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

Gain-of-Function Mutation in STIM1 (P.R304W) Is Associated with Stormorken Syndrome

Stormorken syndrome is a rare autosomal dominant disorder characterized by a phenotype that includes miosis, thrombocytopenia/thrombocytopathy with bleeding time diathesis, intellectual disability, mild hypocalcemia, muscle fatigue, asplenia, and ichthyosis. Using targeted sequencing and whole-exome sequencing, we identified the c.910C > T transition in a STIM1 allele (p.R304W) only in patients and not in their unaffected family members. STIM1 encodes stromal interaction molecule 1 protein (STIM1), which is a finely tuned endoplasmic reticulum Ca(2+) sensor. The effect of the mutation on the structure of STIM1 was investigated by molecular modeling, and its effect on function was explored by calcium imaging experiments. Results obtained from calcium imaging experiments using transfected cells together with fibroblasts from one patient are in agreement with impairment of calcium homeostasis. We show that the STIM1 p.R304W variant may affect the conformation of the inhibitory helix and unlock the inhibitory state of STIM1. The p.R304W mutation causes a gain of function effect associated with an increase in both resting Ca(2+) levels and store-operated calcium entry. Our study provides evidence that Stormorken syndrome may result from a single-gene defect, which is consistent with Mendelian-dominant inheritance.

Orai3 constitutes a native store-operated calcium entry that regulates non small cell lung adenocarcinoma cell proliferation

Orai channels have been associated with cell proliferation, survival and metastasis in several cancers. The present study investigates the expression and the role of Orai3 in cell proliferation in non-small cell lung cancer (NSCLC). We show that Orai3 is over-expressed in cancer tissues as compared to the non-tumoral ones. Furthermore, Orai3 staining is stronger in high grade tumors. Pharmacological inhibition or knockdown of Orai3 significantly reduced store operated calcium entry (SOCE), inhibited cell proliferation and arrested cells of two NSCLC cell lines in G0/G1 phase. These effects were concomitant with a down-regulation of cyclin D1, cyclin E, CDK4 and CDK2 expression. Moreover, Orai3 silencing decreased Akt phosphorylation levels. In conclusion, Orai3 constitutes a native SOCE pathway in NSCLC that controls cell proliferation and cell cycle progression likely via Akt pathway.

Glucose enhances expression of TRPC1 and calcium entry in endothelial cells

Hyperglycemia is a major risk factor for endothelial dysfunction and vascular disease, and in the current study, the link to glucose-induced abnormal intracellular Ca(2+) (Ca(i)(2+)) homeostasis was explored in bovine aortic endothelial cells in high glucose (HG; 25 mmol/l) versus low glucose (LG; 5.5 mmol/l; control). Transient receptor potential 1 (TRPC1) ion channel protein, but not TRPC3, TRPC4, or TRPC6 expression, was significantly increased in HG versus LG at 72 h. HG for 4, 24, and 72 h did not change basal Ca(i)(2+) or ATP-induced Ca(i)(2+) release; however, the amplitude of sustained Ca(i)(2+) was significantly increased at 24 and 72 h and reduced by low concentration of the putative, but nonspecific, TRPC blockers, gadolinium, SKF-96365, and 2-aminoethoxydiphenyl borate. Treatment with TRPC1 antisense significantly reduced TRPC1 protein expression and ATP-induced Ca(2+) entry in bovine aortic endothelial cells. Although the link between HG-induced changes in TRPC1 expression, enhanced Ca(2+) entry, and endothelial dysfunction require further study, the current data are suggestive that targeting these pathways may reduce the impact of HG on endothelial function.

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.

Capacitative Ca2+ entry is involved in regulating soluble amyloid precursor protein (sAPPalpha) release mediated by muscarinic acetylcholine receptor activation in neuroblastoma SH-SY5Y cells

