The liver cell plasma membrane Ca2+ inflow systems exhibit a broad specificity for divalent metal ions

Involvement of calcium channels in the sexual dimorphism of cadmium-induced hepatotoxicity

Cadmium toxicity has been evaluated in a number of in vivo and in vitro toxicological studies. In vivo Cd toxicity exhibits sexual dimorphism with females being more susceptible to Cd uptake, accumulation, and toxicity in the liver. Research to date does not explain why females are more sensitive to Cd-induced hepatotoxicity. Recent studies demonstrate that progesterone sensitizes female F(344) rats and TRL-1215 cells to Cd toxicity, however the mode of action is still unclear. Approximately one half of the Cd entering the cytoplasm does so through receptor operated Ca(2+) channels. Progesterone treatment of human spermatozoa and Xenopus laevis oocytes causes a rapid influx of Ca(2+) suggesting a possible mechanism. Since hepatocytes have progesterone receptors on their cellular membrane and Ca(2+) influx into the cytoplasm occurs following progesterone treatment we evaluated the hypothesis that progesterone facilitates the uptake and accumulation of Cd via Ca(2+) channels, leading to enhanced toxicity. Primary isolated rat hepatocytes were treated with Cd, progesterone, and/or verapamil for 4 h and cytolethality was measured. Pretreatment with the Ca(2+) channel blocker verapamil increased the Cd concentration producing 50% lethality (LC(50)) by 2-fold, thus decreasing Cd cytolethality. In contrast, pretreatment with progesterone decreased the Cd LC(50) by 2-fold resulting in enhanced Cd cytolethality. Verapamil treatment reversed the progesterone enhanced Cd cytolethality. Verapamil and/or progesterone in the absence of Cd did not affect hepatocyte viability. Overall, the results of this study demonstrate that inhibition of progesterone-induced Ca(2+) influx with the Ca(2+) channel blocker verapamil, decreases Cd cytolethality in primary isolated rat hepatocytes. These findings indicate that progesterone activation of receptor-mediated Ca(2+) channels is involved in the sexually dimorphic hepatotoxicity seen following acute Cd exposure.

D-galactosamine induced hepatocyte apoptosis is inhibited in vivo and in cell culture by a calcium calmodulin antagonist, chlorpromazine, and a calcium channel blocker, verapamil

Studies were conducted in C57BL/6N Crj male mice and in cultured hepatocytes to clarify the relationship between galactosamine (GaIN) induced apoptosis and [Ca2+]i kinetics. Chlorpromazine (CPZ), a Ca(2+)-calmodulin antagonist, and verapamil (VR), a Ca(2+)-channel blocker each inhibited GaIN-induced DNA fragmentation and the appearance of apoptotic bodies. The kinetics of calcium uptake were evaluated using a calcium analyzer with the acetoxymethyl ester of fura-PE3 (fura-PE3/AM, 2.5 microM) as the calcium reporter. An increase in [Ca2+]i was detected in the cultured hepatocytes within 3 hours after treatment with 20 mM GaIN; this increase was inhibited by pretreatment with either 20 microM CPZ or 30 microM VR. Ca2+ imaging by confocal laser scanning microscopy showed that increase in [Ca2+]i after treatment with GaIN was initially localized around nuclei, while [Ca2+]i signals were later diffuse and observed throughout the cytoplasm. The activities of lactate dehydrogenase (LDH) and serum glutamate-pyruvate transaminase (sGPT), used as indicators of plasma membrane damage and leakage, however, were not reduced by pretreatment with CPZ or VR. From these findings, we infer that the DNA fragmentation in GaIN-induced hepatocyte apoptosis is associated with an elevation in the perinuclear concentration of Ca2+, but GaIN-induced necrotic cell death is triggered through pathway(s) that are insensitive to blockage of Ca2+ influx and therefore appear to occur independently of elevation in [Ca2+]i. These results help to clarify the role of calcium flux in hepatocyte apoptosis and necrosis induced by exposure to hepatotoxins in vivo and in vitro.

