Inositol 1,4,5-trisphosphate activates receptor-mediated calcium entry by two different pathways in hepatocytes

Expression and function of TRP channels in liver cells

The liver plays a central role in whole body homeostasis by mediating the metabolism of carbohydrates, fats, proteins, drugs and xenobiotic compounds, and bile acid and protein secretion. Hepatocytes together with endothelial cells, Kupffer cells, smooth muscle cells, stellate and oval cells comprise the functioning liver. Many members of the TRP family of proteins are expressed in hepatocytes. However, knowledge of their cellular functions is limited. There is some evidence which suggests the involvement of TRPC1 in volume control, TRPV1 and V4 in cell migration, TRPC6 and TRPM7 in cell proliferation, and TRPPM in lysosomal Ca(2+) release. Altered expression of some TRP proteins, including TRPC6, TRPM2 and TRPV1, in tumorigenic cell lines may play roles in the development and progression of hepatocellular carcinoma and metastatic liver cancers. It is likely that future experiments will define important roles for other TRP proteins in the cellular functions of hepatocytes and other cell types of which the liver is composed.

Vasorelaxant effects of extracts of the stem bark of Terminalia superba Engler & Diels (Combretaceae)

AIM OF THE STUDY

The stem bark of Terminalia superba (Combretaceae) (TS) is used in traditional Cameroonian medicine as antihypertensive remedy. In the present study, we investigated the vasorelaxant properties of different extracts of TS and their underlying mechanisms.

MATERIALS AND METHODS

Activities of aqueous (AQU), methanolic (MET), methylene chloride (MC), and methylene chloride-methanol (MCM) extracts of TS were evaluated on isolated rat aortic rings precontracted with phenylephrine (PE) or high KCl.

RESULTS

All extracts induced a vasodilating effect both on KCl- and PE-induced contractions. The effects of MC and MCM extracts were greater than those of AQU or MET extracts (P<0.05). MC had an endothelium-independent effect and reduced Ca(++)-induced contraction following PE or KCl challenge (P<0.05). After incubation with verapamil, MC induced a relaxation in rings precontracted by PE (P<0.001). By contrast, the effect of MCM was endothelium-dependent and decreased by the nitric oxide synthase inhibitor N(W)-nitro-L-arginine methyl ester (P<0.05).

CONCLUSIONS

These data demonstrate that the MC extract exhibits vasorelaxant effects that are partly due to inhibition of extracellular Ca(++) influx and/or inhibition of intracellular Ca(++) release in vascular smooth muscle cells. By contrast, the effect of the MCM extract was found to be endothelium- and nitric oxide dependent.

Glucagon receptor mediates calcium signaling by coupling to G alpha q/11 and G alpha i/o in HEK293 cells

Glucagon induces intracellular Ca(2+) ([Ca(2+)](i)) elevation by stimulating glucagon receptor (GCGR). Such [Ca(2+)](i) signaling plays important physiological roles, including glycogenolysis and glycolysis in liver cells and the survival of beta-cells. Previous studies indicated that phospholipase C (PLC) might be involved in glucagon-mediated [Ca(2+)](i) response. Other studies also debated whether cAMP accumulation mediated by GCGR/G alpha(s) coupling contributes to [Ca(2+)](i) elevation. But the exact mechanisms remain uncertain. In the present study, we found that glucagon induces [Ca(2+)](i) elevation in HEK293 cells expressing GCGR. Removing extracellular Ca(2+) did not affect glucagon-stimulated [Ca(2+)](i) response. But depleting the intracellular Ca(2+) store by thapsigargin completely inhibited glucagon-induced [Ca(2+)](i) response. Experiments with forskolin and adenylyl cyclase inhibitor revealed that cAMP is not the cause of [Ca(2+)](i) response. Further studies with G alpha(q/11) RNAi and pertussis toxin (PTX) indicated that both G alpha(q/11) and G alpha(i/o) are involved. Combination of G alpha(q/11) RNAi and G alpha(i/o) inhibition almost completely abolished glucagon-induced [Ca(2+)](i) signaling.

Store-operated Ca2+ channels and microdomains of Ca2+ in liver cells

1. Oscillatory increases in the cytoplasmic Ca(2+) concentration ([Ca(2+)](cyt)) play essential roles in the hormonal regulation of liver cells. Increases in [Ca(2+)](cyt) require Ca(2+) release from the endoplasmic reticulum (ER) and Ca(2+) entry across the plasma membrane. 2. Store-operated Ca(2+) channels (SOCs), activated by a decrease in Ca(2+) in the ER lumen, are responsible for maintaining adequate ER Ca(2+). Experiments using patch-clamp recording and the fluorescent Ca(2+) reporter fura-2 indicate there is only one type of SOC in rat liver cells. These SOCs have a high selectivity for Ca(2+) and properties essentially indistinguishable from those of Ca(2+) release-activated Ca(2+) (CRAC) channels. 3. Although Orai1, a CRAC channel pore protein, and stromal interaction molecule 1 (STIM1), a CRAC channel Ca(2+) sensor, are components of liver cell SOCs, the mechanism of activation of SOCs, and in particular the role of subregions of the ER, are not well understood. 4. Recent experiments have used the transient receptor potential vanilloid 1 (TRPV1) non-selective cation channel, ectopically expressed in liver cells, and a choleretic bile acid to deplete Ca(2+) from different ER subregions. The results of these studies have provided evidence that only a small component of the ER is required for STIM1 redistribution and the activation of SOCs. 5. It is concluded that different Ca(2+) microdomains in the ER and cytoplasmic space are important in both the activation of SOCs and in the signalling actions of Ca(2+) in liver cells. Future experiments will investigate the nature of these microdomains further.