Previous studies have demonstrated that stimulation of phospholipase C-linked G-protein-coupled receptors, including muscarinic M1 and M3 receptors, increases the release of the soluble form of amyloid precursor protein (sAPPalpha) by alpha-secretase cleavage. In this study, we examined the involvement of capacitative Ca2+ entry (CCE) in the regulation of muscarinic acetylcholine receptor (mAChR)-dependent sAPPalpha release in neuroblastoma SH-SY5Y cells expressing abundant M3 mAChRs. The sAPPalpha release stimulated by mAChR activation was abolished by EGTA, an extracellular Ca2+ chelator, which abolished mAChR-mediated Ca2+ influx without affecting Ca2+ mobilization from intracellular stores. However, mAChR-mediated sAPPalpha release was not inhibited by thapsigargin, which increases basal [Ca2+]i by depletion of Ca2+ from intracellular stores. While these results indicate that the mAChR-mediated increase in sAPPalpha release is regulated largely by Ca2+ influx rather than by Ca2+ mobilization from intracellular stores, we further investigated the Ca2+ entry mechanisms regulating this phenomenon. CCE inhibitors such as Gd3+, SKF96365, and 2-aminoethoxydiphenyl borane (2-APB), dose dependently reduced both Ca2+ influx and sAPPalpha release stimulated by mAChR activation, whereas inhibition of voltage-dependent Ca2+ channels, Na+/Ca2+ exchangers, or Na+-pumps was without effect. These results indicate that CCE plays an important role in the mAChR-mediated release of sAPPalpha.

In rat hepatocytes, different adenosine receptor subtypes use different secondary messengers to increase the rate of ureagenesis

In rat hepatocytes, the role of cAMP and Ca(2+) as secondary messengers in the ureagenic response to stimulation of specific adenosine receptor subtypes was explored. Analyzed receptor subtypes were: A(1), A(2A), A(2B) and A(3). Each receptor subtype was stimulated with a specific agonist while blocking all other receptor subtypes with a battery of specific antagonists. For the A(1) and A(3) adenosine receptor subtypes, the secondary messenger was the cytoplasmic Ca(2+) concentration ([Ca(2+)](cyt)). Accordingly, the A(1) or A(3)-mediated increase in [Ca(2+)](cyt) and in ureagenic activity were both inhibited by chelating Ca(2+) with either EGTA or BAPTA-AM. Also, Gd(3+) blocked both the increase in [Ca(2+)](cyt) and ureagenesis, suggesting that a Ca(2+) channel may be involved in the response to both A(1) and A(3). A partial effect was observed with the sarcoplasmic reticulum Ca(2+)-ATPase inhibitor thapsigargin. The concentration of cyclic AMP ([cAMP]) increased in response to stimulation of either the A(2A) or the A(2B) adenosine receptor subtypes, while it decreased slightly in response to stimulation of either A(1) or A(3). The stimulation of either the A(2A) or A(2B) adenosine receptor subtypes resulted in an increase in [cAMP] and an ureagenic response which were not sensitive to EGTA, BAPTA-AM, Gd(3+) or to thapsigargin. In addition, the adenylyl cyclase inhibitor MDL12,330A blocked the ureagenic response to A(2A) and A(2B), but not the response to either A(1) or A(3). Our results indicate that in the ureagenic liver response to adenosine, the secondary messenger for both, the A(1) and A(3) adenosine receptor subtypes is [Ca(2+)](cyt), while the message from the A(2A) and A(2B) adenosine receptor subtypes is relayed by [cAMP].

Nuclear factor-kappaB activated by capacitative Ca2+ entry enhances muscarinic receptor-mediated soluble amyloid precursor protein (sAPPalpha) release in SH-SY5Y cells

G(q/11) protein-coupled muscarinic receptors are known to regulate the release of soluble amyloid precursor protein (sAPPalpha) produced by alpha-secretase processing; however, their signaling mechanisms remain to be elucidated. It has been reported that a muscarinic agonist activates nuclear factor (NF)-kappaB, a transcription factor that has been shown to play an important role in the Alzheimer disease brain, and that NF-kappaB activation is regulated by intracellular Ca2+ level. In the present study, we investigated whether NF-kappaB activation plays a role in muscarinic receptor-mediated sAPPalpha release enhancement and contributes to a changed capacitative Ca2+ entry (CCE), which was suggested to be involved in the muscarinic receptor-mediated stimulation of sAPPalpha release. Muscarinic receptor-mediated NF-kappaB activation was confirmed by observing the translocation of the active subunit (p65) of NF-kappaB to the nucleus by the muscarinic agonist, oxotremorine M (oxoM), in SH-SY5Y neuroblastoma cells expressing muscarinic receptors that are predominantly of the M3 subtype. NF-kappaB activation and sAPPalpha release enhancement induced by oxoM were inhibited by NF-kappaB inhibitors, such as an NF-kappaB peptide inhibitor (SN50), an IkappaB alpha kinase inhibitor (BAY11-7085), a proteasome inhibitor (MG132), the inhibitor of proteasome activity and IkappaB phosphorylation, pyrrolidine dithiocarbamate, the novel NF-kappaB activation inhibitor (6-amino-4-(4-phenoxyphenylethylamino) quinazoline), and by an intracellular Ca2+ chelator (TMB-8). Furthermore, both oxoM-induced NF-kappaB activation and sAPPalpha release were antagonized by CCE inhibitors (gadolinium or SKF96365) but not by voltage-gated Ca2+-channel blockers. On the other hand, treatment of cells with NF-kappaB inhibitors (SN50, BAY11-7085, MG132, or pyrrolidine dithiocarbamate) did not inhibit muscarinic receptor-mediated CCE. These findings provide evidence for the involvement of NF-kappaB regulated by CCE in muscarinic receptor-mediated sAPPalpha release enhancement.