Transport of toxic metals by molecular mimicry

Intracellular concentrations of essential metals are normally maintained within a narrow range, whereas the nonessential metals generally lack homeostatic controls. Some of the factors that contribute to metal homeostasis have recently been identified at the molecular level and include proteins that mediate import of essential metals from the extracellular environment, those that regulate delivery to specific intracellular proteins or compartments, and those that mediate metal export from the cell. Some of these proteins appear highly selective for a given essential metal; however, others are less specific and interact with multiple metals, including toxic metals. For example, DCT1 (divalent cation transporter-1; also known as NRAMP2 or DMT1) is considered to be a major cellular uptake mechanism for Fe(2+) and other essential divalent metals, but this protein also mediates uptake of Cd(2+), Pb(2+), and possibly of other toxic divalent metals. The ability of nonessential metals to interact with binding sites for essential metals is critical for their ability to gain access to specific cellular compartments and for their ability to disrupt normal biochemical or physiological functions. Another major mechanism by which metals traverse cell membranes and produce cell injury is by forming complexes whose overall structures mimic those of endogenous molecules. For example, it has long been known that arsenate and vanadate can compete with phosphate for transport and metabolism, thereby disrupting normal cellular functions. Similarly, cromate and molybdate can mimic sulfate in biological systems. Studies in our laboratory have focused on the transport and toxicity of methylmercury (MeHg) and inorganic mercury. Mercury has a high affinity for reduced sulfhydryl groups, including those of cysteine and glutathione (GSH). MeHg-l-cysteine is structurally similar to the amino acid methionine, and this complex is a substrate for transport systems that carry methionine across cell membranes. Once MeHg has entered the cell, some of it binds to GSH, and the resulting MeHg-glutathione complex appears to be a substrate for proteins that mediate cellular export of glutathione S-conjugates, including the apically located MRP2 (multidrug resistance-associated protein 2) transporter, a member of the adenosine triphosphate-binding cassette protein superfamily. Because other toxic metals also form complexes with endogenous molecules, comparable mechanisms may be involved in their membrane transport and disposition.

Calcium competes with zinc for a channel mechanism on the brush border membrane of piglet intestine

Interactions between Ca(+2) and Zn(+2) at the intestinal brush border membrane occur via unclear mechanisms. We hypothesized that Zn(+2) and Ca(+2) are transported across the brush border membrane via a multidivalent metal channel. Using brush border membrane vesicles (BBMV) prepared from intestines of 8 sow-fed piglets, we sought to determine whether Ca(+2) competes with Zn(+2) for uptake. Extravesicular Zn(+2) was removed with ethylenediamine-tetraacetic acid. Time curves of Zn(+2) and Ca(+2) uptake by BBMV were conducted with increasing concentrations of Ca(+2) and Zn(+2), respectively. Saturation curves compared kinetic parameters of Zn(+2) uptake with and without Ca(+2). In addition, Zn(+2) uptake was measured in the presence of various classical Ca(+2) channel modulators. Over 20 min, a 0.4x concentration of Zn(+2) lowered Ca(+2) uptake by vesicles, whereas a 30x concentration of Ca(+2) was necessary to lower Zn(+2) uptake. These data suggest that Ca(+2) has lower affinity than Zn(+2) for a brush border membrane transport protein. Kinetic parameters showed higher K(m) values with 4 or 15 mM Ca(+2) but unchanged J(max), suggesting competitive inhibition. The Ca(+2) channel blocking agents, La(+3), Ba(+2), verapamil, and diltiazem, inhibited Zn(+2) uptake, whereas calcitriol, trans 1,2 cyclohexanediol, cis/trans 1,3 cyclohexanediol, and the L-type Ca(+2) channel agonist, Bay K8644, induced Zn(+2) uptake. These data were consistent with competition for a common transport mechanism on the brush border membrane, possibly a novel multimetal channel. Copyright 2001 Elsevier Science Inc.

Store-operated Ca(2+) inflow in Reuber hepatoma cells is inhibited by voltage-operated Ca(2+) channel antagonists and, in contrast to freshly isolated hepatocytes, does not require a pertussis toxin-sensitive trimeric GTP-binding protein

The treatment of H4-IIE cells (an immortalised liver cell line derived from the Reuber rat hepatoma) with thapsigargin, 2, 5-di-(tert-butyl)-1,4-benzohydroquinone, cyclopiazonic acid, or pretreatment with EGTA, stimulated Ca(2+) inflow (assayed using intracellular fluo-3 and a Ca(2+) add-back protocol). No stimulation of Mn(2+) inflow by thapsigargin was detected. Thapsigargin-stimulated Ca(2+) inflow was inhibited by Gd(3+) (maximal inhibition at 2 microM Gd(3+)), the imidazole derivative SK&F 96365, and by relatively high concentrations of the voltage-operated Ca(2+) channel antagonists, verapamil, nifedipine, nicardipine and the novel dihydropyridine analogues AN406 and AN1043. The calmodulin antagonists W7, W13 and calmidazolium also inhibited thapsigargin-induced Ca(2+) inflow and release of Ca(2+) from intracellular stores. No inhibition of either Ca(2+) inflow or Ca(2+) release was observed with calmodulin antagonist KN62. Substantial inhibition of Ca(2+) inflow by calmidazolium was only observed when the inhibitor was added before thapsigargin. Pretreatment of H4-IIE cells with pertussis toxin, or treatment with brefeldin A, did not inhibit thapsigargin-stimulated Ca(2+) inflow. Compared with freshly isolated rat hepatocytes, H4-IIE cells exhibited a more diffuse actin cytoskeleton, and a more granular arrangement of the endoplasmic reticulum (ER). In contrast to freshly isolated hepatocytes, the arrangement of the ER in H4-IIE cells was not affected by pertussis toxin treatment. Western blot analysis of lysates of freshly isolated rat hepatocytes revealed two forms of G(i2(alpha)) with apparent molecular weights of 41 and 43 kDa. Analysis of H4-IIE cell lysates showed only the 41 kDa form of G(i2(alpha)) and substantially less total G(i2(alpha)) than that present in rat hepatocytes. It is concluded that H4-IIE cells possess store-operated Ca(2+) channels which do not require calmodulin for activation and exhibit properties similar to those in freshly isolated rat hepatocytes, including susceptibility to inhibition by relatively high concentrations of voltage-operated Ca(2+) channel antagonists. In contrast to rat hepatocytes, SOCs in H4-IIE cells do not require G(i2(alpha)) for activation. Possible explanations for differences in the requirement for G(i2(alpha)) in the activation of Ca(2+) inflow are briefly discussed.