Ca(2+) -permeable channels in the hepatocyte plasma membrane and their roles in hepatocyte physiology

Hepatocytes are highly differentiated and spatially polarised cells which conduct a wide range of functions, including intermediary metabolism, protein synthesis and secretion, and the synthesis, transport and secretion of bile acids. Changes in the concentrations of Ca(2+) in the cytoplasmic space, endoplasmic reticulum (ER), mitochondria, and other intracellular organelles make an essential contribution to the regulation of these hepatocyte functions. While not yet fully understood, the spatial and temporal parameters of the cytoplasmic Ca(2+) signals and the entry of Ca(2+) through Ca(2+)-permeable channels in the plasma membrane are critical to the regulation by Ca(2+) of hepatocyte function. Ca(2+) entry across the hepatocyte plasma membrane has been studied in hepatocytes in situ, in isolated hepatocytes and in liver cell lines. The types of Ca(2+)-permeable channels identified are store-operated, ligand-gated, receptor-activated and stretch-activated channels, and these may vary depending on the animal species studied. Rat liver cell store-operated Ca(2+) channels (SOCs) have a high selectivity for Ca(2+) and characteristics similar to those of the Ca(2+) release activated Ca(2+) channels in lymphocytes and mast cells. Liver cell SOCs are activated by a decrease in Ca(2+) in a sub-region of the ER enriched in type1 IP(3) receptors. Activation requires stromal interaction molecule type 1 (STIM1), and G(i2alpha,) F-actin and PLCgamma1 as facilitatory proteins. P(2x) purinergic channels are the only ligand-gated Ca(2+)-permeable channels in the liver cell membrane identified so far. Several types of receptor-activated Ca(2+) channels have been identified, and some partially characterised. It is likely that TRP (transient receptor potential) polypeptides, which can form Ca(2+)- and Na(+)-permeable channels, comprise many hepatocyte receptor-activated Ca(2+)-permeable channels. A number of TRP proteins have been detected in hepatocytes and in liver cell lines. Further experiments are required to characterise the receptor-activated Ca(2+) permeable channels more fully, and to determine the molecular nature, mechanisms of activation, and precise physiological functions of each of the different hepatocyte plasma membrane Ca(2+) permeable channels.

Characteristics of odorant elicited calcium fluxes in acutely-isolated chick olfactory neurons

To understand avian olfaction, it is important to characterize the peripheral olfactory system of a representative bird species. This study determined the functional properties of olfactory receptor neurons of the chicken olfactory epithelium. Individual neurons were acutely isolated from embryonic day-18 to newborn chicks by dissection and enzymatic dissociation. We tested single olfactory neurons with behaviorally relevant odorant mixtures and measured their responses using ratiometric calcium imaging; techniques used in this study were identical to those used in other studies of olfaction in other vertebrate species. Chick olfactory neurons displayed properties similar to those found in other vertebrates: they responded to odorant stimuli with either decreases or increases in intracellular calcium, calcium increases were mediated by a calcium influx, and responses were reversibly inhibited by 100 microM L: -cis-diltiazem, 1 mM Neomycin, and 20 microM U73122, which are biochemical inhibitors of second messenger signaling. In addition, some cells showed a complex pattern of responses, with different odorant mixtures eliciting increases or decreases in calcium in the same cell. It appears that there are common features of odorant signaling shared by a variety of vertebrate species, as well as features that may be peculiar to chickens.

Evidence for multiple calcium response mechanisms in mammalian olfactory receptor neurons

Olfactory receptor neurons employ a diversity of signaling mechanisms for transducing and encoding odorant information. The simultaneous activation of subsets of receptor neurons provides a complex pattern of activation in the olfactory bulb that allows for the rapid discrimination of odorant mixtures. While some transduction elements are conserved among many species, some species-specificity occurs in certain features that may relate to their particular physiology and ecological niche. However, studies of olfactory transduction have been limited to a relatively small number of vertebrate and invertebrate species. To better understand the diversity and evolution of olfactory transduction mechanisms, we studied stimulus-elicited calcium fluxes in olfactory neurons from a previously unstudied mammalian species, the domestic cat. Isolated cells from cat olfactory epithelium were stimulated with odorant mixtures and biochemical agents, and cell responses were measured with calcium imaging techniques. Odorants elicited either increases or decreases in intracellular calcium; odorant-induced calcium increases were mediated either by calcium fluxes through the cell membrane or by mobilization of intracellular stores. Individual cells could employ multiple signaling mechanisms to mediate responses to different odorants. The physiological features of these olfactory neurons suggest greater complexity than previously recognized in the role of peripheral neurons in encoding complex odor stimuli. The investigation of novel and unstudied species is important for understanding the mechanisms of odorant signaling that apply to the olfactory system in general and suggests both broadly conserved and species-specific evolutionary adaptations.

2,5-Anhydro-D-mannitol increases hepatocyte calcium: implications for a hepatic hunger stimulus

The fructose analogue, 2,5-anhydro-D-mannitol (2,5-AM), triggers feeding in rats via a mechanism linked to its ability to trap phosphate and deplete hepatic ATP. This metabolic inhibitor is particularly useful in the study of the role of the liver in initiation of feeding as its effects are preferentially localized to the liver, and its metabolic consequences have been extensively characterized. To determine whether changes in intracellular calcium may participate in a mechanism conveying information about hepatic energy status to the nervous system, we studied the effects of 2,5-AM on intracellular calcium in isolated hepatocytes using the ratiometric indicator, fura-2. 2,5-AM elicited a marked elevation of intracellular calcium within 2-3 min of exposure that returned to baseline upon removal of the agent. Removal of external calcium failed to prevent this response, while emptying intracellular stores prevented it. These data are consistent with the hypothesis that hepatic energy status may be conveyed to the nervous system via a calcium-mediated secretion event.

Evidence that TRPC1 (transient receptor potential canonical 1) forms a Ca(2+)-permeable channel linked to the regulation of cell volume in liver cells obtained using small interfering RNA targeted against TRPC1

The TRPC1 (transient receptor potential canonical 1) protein, which is thought to encode a non-selective cation channel activated by store depletion and/or an intracellular messenger, is expressed in a number of non-excitable cells. However, the physiological functions of TRPC1 are not well understood. The aim of these studies was to investigate the function of TRPC1 in liver cells using small interfering RNA (siRNA) to ablate the TRPC1 protein. Treatment of H4-IIE liver cells with siRNA targeted against TRPC1 caused an approx. 50% decrease in expression of the human TRPC1 protein in cells transfected with cDNA encoding human TRPC1, and a 50% decrease in expression of the endogenous TRPC1 protein (assessed by Western blot and immunofluorescence). The decrease in endogenous TRPC1 protein in cells transfected with TRPC1 siRNA was associated with a greater increase in cell volume (compared with the increase observed in control cells) immediately after cells were placed in a hypotonic medium, and an enhanced regulatory cell volume decrease after exposure to hypotonic medium. Treatment with siRNA targeted against TRPC1 also led to a 25% inhibition of thapsigargin-stimulated Ca(2+) inflow, a 40% inhibition of ATP and maitotoxin-stimulated Ca(2+) inflow, and a 50% inhibition of maitotoxin-stimulated Mn(2+) inflow. The idea that, in liver cells, TRPC1 encodes a non-selective cation channel involved directly or indirectly in the regulation of cell volume is consistent with the results obtained.