Arachidonic acid mediates non-capacitative calcium entry evoked by CB1-cannabinoid receptor activation in DDT1 MF-2 smooth muscle cells

Cannabinoid CB1-receptor stimulation in DDT1 MF-2 smooth muscle cells induces a rise in [Ca2+]i, which is dependent on extracellular Ca2+ and modulated by thapsigargin-sensitive stores, suggesting capacitative Ca2+ entry (CCE), and by MAP kinase. Non-capacitative Ca2+ entry (NCCE) stimulated by arachidonic acid (AA) partly mediates histamine H1-receptor-evoked increases in [Ca2+]i in DDT1 MF-2 cells. In the current study, both Ca2+ entry mechanisms and a possible link between MAP kinase activation and increasing [Ca2+]i were investigated. In the whole-cell patch clamp configuration, the CB-receptor agonist CP 55, 940 evoked a transient, Ca2+-dependent K+ current, which was not blocked by the inhibitors of CCE, 2-APB, and SKF 96365. AA, but not its metabolites, evoked a transient outward current and inhibited the response to CP 55,940 in a concentration-dependent manner. CP 55,940 induced a concentration-dependent release of AA, which was inhibited by the CB1 antagonist SR 141716. The non-selective Ca2+ channel blockers La3+ and Gd3+ inhibited the CP 55,940-induced current at concentrations that had no effect on thapsigargin-evoked CCE. La3+ also inhibited the AA-induced current. CP 55,940-induced AA release was abolished by Gd3+ and by phospholipase A2 inhibition using quinacrine; this compound also inhibited the outward current. The CP 55,940-induced AA release was strongly reduced by the MAP kinase inhibitor PD 98059. The data suggest that in DDT1 MF-2 cells, AA is an integral component of the CB1 receptor signaling pathway, upstream of NCCE and, via PLA2, downstream of MAP kinase.

Evaluation, using targeted aequorins, of the roles of the endoplasmic reticulum and its (Ca2++Mg2+)ATP-ases in the activation of store-operated Ca2+ channels in liver cells

The process by which store-operated Ca2+ channels (SOCs) deliver Ca2+ to the endoplasmic reticulum (ER) and the role of (Ca2++Mg2+)ATP-ases of the ER in the activation of SOCs in H4-IIE liver cells were investigated using cell lines stably transfected with apo-aequorin targeted to the cytoplasmic space or the ER. In order to measure the concentration of Ca2+ in the ER ([Ca2+]er), cells were pre-treated with 2,5-di-tert-butylhydroquinone (DBHQ) to deplete Ca2+ in the ER before reconstitution of holo-aequorin. The addition of extracellular Ca2+ (Cao2+) to Ca2+-depleted cells induced refilling of the ER, which was complete within 5 min. This was associated with a sharp transient increase in the cytoplasmic Ca2+ concentration ([Ca2+]cyt) of about 15 s duration (a Cao2+-induced [Ca2+]cyt spike) after which [Ca2+]cyt remained elevated slightly above the basal value for a period of about 2 min (low [Ca2+]cyt plateau). The Cao2+-induced [Ca2+]cyt spike was inhibited by Gd3+, not affected by tetrakis-(2-pyridymethyl) ethylenediamine (TPEN), and broadened by ionomycin and the intracellular Ca2+ chelators BAPTA and EGTA. Refilling of the ER was inhibited by caffeine. Neither thapsigargin nor DBHQ caused a detectable inhibition or change in shape of the Cao2+-induced [Ca2+]cyt spike or the low [Ca2+]cyt plateau whereas each inhibited the inflow of Ca2+ to the ER by about 80%. Experiments conducted with carbonyl cyanide m-chlorophenyl-hydrazone (CCCP) indicated that thapsigargin did not alter the amount of Ca2+ accumulated in mitochondria. The changes in [Ca2+]cyt reported by aequorin were compared with those reported by fura-2. It is concluded that (i) there are significant quantitative differences between the manner in which aequorin and fura-2 sense changes in [Ca2+]cyt and (ii) thapsigargin and DBHQ inhibit the uptake of Ca2+ to the bulk of the ER but this is not associated with inhibition of the activation of SOCs. The possible involvement of a small sub-region of the ER (or another intracellular Ca2+ store), which contains thapsigargin-insensitive (Ca2++Mg2+)ATP-ases, in the activation of SOCs is briefly discussed.