Effects of nickel sulfate on pulmonary natural immunity in Wistar rats

This study was designed to investigate the in vivo effect of nickel sulfate on the pulmonary non-specific immune defences. Groups of four male Wistar rats were treated with a single intratracheal instillation of NiSO(4) at different doses: 1, 2, 4 and 8 micromol of NiSO(4) per rat. Control rats received a corresponding instillation of the saline vehicle. The effect of NiSO(4) on the cytotoxic activity of the pulmonary natural killer (NK) cells and alveolar macrophages (AM), as well as the pulmonary production of cytokines such as alpha-tumor necrosis factor (TNF-alpha) and gamma-interferon (IFN-gamma), were examined 1, 2 and 7 days later. Spontaneous NK-cytotoxicity towards mouse-derived tumor cell line, Yac-1 was suppressed 1 day after treatment at doses of 2 micromol/rat and above with only one result significant (P<0.05); 2 days after treatment the suppression was increased with all results significant at the same doses; 1 week after treatment NK activity restoration was observed except for the highest dose, 8 micromol/rat. AM-mediated cytotoxicity towards mouse-derived tumor cell line, 3T12, did not show any significant difference in treated and untreated animals. In contrast, whereas moderate levels of TNF-alpha were detected in the broncho-alveolar lavage (BAL) fluid supernatants of controls, the NiSO(4) treatment highly suppressed TNF-alpha production with a maximum observed after 2 days. TNF-alpha suppression was found to be transient, at least with the lowest NiSO(4) dose, with levels returning to normal after 7 days. A non-significant increase in IFN-gamma was observed in the BAL fluids of treated animals at each time of examination. Taken together, these results indicate that NK cell activity and TNF-alpha secretion are sensitive targets for instilled NiSO(4) in Wistar rats.

Extracellular heavy-metal ions stimulate Ca2+ mobilization in hepatocytes

Populations of hepatocytes in primary culture were loaded with fura 2 and the effects of extracellular heavy-metal ions were examined under conditions that allowed changes in fura 2 fluorescence (R340/360, the ratio of fluorescence recorded at 340 and 360 nm) to be directly attributed to changes in cytosolic free [Ca2+] ([Ca2+]i). In Ca2+-free media, Ni2+ [EC50 (concentration causing 50% stimulation) approximately 24+/-9 microM] caused reversible increases in [Ca2+]i that resulted from mobilization of the same intracellular Ca2+ stores as were released by [Arg8]vasopressin. The effects of Ni2+ were not mimicked by increasing the extracellular [Mg2+], by addition of MnCl2, CoCl2 or CdCl2 or by decreasing the extracellular pH from 7.3 to 6.0; nor were they observed in cultures of smooth muscle, endothelial cells or pituitary cells. CuCl2 (80 microM), ZnCl2 (80 microM) and LaCl3 (5 mM) mimicked the ability of Ni2+ to evoke Ca2+ mobilization. The response to La3+ was sustained even in the absence of extracellular Ca2+, probably because La3+ also inhibited Ca2+ extrusion. Although Ni2+ entered hepatocytes, from the extent to which it quenched fura 2 fluorescence the free cytosolic [Ni2+] ([Ni2+]i) was estimated to be <5 nM at the peak of the maximal Ni2+-evoked Ca2+ signals and there was no correlation between [Ni2+]i and the amplitude of the evoked increases in [Ca2+]i. We conclude that extracellular Ni2+, Zn2+, Cu2+ and La3+, but not all heavy-metal ions, evoke an increase in [Ca2+]i in hepatocytes by stimulating release of the hormone-sensitive intracellular Ca2+ stores and that they may do so by interacting with a specific cell-surface ion receptor. This putative ion receptor may be important in allowing hepatocytes to contribute to regulation of plasma heavy-metal ions and may mediate responses to Zn2+ released into the portal circulation with insulin.