Polarized calcium and calmodulin signaling in secretory epithelia

This review examines polarized calcium and calmodulin signaling in exocrine epithelial cells. The calcium ion is a simple, evolutionarily ancient, and universal second messenger. In exocrine epithelial cells, it regulates essential functions such as exocytosis, fluid secretion, and gene expression. Exocrine cells are structurally polarized, with the apical region usually dedicated to secretion. Recent advances in technology, in particular the development of videoimaging and confocal microscopy, have led to the discovery of polarized, subcellular calcium signals in these cell types. The properties of a rich variety of local and global calcium signals have now been described in secretory epithelial cells. Secretagogues stimulate apical-to-basal waves of calcium in many exocrine cell types, but there are some interesting exceptions to this rule. The shapes of intracellular calcium signals are determined by the distribution of calcium-releasing channels and mechanisms that limit calcium elevation. Polarized distribution of calcium-handling mechanisms also leads to transcellular calcium transport in exocrine epithelial cells. This transport can deliver considerable amounts of calcium into secreted fluids. Multicellular polarized calcium signals can coordinate the activity of many individual cells in epithelial secretory tissue. Certain particularly sensitive cells serve as pacemakers for initiation of intercellular calcium waves. Many calcium signaling pathways involve activation of calmodulin. This ubiquitous protein regulates secretion in exocrine cells and also activates interesting feedback interactions with calcium channels and transporters. Very recently it became possible to directly study polarized calcium-calmodulin reactions and to visualize the process of hormone-induced redistribution of calmodulin in live cells. The structural and functional polarity of secretory epithelia alongside the polarity of its calcium and calmodulin signaling present an interesting lesson in tissue organization.

Maintenance of the filamentous actin cytoskeleton is necessary for the activation of store-operated Ca2+ channels, but not other types of plasma-membrane Ca2+ channels, in rat hepatocytes

The roles of the filamentous actin (F-actin) cytoskeleton and the endoplasmic reticulum (ER) in the mechanism by which store-operated Ca(2+) channels (SOCs) and other plasma-membrane Ca(2+) channels are activated in rat hepatocytes in primary culture were investigated using cytochalasin D as a probe. Inhibition of thapsigargin-induced Ca(2+) inflow by cytochalasin D depended on the concentration and time of treatment, with maximum inhibition observed with 0.1 microM cytochalasin D for 3 h. Cytochalasin D (0.1 microM for 3 h) did not inhibit the total amount of Ca(2+) released from the ER in response to thapsigargin but did alter the kinetics of Ca(2+) release. The effects of cytochalasin D (0.1 microM) on vasopressin-induced Ca(2+) inflow were similar to those on thapsigargin-induced Ca(2+) inflow, except that cytochalasin D did inhibit vasopressin-induced release of Ca(2+) from the ER. Cytochalasin D (0.1 microM) inhibited vasopressin-induced Mn(2+) inflow (predominantly through intracellular messenger-activated non-selective cation channels), but the degree of inhibition was less than that of vasopressin-induced Ca(2+) inflow (predominantly through Ca(2+)-selective SOCs). Maitotoxin- and hypotonic shock-induced Ca(2+) inflow were enhanced rather than inhibited by 0.1 microM cytochalasin D. Treatment with 0.1 microM cytochalasin D substantially reduced the amount of F-actin at the cell cortex, whereas 5 microM cytochalasin D increased the total amount of F-actin and caused an irregular distribution of F-actin at the cell cortex. Cytochalasin D (0.1 microM) caused no significant change in the overall arrangement of the ER [monitored using 3',3'-dihexyloxacarbocyanine iodide [DiOC(6)(3)] in fixed cells] but disrupted the fine structure of the smooth ER and reduced the diffusion of DiOC(6)(3) in the ER in live hepatocytes after photobleaching. It is concluded that (i) the concentration of cytochalasin D is a critical factor in the use of this agent as a probe to disrupt the cortical F-actin cytoskeleton in rat hepatocytes, (ii) a reduction in the amount of cortical F-actin inhibits SOCs but not intracellular messenger-activated non-selective cation channels, and (iii) inhibition of the activation of SOCs and reduction in the amount of cortical F-actin is associated with disruption of the organization of the ER.

Intracellular calcium in the isolated rat liver: correlation to glucose release, K(+) balance and bile flow

This study correlates whole organ measurements of intracellular calcium concentration ([Ca(2+)](i)) with hormone-induced (epinephrine, vasopressin) changes of liver functions (glucose release, K(+) balance and bile flow). [Ca(2+)](i) was measured in the isolated perfused rat liver using the sensor Fura-2 and applying liver surface fluorescence spectroscopy. The technique was improved by (i) minimizing biliary elimination of the sensor by employing a rat strain deficient in canalicular organic anion transport (TR(-) mutation) and (ii) by correcting for changes of interfering intrinsic organ fluorescence that was shown to depend on the oxidation-reduction state (NAD(P)H content) of the organ. Epinephrine (50 nM) elicits an instantaneous peak rise of [Ca(2+)](i) to approx. 400 nM, followed by a sustained elevation that depends on the presence of extracellular Ca(2+). The rise of [Ca(2+)](i) coincides with initiation of glucose release, transient K(+) uptake, and transient stimulation of bile flow. Vasopressin (2 nM) exerts qualitatively similar effects. The transient rise of bile flow is attributed to Ca(2+)-mediated contraction of the pericanalicular actin-myosin web of hepatocytes.

Abnormal calcium homeostasis in fibroblasts from patients with Leigh disease

Recently, we reported that in various cell lines under conditions of deenergization of the mitochondrial membrane, the release of Ca(2+) from the endoplasmic reticulum (ER) does not produce the expected activation of store-operated calcium channels (SOCs) in the plasma membrane. In the present work, we examined the activation of SOCs in fibroblasts derived from three patients with Leigh disease (LS). We identified mutations in the SURF-1 gene in all these cells. Consequently, cytochrome oxidase (COX) deficiency was found in all these (LS(COX)) cell lines and, thus, the main mitochondrial mechanism of generation of the electrochemical proton gradient on the mitochondrial membrane was naturally depressed. We demonstrated that, in untreated LS(COX) fibroblasts, the rate of Ca(2+)-inflow through SOCs was low compared to the fibroblasts from healthy individuals even after thapsigargin-induced maximal release of Ca(2+) from the ER. Moreover, the pretreatment of LS(COX) fibroblasts with a protonophore did not modify this rate. Thus, in LS(COX) fibroblasts, the activation of SOCs was naturally impaired. Our findings suggest that altered calcium metabolism, apart from severe energy production failure, may also contribute to developing pathological conditions in patients with COX-deficient Leigh disease related to SURF-1 gene mutation.