Synergistic activation of mitogen-activated protein kinase by insulin and adenosine triphosphate in liver cells: permissive role of Ca2+

We have previously demonstrated that insulin and G(q)-coupled receptor agonists individually activate mitogen-activated protein kinase (MAPK) in liver cells and both effects involve an influx of extracellular Ca(2+). Yet, these agonists have opposing physiological actions on hepatocyte glucose metabolism. We thus investigated the interaction between insulin and the P2Y(2) purinergic agonist adenosine triphosphate (ATP) on MAPK in HTC cells, a model hepatocyte cell line, and determined the involvement of cytosolic Ca(2+). Insulin and ATP each induced a dose-dependent phosphorylation of p44/42 MAPK that was partially inhibited by EGTA. However, pretreatment with insulin markedly increased the MAPK phosphorylation response to ATP. This potentiation was canceled by chelation of extracellular Ca(2+) with EGTA. We used patch clamp electrophysiology and fluorescence microscopy to understand the role of intracellular Ca(2+) in this effect. Insulin and ATP, respectively, induced monophasic and multiphasic changes in membrane potential and intracellular Ca(2+) as expected. Pretreatment with 10 nmol/L insulin significantly decreased the initial rapid depolarization (inward nonselective cation current [NSCC]), as well as the compounded Ca(2+) response induced by 100 micro mol/L ATP. However, in Ca(2+)-free conditions, insulin did not modify the Ca(2+) mobilized from internal pools after stimulation with ATP. Upon Ca(2+) readmission, internal store depletion by ATP or thapsigargin doubled the rate of capacitative Ca(2+) influx, whereas insulin increased this influx 1.32-fold. On the other hand, insulin pretreatment counteracted the increased rate of Ca(2+) influx induced by ATP but not by thapsigargin. In summary, insulin counteracts the membrane potential and Ca(2+) responses to ATP in HTC cells. However, insulin and ATP effects on MAPK activation are synergistic and Ca(2+) influx plays a permissive role. Therefore, the opposing metabolic actions of insulin and G(q)-coupled receptor agonists involve an interaction in signaling pathways that resides downstream of Ca(2+) influx.

Arachidonic acid in astrocytes blocks Ca(2+) oscillations by inhibiting store-operated Ca(2+) entry, and causes delayed Ca(2+) influx

ATP-elicited oscillations of the concentration of free intracellular Ca(2+) ([Ca(2+)](i)) in rat brain astrocytes were abolished by simultaneous arachidonic acid (AA) addition, whereas the tetraenoic analogue 5,8,11,14-eicosatetraynoic acid (ETYA) was ineffective. Inhibition of oscillations is due to suppression by AA of intracellular Ca(2+) store refilling. Short-term application of AA, but not ETYA, blocked Ca(2+) influx, which was evoked by depletion of stores with cyclopiazonic acid (CPA) or thapsigargin (Tg). Addition of AA after ATP blocked ongoing [Ca(2+)](i) oscillations. Prolonged AA application without or with agonist could evoke a delayed [Ca(2+)](i) increase. This AA-induced [Ca(2+)](i) rise developed slowly, reached a plateau after 5 min, could be reversed by addition of bovine serum albumin (BSA), that scavenges AA, and was blocked by 1 microM Gd(3+), indicative for the influx of extracellular Ca(2+). Specificity for AA as active agent was demonstrated by ineffectiveness of C16:0, C18:0, C20:0, C18:2, and ETYA. Moreover, the action of AA was not affected by inhibitors of oxidative metabolism of AA (ibuprofen, MK886, SKF525A). Thus, AA exerted a dual effect on astrocytic [Ca(2+)](i), firstly, a rapid reduction of capacitative Ca(2+) entry thereby suppressing [Ca(2+)](i) oscillations, and secondly inducing a delayed activation of Ca(2+) entry, also sensitive to low Gd(3+) concentration.