Receptor-activated Ca2+ inflow in animal cells: a variety of pathways tailored to meet different intracellular Ca2+ signalling requirements

Receptor-activated Ca2+ channels (RACCs) play a central role in regulation of the functions of animal cells. Together with voltage-operated Ca2+ channels (VOCCs) and ligand-gated non-selective cation channels, RACCs provide a variety of pathways by which Ca2+ can be delivered to the cytoplasmic space and the endoplasmic reticulum (ER) in order to initiate or maintain specific types of intracellular Ca2+ signal. Store-operated Ca2+ channels (SOCs), which are activated by a decrease in Ca2+ in the ER, are a major subfamily of RACCs. A careful analysis of the available data is required in order to discern the different types of RACCs (differentiated chiefly on the basis of ion selectivity and mechanism of activation) and to properly develop hypotheses for structures and mechanisms of activation. Despite much intensive research, the structures and mechanisms of activation of RACCs are only now beginning to be understood. In considering the physiological functions of the different RACCs, it is useful to consider the specificity for Ca2+ of each type of cation channel and the rate at which Ca2+ flows through a single open channel; the locations of the channels on the plasma membrane (in relation to the ER, cytoskeleton and other intracellular units of structure and function); the Ca2+-responsive enzymes and proteins; and the intracellular buffers and proteins that control the distribution of Ca2+ in the cytoplasmic space. RACCs which are non-selective cation channels can deliver Ca2+ directly to specific regions of the cytoplasmic space, and can also admit Na+, which induces depolarization of the plasma membrane, the opening of VOCCs and the subsequent inflow of Ca2+. SOCs appear to deliver Ca2+ specifically to the ER, thereby maintaining oscillating Ca2+ signals.

Rapid changes in intracellular Zn2+ in rat hepatocytes

Changes in the concentration of free Zn2+ were monitored in isolated rat hepatocytes using the fluorescent indicator zinquin (ethyl[2-methyl-8-p-toluenesulphonamido-6-quinolyloxy]acetat e). The concentration of Zn2+ in freshly isolated hepatocytes was 1.3 x 10(-6) M (range 0.61-2.7 x 10[-6] M). This value decreased by about 10%-15% during incubation in the absence of zinc and increased in a time- and concentration-dependent manner in the presence of exogenous zinc (Km approximately 10 microM). IIb group metal ions led to a concentration-dependent increase in zinquin fluorescence. The rank of efficacy was Hg approximately Cd > Pb (IVa) >> Cu (Ib) >>> Ni (VIII). This rank resembles their ability to mobilize zinc from metallothioneins. 8-Br-3',5'-cAMP (10[-4]M) caused a rapid decrease in Zn2+ epifluorescence which was apparent within 10 min and was sustained throughout the experiment. This effect was gradually obliterated in the presence of external ZnCl2. The effect was specific for cAMP (or cAMP generating hormones) as the calcium-dependent hormone [arg8]vasopressin (5 x 10[-8] M) did not affect intracellular Zn2+. An integrated role of zinc as a possible mediator in signal transduction is discussed.

Rapid Ca2+ influx induced by the action of dibutylhydroquinone and glucagon in the perfused rat liver

Glucagon induces a slight Ca2+ efflux when administered to the perfused rat liver. However, the hormone promotes rapid and significant Ca2+ influx after the prior administration of 2, 5-di(t-butyl)-1,4-hydroquinone (BHQ), an agent that promotes Ca2+ release from the endoplasmic reticulum (ER). The concentrations of glucagon that promote Ca2+ influx are similar to those that promote glycogenolysis and gluconeogenesis in isolated hepatocytes. The permeable analogue of cAMP, but not that of cGMP, is able to duplicate the Ca2+-mobilizing effects of glucagon. The influx of Ca2+ into liver is blocked by Ni2+. Administration of sodium azide, an inhibitor of mitochondrial electron transport, also blocks the BHQ plus glucagon-induced Ca2+ influx and this is reversed when azide administration is terminated. The actions of azide are evident within 60 s after administration or withdrawal, and also occur when either oligomycin or fructose is co-administered; this provides evidence for an effect of azide independent of cellular ATP depletion. Measurement of total calcium in mitochondria that were isolated rapidly from perfused livers after the combined administration of glucagon and BHQ confirmed that large quantities of extracellular Ca2+ had entered these organelles. These experiments provide evidence that in the perfused rat liver the artificial emptying of the ER Ca2+ pool allows glucagon to promote rapid and sustained Ca2+ influx that seems to terminate in mitochondria.