Characteristics of odorant elicited calcium changes in cultured human olfactory neurons

An important step in establishing and utilizing a cell culture system for the in vitro study of olfaction is assessing whether the cultured cells possess physiological properties similar to those of mature olfactory neurons. Various investigators have successfully established proliferating cell lines from olfactory tissue, but few have demonstrated the characteristics of odor sensitivity of these cells. We successfully established cultured cell lines from adult human olfactory tissue obtained using an olfactory biopsy procedure and measured their ability to respond to odor stimulation using calcium imaging techniques. A subset of the human olfactory cells in culture displayed a distinct morphology and specifically expressed immunocytochemical markers characteristic of mature human olfactory neurons such as OMP, G(olf), NCAM and NST. Under defined growth conditions, these cultured cells responded to odorant mixes that have been previously shown to elicit intracellular calcium changes in acutely-isolated human olfactory neurons. These odorant-elicited calcium responses displayed characteristics similar to those found in mature human olfactory neurons. First, cultured cells responded with either increases or decreases in intracellular calcium. Second, increases in calcium were abolished by removal of extracellular calcium. Third, inhibitors of the olfactory signal transduction cascades reversibly blocked these odorant-elicited intracellular calcium changes. Our results demonstrate that cultures of adult human olfactory cells established from olfactory biopsies retain some of the in vivo odorant response characteristics of acutely isolated cells from the adult olfactory epithelium. This work has important ramifications for investigation of olfactory function and dysfunction using biopsy procedures and in vitro assays of odor sensitivity.

Switching from simple to complex oscillations in calcium signaling

We present a new model for calcium oscillations based on experiments in hepatocytes. The model considers feedback inhibition on the initial agonist receptor complex by calcium and activated phospholipase C, as well as receptor type-dependent self-enhanced behavior of the activated G(alpha) subunit. It is able to show simple periodic oscillations and periodic bursting, and it is the first model to display chaotic bursting in response to agonist stimulations. Moreover, our model offers a possible explanation for the differences in dynamic behavior observed in response to different agonists in hepatocytes.

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.

Ca2+ depletion and inositol 1,4,5-trisphosphate-evoked activation of Ca2+ entry in single guinea pig hepatocytes

Store-operated Ca(2+) entry was investigated by monitoring the Ca(2+)-dependent K(+) permeability in voltage-clamped guinea pig hepatocytes. In physiological conditions, intracellular Ca(2+) stores are discharged following agonist stimulation, but depletion of this stores can be achieved using Ca(2+)-Mg(2+)-ATPase inhibitors such as 2,5-di(tert-butyl)-1,4-benzohydroquinone and thapsigargin. The effect of internal Ca(2+) store depletion on Ca(2+) influx was tested in single cells using inositol 1,4,5-trisphosphate (InsP(3)) release from caged InsP(3) after treatment of the cells with 2, 5-di(tert-butyl)-1,4-benzohydroquinone or thapsigargin in Ca(2+)-free solutions. We show that the photolytic release of 1-d-myo-inositol 1,4-bisphosphate 5-phosphorothioate, a stable analog of InsP(3), and Ca(2+) store depletion have additive effects to activate a high level of Ca(2+) entry in single guinea pig hepatocytes. These results suggest that there is a direct functional interaction between InsP(3) receptors and Ca(2+) channels in the plasma membrane, although the nature of these Ca(2+) channels in hepatocytes is unclear.

The role of mitochondria in the regulation of calcium influx into Jurkat cells

In electrically nonexcitable cells the activity of the plasma membrane calcium channels is controlled by events occurring in mitochondria, as well as in the lumen of the endoplasmic reticulum. Thapsigargin, a specific inhibitor of endoplasmic reticulum Ca2+-ATPase, produces the release of calcium from the endoplasmic reticulum and thus, activation of store-operated calcium channels in the plasma membrane. However, thapsigargin failed to produce significant activation of the channels in Jurkat cells that had been pretreated with mitochondria-directed agents: an uncoupler (carbonyl cyanide m-chlorophenylhydrazone) and oligomycin. This is in spite of the fact that Jurkat cells pretreated with carbonyl cyanide m-chlorophenylhydrazone plus oligomycin are otherwise energetically competent, due to a high rate of glycolysis and the inhibition of mitochondrial F1Fo-ATPase by oligomycin. The pool of intracellular ATP was found not to be influenced by the pretreatments of cells with oligomycin or with oligomycin plus carbonyl cyanide m-chlorophenylhydrazone. In the control cells, we found that the ATP pool amounted to 23.2 +/- 1.9 nmoles per 107 cells (n = 4). In cells pretreated with oligomycin the level of ATP was 21.8 +/- 1.9 nmoles per 107 cells (n = 4), and in cells pretreated with both oligomycin and an uncoupler the level of ATP was 22.1 +/- 0.2 nmoles per 107 cells (n = 3). Moreover, in cells pretreated with oligomycin plus carbonyl cyanide m-chlorophenylhydrazone and suspended in a nominally calcium-free medium, thapsigargin produces transient increases in cytosolic calcium identical to those in the control cells. Thus, this pretreatment does not modify either the content of intracellular calcium stores and/or the activity of calcium ATPase in the plasma membrane. Similar results were obtained when Jurkat cells were challenged by myxothiazol, a potent inhibitor of mitochondrial cytochrome bc1 oxidoreductase. Thapsigargin, although producing calcium release from intracellular stores, was ineffective in triggering the activation of calcium channels in the plasma membrane in the case of cells pretreated with myxothiazol and oligomycin. Our results suggest that coupled mitochondria participate directly in the control of calcium channel activity in the plasma membrane of Jurkat cells. When the mitochondrial protonmotive force is collapsed, either by carbonyl cyanide m-chlorophenylhydrazone or myxothiazol, the channel remains inactive even under conditions of empty intracellular calcium stores.