Analysis of vasopressin-induced Ca2+ increase in rat hepatocytes

To analyze vasopressin-induced Ca2+ increase in liver cells, rat hepatocytes were isolated and attached to collagen-coated cover slips. Using fura-2, a Ca2+-sensing dye, changes in intracellular Ca2+ concentration by vasopressin were monitored. Results in this communication suggested that vasopressin-induced Ca2+ increase were composed of both Ca2+ release from internal Ca2+ stores and influx from the plasma membrane. The Ca2+ influx consisted of two distinguishable components. One was dependent on the presence of vasopressin and the other was not. SK&F96365 blocked vasopressin-induced Ca2+ influx in a dose-dependent manner. Vasopressin-induced Ca2+ release from internal stores diminished in a primary culture of hepatocytes according to the culture time. However, changes in vasopressin-induced Ca2+ influx across the plasma membrane differed from changes in the Ca2+ release from internal stores, suggesting two separate signalings from receptor activation to internal stores and to the plasma membrane.

Atrial natriuretic peptide attenuates Ca2+ oscillations and modulates plasma membrane Ca2+ fluxes in rat hepatocytes

BACKGROUND & AIMS

Oscillations in cytosolic free Ca2+ concentration are a fundamental mechanism of intracellular signaling in hepatocytes. The aim of this study was to examine the effects of atrial natriuretic peptide (ANP) on cytosolic Ca2+ oscillations in rat hepatocytes.

METHODS

Cyclic guanosine monophosphate (cGMP) was measured by enzyme immunoassay. Cytosolic Ca2+ oscillations were recorded from single aequorin-injected hepatocytes. Ca2+ efflux from hepatocyte populations was measured by using extracellular fura-2. Ca2+ influx was estimated by Mn2+ quench of fluorescence of fura-2 dextran injected into single hepatocytes.

RESULTS

ANP attenuated cytosolic Ca2+ oscillations through a decrease in their frequency. In addition, ANP dramatically stimulated plasma membrane Ca2+ efflux and modestly inhibited basal Ca2+ influx. All of the observed effects of ANP were mimicked by the cGMP analogue 8-bromo-cGMP (8-Br-cGMP), and were prevented by inhibition of protein kinase G. In contrast, activation of cytosolic guanylyl cyclase by sodium nitroprusside had no effect on Ca2+ efflux, Ca2+ influx, or Ca2+ oscillations.

CONCLUSIONS

ANP decreases the frequency of Ca2+ oscillations and modulates plasma membrane Ca2+ fluxes in rat hepatocytes. Attenuation of oscillatory Ca2+ signaling in hepatocytes may represent a key role for ANP in vivo.

Ca2+-mediated activation of ERK in hepatocytes by norepinephrine and prostaglandin F2 alpha: role of calmodulin and Src kinases

BACKGROUND

Previous studies have shown that several agents that stimulate heptahelical G-protein coupled receptors activate the extracellular signal regulated kinases ERK1 (p44mapk) and ERK2 (p42mapk) in hepatocytes. The molecular pathways that convey their signals to ERK1/2 are only partially clarified. In the present study we have explored the role of Ca2+ and Ca2+-dependent steps leading to ERK1/2 activation induced by norepinephrine and prostaglandin (PG)F2alpha.

RESULTS

Pretreatment of the cells with the Ca2+ chelators BAPTA-AM or EGTA, as well as the Ca2+ influx inhibitor gadolinium, resulted in a partial decrease of the ERK response. Furthermore, the calmodulin antagonists W-7, trifluoperazine, and J-8 markedly decreased ERK activation. Pretreatment with KN-93, an inhibitor of the multifunctional Ca2+/calmodulin-dependent protein kinase, had no effect on ERK activation. The Src kinase inhibitors PP1 and PP2 partially diminished the ERK responses elicited by both norepinephrine and PGF2alpha.

CONCLUSION

The present data indicate that Ca2+ is involved in ERK activation induced by hormones acting on G protein-coupled receptors in hepatocytes, and suggest that calmodulin and Src kinases might play a role in these signaling pathways.

