the rise of the cytosolic Ca+ concentration induced by the monohydroxy bile acid taurolithocholate in isolated rat liver cells. The results showed that: (a) the bile acid promotes the same dose-dependent increase in the cytosolic Ca+ concentration (half-maximal effect at 23 microM) in hepatocytes incubated in the presence of 1.2 mM Ca2+ or 6 microM Ca2+; (b) taurolithocholate is able to activate the Ca2(+)-dependent glycogen phosphorylase a by 6.3-fold and 6.0-fold in high and low Ca2+ media, respectively; (c) [14C]taurolithocholate influx is not affected by external Ca2+, and 45Ca2+ influx is not altered by taurolithocholate. These results establish that the effects of taurolithocholate on cell Ca2+ do not require extracellular Ca2+ and are consistent with the view that monohydroxy bile acids primarily release Ca2+ from the endoplasmic reticulum in the liver.

Hepatic circulation: potential for therapeutic intervention

In recent years, knowledge of the physiology and pharmacology of hepatic circulation has grown rapidly. Liver microcirculation has a unique design that allows very efficient exchange processes between plasma and liver cells, even when severe constraints are imposed upon the system, i.e. in stressful situations. Furthermore, it has been recognized recently that sinusoids and their associated cells can no longer be considered only as passive structures ensuring the dispersion of molecules in the liver, but represent a very sophisticated network that protects and regulates parenchymal cells through a variety of mediators. Finally, vascular abnormalities are a prominent feature of a number of liver pathological processes, including cirrhosis and liver cell necrosis whether induced by alcohol, ischemia, endotoxins, virus or chemicals. Although it is not clear whether vascular lesions can be the primary events that lead to hepatocyte injury, the main interest of these findings is that liver microcirculation could represent a potential target for drug action in these conditions.

The measurement of Ca2+ inflow across the liver cell plasma membrane by using quin2 and studies of the roles of Na+ and extracellular Ca2+ in the mechanism of Ca2+ inflow

1. Rates of Ca2+ inflow across the hepatocyte plasma membrane in the presence of vasopressin were estimated by using quin2. 2. Plots of the rate of Ca2+ inflow as a function of the intracellular quin2 concentration reached a plateau at about 1.7 mM intracellular quin2. Ca2+ inflow was inhibited by 60% in the presence of 400 microM-verapamil. 3. A plot of the rate of Ca2+ inflow as a function of the concentration of extracellular Ca2+ ([Ca2+]o) was biphasic. The second (slower) phase showed no sign of saturation at values of [Ca2+]o up to 5 mM. It is concluded that, in the presence of vasopressin, Ca2+ flows into the liver cell by two different processes, one of which is not readily saturated by Ca2+o. 4. The effect of the replacement of extracellular NaCl by choline or tetramethylammonium chloride on cellular Ca2+ movement was found to depend on the presence or absence of intracellular quin2. 5. In cells loaded with quin2 and incubated in the presence of choline or tetramethylammonium chloride, a small decrease in the basal intracellular free Ca2+ concentration ([Ca2+]i) was observed, and the increase in [Ca2+]i caused by the addition of vasopressin was considerably diminished when compared with cells incubated in the presence of NaCl. In cells loaded with quin2, replacement of NaCl by choline chloride caused a decrease in Ca2+ inflow in the presence of vasopressin, as measured by using quin2 or 45Ca2+ exchange, whereas no change in Ca2+ inflow was observed in the absence of vasopressin. 6. In cells not loaded with quin2, replacement of NaCl by choline chloride did not alter Ca2+ inflow either in the presence or in the absence of vasopressin. 7. It is concluded that (i) Ca2+ inflow through the basal and receptor-activated Ca2+ inflow systems does not involve the inward movement of Ca2+ in exchange for Na+ or the induction of Ca2+ inflow by intracellular Na+, and (ii) the presence of both intracellular quin2 and extracellular choline or tetramethylammonium chloride (in place of NaCl) inhibits Ca2+ inflow through the receptor-activated Ca2+ inflow system but not through the basal Ca2+ inflow system, and inhibits the release of Ca2+ from intracellular stores.