Disruption of filamentous actin diminishes hormonally evoked Ca2+ responses in rat liver

Previous studies have suggested a role for the actin cytoskeleton in hormonally evoked Ca2+ signaling in the liver. Here, we present evidence supporting a connection between filamentous actin (F-actin) organization and the ability of vasopressin and glucagon to increase cytosolic free-Ca2+ ([Ca2+]i) levels. F-actin was disrupted in hepatic cells by perfusion of rat liver with cytochalasin D. Epifluorescence microscopy of subsequently isolated cells showed reduced cortical fluorescent phalloidin staining in cytochalasin D-treated liver cells. Cytochalasin D pretreatment of liver cells reduced the vasopressin-stimulated elevation of [Ca2+]i by 60% and of glucagon by 50%. Experiments performed on cytochalasin D-treated cells using Mn2+ as an indicator of Ca2+ influx quenched fura-2 fluorescence signals following vasopressin administration. This indicates that a structurally intact cortical F-actin web is not a prerequisite for the influx of calcium. Therefore, the attenuation of the increase in cytosolic calcium observed in cytochalasin D-treated liver cells was likely caused either by the depletion of the calcium store by treatment with cytochalasin D or by the need for an intact cytoskeletal structure for its release. Because the resting level of calcium did not change in cells exposed to cytochalasin D, the latter is likely. The reduced [Ca2+]i response may be the mechanism by which cytochalasin D pretreatment inhibits vasopressin-induced metabolic effects. Cytochalasin D pretreatment also decreased the ability of glucagon to stimulate gluconeogenesis and reduced the stimulation of O2 uptake usually observed following glucagon administration. In conclusion, these results suggest that the hormonal elevation of [Ca2+]i and resultant activation of specific metabolic pathways require normal F-actin organization.

Inhibition of EGF-dependent calcium influx by annexin VI is splice form-specific

Annexin VI is a widely expressed calcium- and phospholipid-binding protein that lacks a clear physiological role. We now report that A431 cells expressing annexin VI are defective in their ability to sustain elevated levels of cytosolic Ca(2+) following stimulation with EGF. Other aspects of EGF receptor signaling, such as protein tyrosine phosphorylation and induction of c-fos are normal in these cells. However, EGF-mediated membrane hyperpolarization is attenuated and Ca(2+) entry abolished in cells expressing annexin VI. This effect of annexin VI was only observed for the larger of the two annexin VI splice forms, the smaller splice variant had no discernable effect on either cellular phenotype or growth rate. Inhibition of Ca(2+) influx was specific for the EGF-induced pathway; capacitative Ca(2+) influx initiated by emptying of intracellular stores was unaffected. These results provide the first evidence that the two splice forms of annexin VI have different functions.

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.

Purinergic receptor-induced changes in paracellular resistance across cultures of human cervical cells are mediated by two distinct cytosolic calcium-related mechanisms

In human cervical (CaSki) cells, extracellular adenosine triphosphate (ATP) induces an acute decrease in the resistance of the lateral intercellular space (RLIS), phase I response, followed by an increase in tight junctional resistance (RTJ), phase II response. ATP also stimulates release of calcium from intracellular stores, followed by augmented calcium influx, and both effects have similar sensitivities to ATP (EC50 of 6 microM). The objective of the study was to determine the degree to which the changes in [Ca2+]i mediate the responses to ATP. 1,2-bis (2-aminophenoxy) ethane-N,N,N1,N1-tetraacetic acid (BAPTA) abrogated calcium mobilization and phase I response; in contrast, nifedipine and verapamil inhibited calcium influx and attenuated phase II response. Barium, La3+, and Mn2+ attenuated phase I response and attenuated and shortened the ionomycin-induced phase I-like decrease in RLIS, suggesting that store depletion-activated calcium entry was inhibited. Barium and La3+ also inhibited the ATP-induced phase II response, but Mn2+ had no effect on phase II response, and in the presence of low extracellular calcium it partly restored the increase in RTJ. KCl-induced membrane depolarization stimulated an acute decrease in RLIS and a late increase in RTJ similar to ATP, but only the latter was inhibited by nifedipine. KCl also induced a nifedipine-sensitive calcium influx, suggesting that acute increases in [Ca2+]i, regardless of mobilization or influx, mediate phase I response. Phase II-like increases in RTJ could be induced by treatment with diC8, and were not affected by nifedipine. Biphasic, ATP-like changes in RTE could be induced by treating the cells with ionomycin plus diC8. We conclude that calcium mobilization mediates the early decrease in RLIS, and calcium influx via calcium channels activates protein kinase C and mediates the late increase in RTJ.

Transduction mechanisms in vertebrate olfactory receptor cells

Considerable progress has been made in the understanding of transduction mechanisms in olfactory receptor neurons (ORNs) over the last decade. Odorants pass through a mucus interface before binding to odorant receptors (ORs). The molecular structure of many ORs is now known. They belong to the large class of G protein-coupled receptors with seven transmembrane domains. Binding of an odorant to an OR triggers the activation of second messenger cascades. One second messenger pathway in particular has been extensively studied; the receptor activates, via the G protein Golf, an adenylyl cyclase, resulting in an increase in adenosine 3',5'-cyclic monophosphate (cAMP), which elicits opening of cation channels directly gated by cAMP. Under physiological conditions, Ca2+ has the highest permeability through this channel, and the increase in intracellular Ca2+ concentration activates a Cl- current which, owing to an elevated reversal potential for Cl-, depolarizes the olfactory neuron. The receptor potential finally leads to the generation of action potentials conveying the chemosensory information to the olfactory bulb. Although much less studied, other transduction pathways appear to exist, some of which seem to involve the odorant-induced formation of inositol polyphosphates as well as Ca2+ and/or inositol polyphosphate -activated cation channels. In addition, there is evidence for odorant-modulated K+ and Cl- conductances. Finally, in some species, ORNs can be inhibited by certain odorants. This paper presents a comprehensive review of the biophysical and electrophysiological evidence regarding the transduction processes as well as subsequent signal processing and spike generation in ORNs.

A novel type of Ca2+ channel in U-87 MG cells activated by anti-galactocerebroside

Antibody to galactocerebroside (GalC) evokes a Ca2+ response in cultured glioma U-87 MG cells. The rise in intracellular calcium [Ca2+]i occurs largely due to the influx of Ca2+ through a plasma membrane channel, though the release of Ca2+ from intracellular stores also contributes. We characterized the channel activated by anti-GalC. The channel activity was transient and the inactivation appeared to be Ca2+ dependent. The channel was impermeant to monovalent ions Na+ and K+ and also to Mn2+. Ni2+ and Co2+ neither permeate through the channel nor inhibit the Ca2+ influx. In contrast Cd2+ the most potent inorganic blocker of Ca2+ channels permeated through this channel. The Ca2+ influx was inhibited by verapamil with IC50 of 65 +/- 8 microM. The Ca2+ influx as well as the intracellular release were markedly inhibited by neomycin sulfate and phorbol dibutyrate, suggesting that the Ca2+ influx may be mediated by IP3 (1). Depletion of intracellular Ca2+ stores by thapsigargin was followed by Ca2+ influx. This represents the capacitative Ca2+ entry pathway and is distinct from the channel activated by anti -GalC.