Pinocytosis in 2,5-di-tert-butylhydroquinone-stimulated hepatocytes and evaluation of its role in Ca2+ inflow

In order to evaluate the contribution of pinocytosis to basal (no agonist) and lanthanide-insensitive store-activated Ca2+ inflow in freshly-isolated rat hepatocytes, the uptake of extracellular fluid by pinocytosis was measured at 20 degrees C and used to predict the amount of extracellular Ca2+ taken up by pinocytosis. This was compared with the measured rate of Ca2+ uptake in the basal state, and with the measured lanthanide-insensitive component of divalent cation uptake stimulated by 2,5-di-tert-butylhydroquinone (DBHQ), an inhibitor of the smooth endoplasmic reticulum (Ca2+ + Mg2+)ATP-ase. Fluid uptake by pinocytosis was measured using [14C]sucrose. In hepatocytes incubated at 20 degrees C, DBHQ increased the initial rate of sucrose uptake by about 35%. The data for sucrose uptake were used to calculate the volume of extracellular fluid taken up by pinocytosis which, in turn, was used to predict the amount of extracellular Ca2+ taken up through pinocytosis in the basal and DBHQ-stimulated states. Rates of divalent cation inflow in the basal state were determined at 20 degrees C by measuring the uptake of 45Ca2+. The degree of stimulation of Ca2+ inflow by DBHQ and the lanthanide-insensitive component of DBHQ-stimulated divalent cation inflow were determined by measuring the rate of Mn(2+)-induced quenching of intracellular quin-2 in the absence of an agonist, and in the presence of DBHQ or DBHQ plus Gd3+. It was calculated that the process of pinocytosis accounts for at least 15% of Ca2+ uptake in the basal (no agonist) state, and for about 10% of DBHQ-stimulated lanthanide-insensitive Ca2+ uptake. It is concluded that in isolated hepatocytes (i) the release of Ca2+ from intracellular stores stimulates pinocytosis and (ii) the process of pinocytosis can account for a substantial proportion of basal Ca2+ inflow and a small proportion of DBHQ-stimulated lanthanide-insensitive Ca2+ inflow.

Effect of cadmium on calcium transport in a human fetal hepatic cell line (WRL-68 cells)

Toxic metals appear to use the transport pathways that exist for biologically essential metals. Calcium uptake in cells occurs through specific membrane channels. Since cadmium inhibits calcium uptake, this study was carried on to elucidate the mechanism of Cd interference with calcium transport using the fetal hepatic cell line WRL-68 as an in vitro model. Ca accumulation by WRL-68 cells presented an initial rapid phase, followed by a sustained phase of slower accumulation over a 60 min period. A concentration of 50 microM CdCl2 produced 39% inhibition of the uptake of CaCl2 (100 microM), while 100 microM nifedipine or verapamil decreased Ca accumulation by 35 and 63%, respectively. All Cd concentrations tested produced significant decrease in Ca uptake in a concentration-dependent manner at 1 min and thereafter, although with 10 microM CdCl2 no significant difference was found after 30 min of incubation. From the Lineweaver-Burk plot, we found that Cd exerted a competitive inhibition on Ca uptake, since there was no significant effect on the Vmax but an increased K(m). A second order rate constant of Cd inactivation of 0.061 mM-1.s-1 was determined from the course of Ca uptake during Cd inhibition. SH groups seemed to play an essential role in Ca inhibition uptake by Cd because the inhibition of Ca accumulation by 50 microM Cd was practically reversed after the addition of dithiothreitol.

Expression of Drosophila trpl cRNA in Xenopus laevis oocytes leads to the appearance of a Ca2+ channel activated by Ca2+ and calmodulin, and by guanosine 5'[gamma-thio]triphosphate