Evidence that 2-aminoethyl diphenylborate is a novel inhibitor of store-operated Ca2+ channels in liver cells, and acts through a mechanism which does not involve inositol trisphosphate receptors

The compound 2-aminoethyl diphenylborate (2-APB), an inhibitor of Ins(1,4,5)P(3) receptor action in some cell types, has been used to assess the role of Ins(1,4,5)P(3) receptors in the activation of store-operated Ca2+ channels (SOCs) [Ma, Patterson, van Rossum, Birnbaumer, Mikoshiba and Gill (2000) Science 287, 1647-1651]. In freshly-isolated rat hepatocytes, 2-APB inhibited thapsigargin- and vasopressin-stimulated Ca2+ inflow (measured using fura-2) with no detectable effect on the release of Ca2+ from intracellular stores. The concentration of 2-APB which gave half-maximal inhibition of Ca2+ inflow was approx. 10 microM. 2-APB also inhibited Ca2+ inflow initiated by a low concentration of adenophostin A but had no effect on maitotoxin-stimulated Ca2+ inflow through non-selective cation channels. The onset of the inhibitory effect of 2-APB on thapsigargin-stimulated Ca2+ inflow was rapid. When 2-APB was added to rat hepatocytes in the presence of extracellular Ca2+ after a vasopressin-induced plateau in the cytoplasmic free Ca2+ concentration ([Ca2+](cyt)) had been established, the kinetics of the decrease in [Ca2+](cyt) were identical with those induced by the addition of 50 microM Gd(3+) (gadolinium). 2-APB did not inhibit the release of Ca2+ from intracellular stores induced by the addition of Ins(1,4,5)P(3) to permeabilized hepatocytes. In the H4-IIE rat hepatoma cell line, 2-APB inhibited thapsigargin-stimulated Ca2+ inflow (measured using fura-2) and, in whole-cell patch-clamp experiments, the Ins(1,4,5)P(3)-induced inward current carried by Ca2+. It was concluded that, in liver cells, 2-APB inhibited SOCs through a mechanism which involved the binding of 2-APB to either the channel protein or an associated regulatory protein. 2-APB appeared to be a novel inhibitor of SOCs in liver cells with a mechanism of action which, in this cell type, is unlikely to involve an interaction of 2-APB with Ins(1,4,5)P(3) receptors. The need for caution in the use of 2-APB as a probe for the involvement of Ins(1,4,5)P(3) receptors in the activation of SOCs in other cell types is briefly discussed.

Evidence that 2-aminoethyl diphenylborate is a novel inhibitor of store-operated Ca2+ channels in liver cells, and acts through a mechanism which does not involve inositol trisphosphate receptors

The compound 2-aminoethyl diphenylborate (2-APB), an inhibitor of Ins(1,4,5)P(3) receptor action in some cell types, has been used to assess the role of Ins(1,4,5)P(3) receptors in the activation of store-operated Ca2+ channels (SOCs) [Ma, Patterson, van Rossum, Birnbaumer, Mikoshiba and Gill (2000) Science 287, 1647-1651]. In freshly-isolated rat hepatocytes, 2-APB inhibited thapsigargin- and vasopressin-stimulated Ca2+ inflow (measured using fura-2) with no detectable effect on the release of Ca2+ from intracellular stores. The concentration of 2-APB which gave half-maximal inhibition of Ca2+ inflow was approx. 10 microM. 2-APB also inhibited Ca2+ inflow initiated by a low concentration of adenophostin A but had no effect on maitotoxin-stimulated Ca2+ inflow through non-selective cation channels. The onset of the inhibitory effect of 2-APB on thapsigargin-stimulated Ca2+ inflow was rapid. When 2-APB was added to rat hepatocytes in the presence of extracellular Ca2+ after a vasopressin-induced plateau in the cytoplasmic free Ca2+ concentration ([Ca2+](cyt)) had been established, the kinetics of the decrease in [Ca2+](cyt) were identical with those induced by the addition of 50 microM Gd(3+) (gadolinium). 2-APB did not inhibit the release of Ca2+ from intracellular stores induced by the addition of Ins(1,4,5)P(3) to permeabilized hepatocytes. In the H4-IIE rat hepatoma cell line, 2-APB inhibited thapsigargin-stimulated Ca2+ inflow (measured using fura-2) and, in whole-cell patch-clamp experiments, the Ins(1,4,5)P(3)-induced inward current carried by Ca2+. It was concluded that, in liver cells, 2-APB inhibited SOCs through a mechanism which involved the binding of 2-APB to either the channel protein or an associated regulatory protein. 2-APB appeared to be a novel inhibitor of SOCs in liver cells with a mechanism of action which, in this cell type, is unlikely to involve an interaction of 2-APB with Ins(1,4,5)P(3) receptors. The need for caution in the use of 2-APB as a probe for the involvement of Ins(1,4,5)P(3) receptors in the activation of SOCs in other cell types is briefly discussed.