Evidence that a low-molecular-mass GTP-binding protein is required for store-activated Ca2+ inflow in hepatocytes

The roles of a monomeric GTP-binding regulatory protein in the activation of store-activated plasma membrane Ca2+ channels and in the release of Ca2+ from the smooth endoplasmic reticulum (SER) in rat liver parenchymal cells were investigated with the use of freshly isolated rat hepatocytes and rat liver microsomes. A low concentration (approx. 130 microM intracellular) of guanosine 5'-[gamma-thio]triphosphate (GTP[S]) activated Ca2+ inflow in intact hepatocytes in the absence of an agonist, whereas a high concentration (approx. 530 microM intracellular) of GTP-S- or guanosine 5'-[betagamma-imido]triphosphate (p[NH]ppG) inhibited the Ca2+ inflow induced by inhibitors of the activity of the endoplasmic-reticulum Ca2+-ATPase (SERCA) and by vasopressin. GTP (530 microM) prevented the inhibition of Ca2+ inflow by GTP-S- and p[NH]ppG. Brefeldin A and the peptide human Arf-1-(2-17), which inhibit many functions of ADP ribosylation factor (Arf) proteins, inhibited the Ca2+ inflow induced by SERCA inhibitors and vasopressin, and altered the profile of Ca2+ release from the SER. These effects were observed at concentrations of Brefeldin A and Arf-1-(2-17) comparable with those that inhibit the functions of Arf proteins in other systems. Succinylated Arf-1-(2-17) had a negligible effect on Ca2+ inflow. GTP[S] and Arf-1-(2-17) completely inhibited the synergistic action of GTP and Ins(1,4,5)P3 in releasing 45Ca2+ from rat liver microsomes loaded with 45Ca2+. AlF4(-) (under conditions expected to activate trimeric G-proteins) and succinylated Arf-1-(2-17) had no effect on GTP/Ins(1,4,5))3-induced 45Ca2+ release, and a mastoparan analogue caused partial inhibition. Arf-1-(2-17) did not inhibit 45Ca2+ release induced by either thapsigargin or ionomycin. It is concluded that a low-molecular-mass G-protein, most probably a member of the Arf protein family, is required for store-activated Ca2+ inflow in rat hepatocytes. The idea that the role of this G-protein is to maintain a region of the SER in the correct intracellular location is discussed briefly.

Selectivity and response characteristics of human olfactory neurons

Transduction mechanisms were investigated in human olfactory neurons by determining characteristics of odorant-induced changes in intracellular calcium concentration ([Ca2+]i). Olfactory neurons were freshly isolated from nasal biopsies, allowed to attach to coverslips, and loaded with the calcium-sensitive indicator fura-2. Changes in [Ca2+]i were studied in response to exposure to individual odors, or odorant mixtures composed to distinguish between transduction pathways mediated by adenosine 3'5'-monophosphate (cAMP; mix A) or inositol 1,4,5-trisphosphate (InsP3; mix B). Overall, 52% of biopsies produced one or more odorant-responsive olfactory neurons, whereas 24% of all olfactory neurons tested responded to odorant exposure with a change in [Ca2+]i. As in olfactory neurons from other species, the data suggest that odorant exposure elicited calcium influx via second-messenger pathways involving cAMP or InsP3. Unlike olfactory neurons from other species that have been tested, some human olfactory neurons responded to odorants with decreases in [Ca2+]i. Also in contrast with olfactory neurons from other species, human olfactory neurons were better able to discriminate between odorant mixtures in that no neuron responded to more than one type of odor or mixture. These results suggest the presence of a previously unreported type of olfactory transduction mechanism, and raise the possibility that coding of odor qualities in humans may be accomplished to some degree differently than in other vertebrates, with the olfactory neuron itself making a greater contribution to the discrimination process.

Regulation of motility of cells from marine sponges by calcium ions

Sponges are known not to contain muscle and nerve cells. Since sponge cells are characterized by high motility we determined the effect of intracellular calcium ion concentration ([Ca2+]i) on their motility. Addition of the Ca2+ ionophore ionomycin to dissociated cells from the marine sponge Suberites domuncula caused in Ca(2+)-containing artificial seawater (ASW) an increase in motility from 0.2 micron/min (absence of the ionophore) to 3.7 microns/min (presence of ionomycin). When the experiments were performed in Ca(2+)-free medium, no effect of ionomycin could be observed. In parallel experiments the changes of [Ca2+]i using the dye Fura-2 were measured. The experiments revealed that ionomycin causes an influx of Ca2+ into the cytosol of cells suspended in Ca(2+)-containing artificial seawater. In contrast, if cells were suspended in Ca(2+)-free artificial seawater, no increase of [Ca2+]i occurred. Incubation of cells in the presence of inhibitors, specific for endoplasmatic Ca(2+)-ATPase in mammals such as thapsigargin, cyclopiazonic acid, or 2,5 di-t-butylhydrochinone, did not influence the [Ca2+]i if cells were suspended in Ca(2+)-free artificial seawater. From these data we conclude that the [Ca2+]i is primarily regulated through channels in the plasma membrane. In addition we summarize experimental evidence indicating that the [Ca2+]i is involved in the control of cell motility. From the marine sponge Geodia cydonium a partial sequence of the myosin cDNA has been cloned. The deduced amino acid sequence comprises highest homology to nonmuscle myosin type II found in higher invertebrates and vertebrates. Taken together, these data show that the [Ca2+]i level in sponge cells can be modulated by incubation with ionomycin. An increase of the Ca2+ level parallels with higher motility of cells, suggesting an activation of Ca(2+)-dependent protein kinases of myosin type II. Investigations on the ionomycin-activated influx of Ca2+ into the cytosol revealed that predominantly the Ca2+ channels in plasma membrane control the level of [Ca2+]i.