The effects of expression of the Drosophila melanogaster Trpl protein, which is thought to encode a putative Ca2+ channel [Phillips, Bull and Kelly (1992) Neuron 8, 631-642], on divalent cation inflow in Xenopus laevis oocytes were investigated. The addition of extracellular Ca2+ ([Ca2+]0) to oocytes injected with trpl cRNA and to mock-injected controls, both loaded with the fluorescent Ca2+ indicator fluo-3, induced a rapid initial and a slower sustained rate of increase in fluorescence, which were designated the initial and sustained rates of Ca2+ inflow respectively. Compared with mock-injected oocytes, trpl-cRNA-injected oocytes exhibited a higher resting cytoplasmic free Ca2+ concentration ([Ca2+]i), and higher initial and sustained rates of Ca2+ inflow in the basal (no agonist) states. The basal rate of Ca2+ inflow in trpl-cRNA-injected oocytes increased with (1) an increase in the time elapsed between injection of trpl cRNA and the measurement of Ca2+ inflow, (2) an increase in the amount of trpl cRNA injected and (3) an increase in [Ca2+]0. Gd3+ inhibited the trpl cRNA-induced basal rate of Ca2+ inflow, with a concentration of approx. 5 microM Gd3+ giving half-maximal inhibition. Expression of trpl cRNA also caused an increase in the basal rate of Mn2+ inflow. The increases in resting [Ca2+]1 and in the basal rate of Ca2+ inflow induced by expression of trpl cRNA were inhibited by the calmodulin inhibitors W13, calmodazolium and peptide (281-309) of (Ca2+ and calmodulin)-dependent protein kinase II. A low concentration of exogenous calmodulin (introduced by microinjection) activated, and a higher concentration inhibited, the trpl cRNA-induced increase in basal rate of Ca2+ inflow. The action of the high concentration of exogenous calmodulin was reversed by W13 and calmodazolium. When rates of Ca2+ inflow in trpl-cRNA-injected oocytes were compared with those in mock-injected oocytes, the guanosine 5'-[beta-thio]diphosphate-stimulated rate was greater, the onset of thapsigargin-stimulated initial rate somewhat delayed and the inositol 1,4,5-trisphosphate-stimulated initial rate markedly inhibited. It is concluded that (1) the divalent cation channel activity of the Drosophila Trpl protein can be detected in Xenopus oocytes: (2) in the environment of the Xenopus oocyte the Trpl channel admits some Mn2+ as well as Ca2+, is activated by cytoplasmic free Ca2+ (through endogenous calmodulin) and by a trimeric GTP-binding regulatory protein, but does not appear to be activated by depletion of Ca2+ in the endoplasmic reticulum; and (3) expression of the Trpl protein inhibits the process by which the release of Ca2+ from intracellular stores activates endogenous store-activated Ca2+ channels.

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.

Characterisation of the divalent cation channels of the hepatocyte plasma membrane receptor-activated Ca2+ inflow system using lanthanide ions

The ability of Gd3+ to inhibit vasopressin-stimulated Ca2+ inflow to hepatocytes was compared with its effect on Mn2+ inflow. In the absence of Gd3+, the stimulation of Mn2+ inflow by vasopressin increased with increasing pH of the extracellular medium. Maximal inhibition of vasopressin-stimulated Ca2+ and Mn2+ inflow by saturating concentrations of Gd3+ was 70 and 30%, respectively. Gd3+ also inhibited thapsigargin-stimulated Ca2+ and Mn2+ inflow with maximal inhibition of 70 and 40%, respectively. It is concluded that vasopressin and thapsigargin each activate two types of Ca2+ inflow processes, one which is sensitive and one which is insensitive to lanthanides. The nature of the pore of the lanthanide-sensitive Ca2+ channel was investigated further using different lanthanides as inhibitors. Tm3+, Gd3+, Eu3+, Nd3+ and La3+ each inhibited vasopressin-stimulated Ca2+ and Mn2+ inflow but had no effect on Ca2+ inflow in the absence of an agonist, or on vasopressin-stimulated release of Ca2+ from intracellular stores. Maximal inhibition of vasopressin-stimulated Ca2+ inflow in the presence of a saturating concentration of each lanthanide ranged from 70-90%. An equation which describes a 1:1 interaction of the lanthanide with a putative binding site in the Ca2+ channel gave a good fit to dose-response curves for the inhibition of vasopressin-stimulated Ca2+ inflow by each lanthanide. Lanthanides in the middle of the series exhibited the lowest dissociation constant (Kd) values. The Kd for Gd3+ increased with increasing extracellular Ca2+ concentration, suggesting competitive inhibition of Ca2+ binding by Gd3+. In the absence of lanthanide, vasopressin-stimulated Mn2+ inflow was substantially reduced when the plasma membrane was depolarised by increasing the extracellular K+ concentration. Changing the membrane potential had little effect on the maximum inhibition by Gd3+ of vasopressin-stimulated Mn2+ inflow. The Kd for inhibition of vasopressin-stimulated Ca2+ inflow by Gd3+, measured at the lowest attainable membrane potential, was about 6-fold lower than the Kd measured at the highest attainable membrane potential. The idea that there is a site in the vasopressin-stimulated lanthanide-sensitive Ca2+ channel composed of carboxylic acid groups which bind Ca2+, Mn2+ or a lanthanide ion is consistent with the data obtained using the different lanthanides.

Regulation by purinergic agonists of zinc uptake by rat submandibular glands

The zinc uptake in rat submandibular cells was measured using fura2 as a fluorescent probe. Basal zinc uptake was observed in a 100 microM - 1 mM concentration range. Carbachol and isoproterenol had no effect but ATP4- dose-dependently increased the basal zinc uptake (half-maximal concentration: 250 microM). The purinergic agonist shifted the concentration curve for zinc to the left by one order of magnitude. The response to ATP was not reproduced by adenosine or ADP and was blocked by Coomassie blue. Calcium, nickel or lanthanum were inhibitors of zinc uptake, while the substitution of extracellular sodium by potassium or lithium increased the basal zinc uptake. We conclude that in submandibular cells zinc can permeate through the non-specific cation channel coupled to ATP-sensitive purinergic receptors.