Signaling pathways underlying muscarinic receptor-induced [Ca2+]i oscillations in HEK293 cells

We have investigated the signaling pathways underlying muscarinic receptor-induced calcium oscillations in human embryonic kidney (HEK293) cells. Activation of muscarinic receptors with a maximal concentration of carbachol (100 microm) induced a biphasic rise in cytoplasmic calcium ([Ca2+]i) comprised of release of Ca2+ from intracellular stores and influx of Ca2+ from the extracellular space. A lower concentration of carbachol (5 microm) induced repetitive [Ca2+]i spikes or oscillations, the continuation of which was dependent on extracellular Ca2+. The entry of Ca2+ with 100 microm carbachol and with the sarcoplasmic-endoplasmic reticulum calcium ATPase inhibitor, thapsigargin, was completely blocked by 1 microm Gd3+, as well as 30-100 microm concentrations of the membrane-permeant inositol 1,4,5-trisphosphate receptor inhibitor, 2-aminoethyoxydiphenyl borane (2-APB). Sensitivity to these inhibitors is indicative of capacitative calcium entry. Arachidonic acid, a candidate signal for Ca2+ entry associated with [Ca2+]i oscillations in HEK293 cells, induced entry that was inhibited only by much higher concentrations of Gd3+ and was unaffected by 100 microm 2-APB. Like arachidonic acid-induced entry, the entry associated with [Ca2)]i oscillations was insensitive to inhibition by Gd3+ but was completely blocked by 100 microm 2-APB. These findings indicate that the signaling pathway responsible for the Ca2+) entry driving [Ca2+]i oscillations in HEK293 cells is more complex than originally thought, and may involve neither capacitative calcium entry nor a role for PLA2 and arachidonic acid.

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.

Effect of Gd3+ on bradykinin-induced catecholamine secretion from bovine adrenal chromaffin cells

1. The effects of Gd3+ on bradykinin- (BK-) induced catecholamine secretion, 45Ca2+ efflux and cytosolic [Ca2+] were studied using bovine adrenal chromaffin cells. 2. BK increased secretion in a Ca2+-dependent manner. From 1 - 100 microM, Gd3+ progressively inhibited secretion induced by 30 nM BK to near-basal levels, however from 0.3 - 3 mM Gd3+ dramatically enhanced BK-induced secretion to above control levels. Gd3+ also increased basal catecholamine secretion by 2 - 3 fold at 1 mM. These effects were mimicked by Eu3+ and La3+. 3. Gd3+ enhanced secretion induced by other agonists that mobilize intracellular Ca2+ stores, but simply blocked the response to K+. 4. Gd3+ still enhanced basal and BK-induced secretion in Ca2+-free solution or in the presence of 30 microM SKF96365, however both effects of Gd3+ were abolished after depleting intracellular Ca2+ stores. 5. Gd3+ (1 mM) reduced the rate of basal 45Ca2+ efflux by 57%. In Ca2+-free buffer, BK transiently increased cytosolic [Ca2+] measured with Fura-2. The [Ca2+] response to BK was substantially prolonged in the presence of Gd3+ (1 mM). 6. The results suggest that Gd3+ greatly enhances the efficacy of Ca2+ released from intracellular stores in evoking catecholamine secretion, by inhibiting Ca2+ extrusion from the cytosol. This suggests that intracellular Ca2+ stores are fully competent to support secretion in chromaffin cells to levels comparable to those evoked by extracellular Ca2+ entry. Drugs that modify Ca2+ extrusion from the cell, such as lanthanide ions, will be useful in investigating the mechanisms by which intracellular Ca2+-store mobilization couples to Ca2+-dependent exocytosis.

SK&F 96365 inhibits intracellular Ca2+ pumps and raises cytosolic Ca2+ concentration without production of nitric oxide and von Willebrand factor

The effects of the imidazole compound SK&F 96365 on Ca2+ movements and production of nitric oxide (NO) and von Willebrand factor (vWF) have been investigated in human endothelial cells. Changes in cytosolic Ca2+ concentration ([Ca2+]i) were measured with Fura-2. Real-time production of NO was monitored with a porphyrinic microsensor and the release of vWF with an enzyme-linked immunosorbent assay. Irrespective of the transmembrane Ca2+ gradient, 30 microM SK&F 96365 doubled [Ca2+]i suggesting a Ca2+ release from intracellular stores. The SK&F 96365-induced [Ca2+]i rise was not accompanied by detectable NO and vWF production, while 1 microM thapsigargin enhanced [Ca2+]i 2.5 times, doubled the secretion of vWF and increased the NO production to 10 +/- 4 nM (n = 5). Pretreatment with SK&F 96365 prevented thapsigargin from increasing [Ca2+]i, NO production and vWF secretion. To investigate the mechanism by which SK&F 96365 released Ca2+ from internal pools, its effect and that of thapsigargin on the ATP-dependent 45Ca2+ uptake into platelet membrane vesicles were compared. SK&F 96365 as thapsigargin, dose-dependently reduced the initial rate of 45Ca2+ uptake. In conclusion, we demonstrate that, in the absence of Ca2+ entry from the extracellular space, the [Ca2+]i increase elicited by SK&F 96365 or thapsigargin is not sufficient to initiate NO synthesis and vWF secretion. This confirms the important role of Ca2+ influx in endothelial secretion processes.