TLC characterization of small unilamellar liposomes containing D-myo-inositol derivatives

The thin-layer chromatographic (TLC) behaviour of small unilamellar liposomes containing inositol phosphates (IPs) was studied. The vesicles contained different concentrations of D-myo-inositol 1,4,5-triphosphate (IP3), D-myo-inositol 1,2,6-triphosphate (alpha-trinositol, PP 56, a novel Perstorp Pharma derivative), D-myo-inositol 1,3,4,5-tetraphosphate (IP4), D-myo-inositol 1,3,4,5,6-pentakisphosphate (IP5) and D-myo-inositol 1,2,3,4,5,6-hexakisphosphate (IP6). Migration of all liposome batches was compared to that of control liposomes (multilamellar and small unilamellar, both containing only triple-distilled water), and to that of free phosphatidylcholine (PC). The same amount of lipid was used in all situations. Thin-layer chromatography was performed with silica gel as adsorbent. The developing solvent was an n-buthanol:ethanol:water mixture in a 4:3:3 volume ratio. At doses higher than 10(-2) M liposomes containing alpha-trinositol and IP6 had a different migration than PC, MLV or SUV as well as all batches of liposomes. Physiological studies (using as model endothelized rat aorta rings) proved that in this situation they had no effects.

Ligand-gated calcium channels inside and out

Calcium release from intracellular stores occurs through two types of channels associated with intracellular membranes, namely, the ryanodine receptor and the inositol 1,4,5-trisphosphate receptor. Recently, it has been shown that these channels are regulated by allosteric mechanisms and associated proteins. Release of intracellular calcium induces the opening of calcium-permeable channels on the plasma membrane. Current work has focused on the molecular and functional characterization of these channels which have been identified as store-operated channels or calcium release activated channels.

Adenosine stimulates calcium influx in isolated rat hepatocytes

The mechanism of stimulation of Ca2+ entry into hepatocytes by adenosine was investigated. When Fura-2-loaded hepatocytes were suspended in a nominally Ca(2+)-free buffer, adenosine produced only a small transient increase in the cytosolic free Ca2+ concentration ([Ca2+)i). However, on restoration of an extracellular Ca2+ concentration of 1.3 mM, a rapid increase in [Ca2+]i occurred, which indicates activation of a Ca(2+)-influx pathway. Adenosine augmented the rate of Ca2+ influx triggered by maximally effective concentrations of thapsigargin or cAMP, but was without effect on the rate of Ca2+ entry that resulted from phospholipase-C-linked-receptor activation by maximally effective concentrations of vasopressin or ATP. However, in contrast to vasopression and ATP, adenosine did not stimulate Mn2+ entry. The rate of Mn2+ influx after stimulation of the hepatocytes with vasopressin was not increased by adenosine treatment. The stimulation of hepatocytes with adenosine did not result in significant accumulation of inositol phosphates or cAMP. Furthermore, the rate of adenosine-induced Ca2+ entry in hepatocytes was only slightly reduced in the presence of the P1 purinoceptor antagonist 8-phenyltheophylline. In contrast, the receptor-mediated-Ca(2+)-entry antagonist SK&F 96365 nearly completely blocked the Ca(2+)-entry response without any effect on internal-Ca(2+)-pool mobilisation by adenosine. It is concluded that adenosine activates the internal-pool-regulated pathway of Ca2+ entry and an additional pathway that appears comparable to the previously reported receptor-dependent pathway, except that Mn2+ entry is not stimulated.

Injection of rat hepatocyte poly(A)+ RNA to Xenopus laevis oocytes leads to expression of a constitutively-active divalent cation channel distinguishable from endogenous receptor-activated channels

The expression of hepatocyte plasma membrane receptor-activated divalent cation channels in immature (stages V and VI) Xenopus laevis oocytes and the properties which allow these channels to be distinguished from endogenous receptor-activated divalent cation channels were investigated. Divalent cation inflow to oocytes housed in a multiwell plate was measured using the fluorescent dyes Fluo-3 and Fura-2. In control oocytes, ionomycin, cholera toxin, thapsigargin, 3-fluoro-inositol 1,4,5-trisphosphate (InsP3F) and guanosine 5'-[gamma-thio]triphosphate (GTP gamma S) stimulated Ca2+ and Mn2+ inflow following addition of these ions to the oocytes. Ionomycin-, cholera-toxin-, thapsigargin- and InsP3F-stimulated Ca2+ inflow was inhibited by Gd3+ (half maximal inhibition at less thari 5 microM Gd3+ for InsP3F-stimulated Ca2+ inflow). GTP gamma S-stimulated Ca2+ inflow was insensitive to 50 microM Gd3+ and to SK&F 96365. These results indicate that at least three types of endogenous receptor-activated Ca2+ channels can be detected in Xenopus oocytes using Ca(2+)-sensitive fluorescent dyes: lanthanide-sensitive divalent cation channels activated by intracellular Ca2+ store depletion, lanthanide-sensitive divalent cation channels activated by cholera toxin, and lanthanide-insensitive divalent cation channels activated by an unknown trimeric G-protein. Oocytes microinjected with rat hepatocyte poly(A)+ RNA exhibited greater rates of Ca2+ and Mn2+ inflow in the basal (no agonist) state, greater rates of Ca2+ inflow in the presence of vasopressin or InsP3F and greater rates of Ba2+ inflow in the presence of InsP3F, when compared with "mock"-injected oocytes. In poly(A)+ RNA-injected oocytes, vasopressin- and InsP3F-stimulated Ca2+ inflow, but not basal Ca2+ inflow, was inhibited by Gd3+. It is concluded that at least one type of hepatocyte plasma membrane divalent cation channel, which admits Mn2+ as well as Ca2+ and is lanthanide-insensitive, can be expressed and detected in Xenopus oocytes.

Second messenger signaling in olfactory transduction

Olfactory receptor neurons respond to odorants with G-protein mediated increases in the concentration of cyclic adenosine 3',5'-monophosphate (cAMP) and/or inositol 1,4,5-trisphospahte (InsP3). These two second messengers directly regulate opening of cAMP- and InsP3-regulated conductances localized to the apical transduction compartments of the cell (cilia and olfactory knob). In the presence of physiological concentrations of extracellular Ca2+, these second messenger regulated conductances mediate influx of Ca2+ into the olfactory neuron resulting in large, localized increases in intracellular Ca2+ ([Ca2+]i). A significant advance in our understanding of the molecular mechanisms of olfaction is the recent realization that this increase in [Ca2+]i plays an important role as a "third messenger" in olfactory transduction. Second messenger dependent increases in [Ca2+]i cause opening of ciliary Ca(2+)-activated Cl-, cation and/ or K+ channels that can carry a large percentage of the generator current, thus amplifying the signal substantially. As a result of this sequence of events, the generator potential in olfactory neurons can be depolarizing, leading to excitation of the neuron, or hyperpolarizing, leading to suppression of basal action potential firing rate. This dual effect of odorants on olfactory neurons may play an important role in quality coding and in the ability to detect low concentrations of odorants, particularly in complex mixtures.