Evidence from studies with hepatocyte suspensions that store-operated Ca2+ inflow requires a pertussis toxin-sensitive trimeric G-protein

The role of heterotrimeric GTP-binding proteins in the process of store-operated Ca2+ inflow in hepatocytes was investigated by testing the ability of pertussis toxin to inhibit thapsigargin- and 2,5-di-tert-butylhydroquinone (DBHQ)-induced bivalent cation inflow. Hepatocytes isolated from rats treated with pertussis toxin for 24 h exhibited markedly inhibited rates of both Ca2+ and Mn2+ inflow when these were stimulated by vasopressin, angiotension II, epidermal growth factor, thapsigargin and DBHQ. Pertussis toxin had little effect on the basal intracellular free Ca2+ concentration ([Ca2+]i), basal rates of Ca2+ and Mn2+ inflow, the abilities of vasopressin, angiotensin II, thapsigargin and DBHQ to induce the release of Ca2+ from intracellular stores, and the maximum value of [Ca2+]i reached following agonist-induced release of Ca2+ from intracellular stores. It is concluded that store-operated Ca2+ inflow in hepatocytes employs a slowly ADP-ribosylated trimeric GTP-binding protein and is the physiological mechanism, or one of the physiological mechanisms, by which vasopressin and angiotensin stimulate plasma membrane Ca2+ inflow in this cell type.

Characterisation of the inhibition of the hepatocyte receptor-activated Ca2+ inflow system by gadolinium and SK&F 96365

The inhibition of agonist-stimulated divalent cation inflow in hepatocytes by Gd3+ and compound SK&F 96365 (1-[beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole hydrochloride) was investigated. Gd3+ and SK&F 96365 inhibited Ca2+ and Mn2+ inflow stimulated by vasopressin, angiotensin II or phenylephrine. The concentrations of Gd3+ and SK&F 96365 which gave half-maximal inhibition of vasopressin-stimulated Ca2+ inflow were 2 x 10(-7) M and 16 x 10(-6) M, respectively. The action of Gd3+ on vasopressin-stimulated Ca2+ inflow was rapid (less than 10 s in onset) and reversible. Gd3+ had no effect on Mn2+ inflow in the absence of an agonist and no effect on the ability of vasopressin to release Ca2+ from intracellular stores. SK&F 96365 inhibited Mn2+ inflow in the absence of agonists and vasopressin-induced release of Ca2+ from intracellular stores, but at approximately a 5-fold higher concentration than that which inhibited vasopressin-stimulated divalent cation inflow. It is concluded that Gd3+ and SK&F 96365 (at concentrations below 20 microM) inhibit, in a selective manner, divalent cation movement through the putative cation channel of the hepatocyte receptor-activated Ca2+ inflow system. Gd3+ appears to be the most potent inhibitor of this Ca2+ inflow system so far described.

A role for a pertussis toxin-sensitive trimeric G-protein in store-operated Ca2+ inflow in hepatocytes

The mechanism of store-operated Ca2+ inflow in hepatocytes was investigated using fluo-3 and fura-2 to monitor changes in the concentration of intracellular free Ca2+ in single cells, and 1-(alpha-glycerophosphoryl)-myo-inositol 4,5-diphosphate, P4(5)-1-(2-nitrophenyl)ethyl ester ('caged' GPIP2) and 'caged' guanosine 5'-[gamma thio]triphosphate (GTP gamma S) (introduced into the cytoplasmic space by microinjection), thapsigargin and 2,5-di-tert- butylhydroquinone (DBHQ) to stimulate Ca2+ inflow. Photolysis of 'caged' GPIP2 or 'caged' GTP gamma S stimulated Ca2+ inflow. The abilities of GPIP2, thapsigargin and DBHQ to stimulate Ca2+ inflow were inhibited by the pre-treatment of hepatocytes with pertussis toxin in vivo for 36 h. Thapsigargin-stimulated Ca2+ inflow was also inhibited by guanosine 5'-[beta-thio]diphosphate (GDP beta S) (introduced by microinjection). It is concluded that, in hepatocytes, store-operated Ca2+ inflow induced by the actions of either inositol 1,4,5-trisphosphate, thapsigargin or DBHQ requires a pertussis toxin-sensitive trimeric G-protein.