The role of intracellular Ca2+ in the regulation of gluconeogenesis

A hypothesis for the hormonal regulation of gluconeogenesis, in which increases in cytosolic free-Ca2+ levels ([Ca2+]i) play a major role, is presented. This hypothesis is based on the observation that gluconeogenic hormones evoke a common pattern of Ca2+ redistribution, resulting in increases in [Ca2+]i. Current concepts of hormonally evoked Ca2+ fluxes are presented and discussed. It is suggested that the increase in [Ca2+]i is functionally linked to stimulation of gluconeogenesis. The stimulation of gluconeogenesis is accomplished in two ways: (1) by increasing the activities of the Krebs cycle and the electron-transfer chain, thereby supplying adenosine triphosphates (ATP) and reducing equivalents to the process; and (2) by stimulating the activities of key gluconeogenic enzymes, such as pyruvate carboxylase. The hypothesis presents a conceptual framework that ties together two interrelated manifestations of hormone action: signal transduction and metabolism.

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.

T cell receptor-mediated Ca2+ signaling: release and influx are independent events linked to different Ca2+ entry pathways in the plasma membrane

In this study, we showed that cross-linking CD3 molecules on the T cell surface resulted in Ca2+ release from the intracellular stores followed by a sustained Ca2+ influx. Inhibition of release with TMB-8 did not block the influx. However, inhibition of phospholipase C activity suppressed both Ca2+ release and influx. Once activated, the influx pathway remained open in the absence of further hydrolysis of PIP2. Thapsigargin, a microsomal Ca(2+)-ATPase inhibitor, stimulated Ca2+ entry into the cells by a mechanism other than emptying Ca2+ stores. In addition, Ca2+ entry into the Ca(2+)-depleted cells was stimulated by low basal level of cytosolic Ca2+, not by the emptying of intracellular Ca2+ stores. Both the Ca2+ release and influx were dependent on high and low concentrations of extracellular Ca2+. At low concentrations, Mn2+ entered the cell through the Ca2+ influx pathway and quenched the sustained phase of fluorescence; whereas, at higher Mn2+ concentration both the transient and the sustained phases of fluorescence were quenched. Moreover, Ca2+ release was inhibited by low concentrations of Ni2+, La3+, and EGTA, while Ca2+ influx was inhibited by high concentrations. Thus, in T cells Ca2+ influx occurs independently of IP3-dependent Ca2+ release. However, some other PIP2 hydrolysis-dependent event was involved in prolonged activation of Ca2+ influx. Extracellular Ca2+ influenced Ca2+ release and influx through the action of two plasma membrane Ca2+ entry pathways with different pharmacological and biochemical properties.

TLC characterization of liposomes containing D-myo-inositol derivatives

The thin-layer chromatographic (TLC) behaviour of liposomes containing inositol phosphates (IPs) was studied. The liposomes contained different concentrations of D-myo-inositol 1,4,5k-trisphosphate (IP3), D-myo-inositol 1,2,6-trisphosphate (alpha-trinositol, PP 56, a novel Perstorp Pharma derivative), D-myo-inositol 1,3,4,5-tetrakisphosphate (IP4), D-myo-inositol 1,3,4,5,6-pentakisphosphate (IP5) and D-myo-inositol 1,2,3,4,5,6-hexakisphosphate (IP6). Migration of all liposome batches was compared to that of control liposomes (containing only triple distilled water), and to that of free phosphatidylcholine (PC); the same amount of lipid was used in all situations. Thin-layer chromatography was performed on silica gel as adsorbent. As solvent we used an n-buthanol:ethanol:water mixture in a 4:3:3 volume ratio. Significant differences were found between PC and all liposome batches, as well as between control liposomes and the ones containing IP3, alpha-trinositol, IP4, or IP5, in various concentrations. Liposomes containing IP6 migrate completely differently compared not only to phosphatidylcholine and control liposomes, but also to the ones containing other IPs ( < 10(-3) M). Unlike the other IPs studied, liposome-entrapped IP6 elicits dose-dependent contractions of the isolated rat aorta. This suggests that liposomes loaded with IP6 undergo, during or after their preparation, physico-chemical alterations that eventually change their drug-delivery capacity.

Histamine-induced Ca2+ entry precedes Ca2+ mobilization in bovine adrenal chromaffin cells

The relationship between histamine-induced Ca2+ mobilization and Ca2+ entry in bovine adrenal chromaffin cells has been investigated. Stopped-flow fluorimetry of fura-2-loaded chromaffin cell populations revealed that 10 microM histamine promoted entry of Ca2+ or Mn2+ without measurable delay (< or = 20 ms), through a pathway that was insensitive to the dihydropyridine antagonist nifedipine. In the absence of extracellular Ca2+, or in the presence of 100 microM La3+, a blocker of receptor-mediated Ca2+ entry, 10 microM histamine triggered an elevation in intracellular calcium concentration ([Ca2+]i), but only after a delay of approx. 200 ms, which presumably represented the time required to mobilize intracellular Ca2+. These data suggested that histamine-induced bivalent-cation entry precedes extensive Ca2+ mobilization in chromaffin cells. In order to confirm that histamine can promote Ca2+ entry largely independently of mobilizing intracellular Ca2+, the ability of histamine to promote Ca2+ entry into cells whose intracellular Ca2+ store had been largely depleted was assessed. Fura-2-loaded chromaffin cells were treated with 10 microM ryanodine together with 40 mM caffeine, to deplete the hormone-sensitive Ca2+ store. This resulted in an approx. 95% inhibition of histamine-induced Ca2+ release. Under these conditions, histamine was still able to promote an entry of Ca2+ that was essentially indistinguishable from that promoted in control cells. In single cells, introduction of heparin (100 mg/ml), but not de-N-sulphated heparin (100 mg/ml), abolished the histamine-induced rise in [Ca2+]i. All these data suggest that histamine can induce G-protein- or inositol phosphate-dependent rapid (< or = 20 ms) Ca2+ entry without an extensive intracellular mobilization response in chromaffin cells, which points to activation of an entry mechanism distinct from the Ca(2+)-release-activated Ca2+ channel found in non-excitable cells.