A slowly ADP-ribosylated pertussis-toxin-sensitive GTP-binding regulatory protein is required for vasopressin-stimulated Ca2+ inflow in hepatocytes

The roles of heterotrimeric GTP-binding regulatory proteins (G-proteins) and inositol polyphosphates in the mechanism by which vasopressin stimulates Ca2+ inflow in hepatocytes were investigated by using single cells loaded with fura2 by microinjection. Vasopressin-stimulated Ca2+ inflow was mimicked by microinjection of guanosine 5'-[gamma-thio]triphosphate (GTP[S]) or guanosine 5'-[beta gamma-imido]triphosphate to the cells, but not adenosine 5'-[gamma-thio]triphosphate (ATP[S]) or guanosine 5'-[beta-thio]diphosphate (GDP[S]). Extracellular Gd3+ (5 microM) inhibited both vasopressin- and GTP[S]-stimulated Ca2+ inflow. GDP[S], but not GMP, administered to hepatocytes by microinjection, completely inhibited vasopressin-stimulated Ca2+ inflow and partially inhibited vasopressin-induced release of Ca2+ from intracellular stores. The microinjection of pertussis toxin had no effect either on the release of Ca2+ from intracellular stores or on Ca2+ inflow induced by vasopressin, but completely inhibited changes in these processes induced by epidermal growth factor (EGF). Hepatocytes isolated from rats treated with pertussis toxin for 24 h exhibited no vasopressin- or GTP[S]-stimulated Ca2+ inflow, whereas the vasopressin-stimulated release of Ca2+ from intracellular stores was similar to that observed for control cells. Heparin or ATP[S] inhibited, or delayed the onset of, both vasopressin-induced release of Ca2+ from intracellular stores and vasopressin-stimulated Ca2+ inflow. Vasopressin-induced oscillations in intracellular [Ca2+] were observed in some heparin-treated cells. It is concluded that the stimulation by vasopressin of Ca2+ inflow to hepatocytes requires inositol 1,4,5-trisphosphate (InsP3) and, by implication, the pertussis-toxin-insensitive G-protein required for the activation of phospholipase C beta [Taylor, Chae, Rhee and Exton (1991) Nature (London) 350, 516-518], and another G-protein which is slowly ADP-ribosylated by pertussis toxin and acts between InsP3 and the putative plasma-membrane Ca2+ channel. EGF-stimulated Ca2+ inflow involves at least one G-protein which is rapidly ADP-ribosylated and is most likely required for InsP3 formation.

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

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Inhibitory effect of zinc on stimulated erythropoietin synthesis in HepG2 cells

The effect of zinc on erythropoietin (EPO) synthesis in HepG2 cells was investigated. The increase in EPO synthesis induced by Co2+ (50 microM), Ni2+ (300 microM) or oxygen (1% O2) was inhibited by the presence of ZnCl2 (50-150 microM) in the tissue-culture medium, whereas basal EPO synthesis was unaffected. The effect was reflected by corresponding changes in the EPO mRNA level. These effects of zinc on EPO synthesis could not be mimicked by CdCl2 (less than or equal to 2 microM). Addition of FeCl3 to the medium appeared to decrease the inhibitory effect of zinc on hypoxia-induced EPO synthesis, implying that zinc may interfere with an iron-dependent step in EPO regulation.

Thyrotropin-releasing hormone-mediated Mn2+ entry in perifused rat anterior pituitary cells

Receptor-mediated Ca2+ influx has been shown to exist in several cell types. Thyrotropin-releasing-hormone (TRH)-stimulated Ca2+ entry has also been postulated to exist in rat anterior pituitary cells, but direct evidence has been lacking. We have measured the fluorescence quenching of indo-1 caused by Mn2+ at a Ca(2+)-insensitive wavelength to investigate the actions of TRH on cation entry in dispersed perifused anterior pituitary cells. In indo-1-loaded cells perifused with Ca(2+)-free medium, Mn2+ caused fluorescence quenching in unstimulated cells; TRH caused further quenching. TRH-stimulated Mn2+ entry was transient, and levelled off within a few minutes in the presence of continuous TRH infusion. TRH-stimulated Mn2+ entry was dependent on the concentration of Mn2+ (50 microM-1 mM). TRH (1 microM) caused a larger effect than TRH (10 nM). La3+ and Ni2+ blocked the quenching stimulated by TRH. The rate of basal quenching was not blocked by dopamine, but TRH-stimulated Mn2+ entry was partially blocked by 1 microM-dopamine and almost completely abolished by 10 microM-dopamine. Thapsigargin (1-5 microM), a tumour promotor which depleted intracellular Ca2+ stores, had little effect on Mn2+. F- (20 mM), which activates G-proteins, also had little effect on Mn2+ entry. We conclude that TRH can transiently stimulate Ca2+ entry through a channel than can pass Mn2+ and be inhibited by dopamine. Depleting Ca2+ stores alone is not sufficient to stimulate Ca2+ entry, and so TRH must do so by other mechanisms.

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.

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)