Store-activated Ca2+ inflow in Xenopus laevis oocytes: inhibition by primaquine and evaluation of the role of membrane fusion

The role of membrane fusion in the activation of store-activated Ca2+ channels (SACCs) in the plasma membrane of Xenopus laevis oocytes was investigated with primaquine, an inhibitor of vesicle trafficking, reagents that disrupt the cytoskeleton, and reagents that activate or inhibit the functions of monomeric and trimeric GTP-binding regulatory proteins. Ca2+ inflow was assessed by measuring the rate of increase in the fluorescence of the intracellular Ca2+ chelator fluo-3 after the addition of extracellular Ca2+ to oocytes previously incubated in the absence of added Ca2+. Primaquine inhibited the 3-deoxy-3-fluoro Ins(1,4,5)P3 (Ins(1,4,5)P3F)-stimulated increase in Ca2+O,-induced fluo-3 fluorescence with no detectable effect on the release of Ca2+ from intracellular stores. The effect of primaquine was observed within 1.5 min, showed similarity to the inhibition induced by Gd3+, was reversible, and was observed when primaquine was added either before or after activation of the SACCs. The degree of inhibition of Ca2+ inflow by primaquine was halved when the extracellular concentration of Ca2+ was increased from 3.1 to 12.5 mM. Primaquine also inhibited Ca2+ inflow through cholera toxin-activated divalent cation channels and Drosophila Trpl channels (expressed in oocytes after injection of trp1 cRNA). These results indicate that primaquine inhibits open SACCs, possibly by directly inhibiting Ca2+ flow through the channel pore. Colchicine plus cytochalasin B, Brefeldin A, the peptide Arf-1 (2-17) (introduced by microinjection), lovastatin or pertussis toxin did not inhibit the Ins(1,4,5)P3F stimulated increase in fluo-3 fluorescence. In contrast, guanosine 5'-[gamma-thio]triphosphate (GTP[S]), guanosine 5'-[beta, gamma-imido]triphosphate (p[NH]ppG) and A1F4-, but not guanosine 5'-[beta-thio]diphosphate, inhibited the Ins(1,4,5)P3F-stimulated increase in fluo-3 fluorescence. Co-administration of GTP did not prevent the inhibition by GTP[S] of FA1F4-. Staurosporine largely prevented the inhibition of store-activated Ca2+ inflow by GTP[S]. It is concluded that membrane fusion processes are unlikely to be involved in the link between the release of Ca2+ from the endoplasmic reticulum and activation of SACCs. The idea that this link is achieved by direct interaction of a protein(s) in the endoplasmic reticulum membrane with the SACC protein is briefly discussed.

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.

SK & F 96365 inhibits carbachol-induced phosphoinositide turnover in human neuroblastoma SH-SY5Y and rat cerebellar granule cells

SK & F 96365, a receptor-mediated Ca2+ entry inhibitor, has been reported to inhibit Ca2+ responses to various agonists without affecting internal Ca2+ release and phosphoinositide turnover. Since muscarinic acetylcholine receptor-mediated phosphoinositide turnover shows a marked dependence on factors affecting cytosolic Ca2+ concentration, the effects of SK & F 96365 on the coupling of muscarinic receptors to the phosphoinositide hydrolysis were examined in human neuroblastoma SH-SY5Y and rat cerebellar granule cells. SK & F 96365 concentration-dependently (3-30 microM) inhibited the inositol phosphate formation elicited by carbachol in both cellular systems. Moreover, SK & F 96365 inhibited the carbachol-induced inositol phosphate formation in the absence of extracellular Ca2+, and similar extent of inhibition was achieved in the presence or absence of extracellular Ca2+. In ligand binding studies, we found that the binding affinities for [3H] N-methyl-scopolamine in both cells were attenuated by SK & F 96365 (3-30 microM), while Bmax values for the ligand were not changed. The competition curves of SK & F 96365 showed a Ki value of 28.4 uM in SH-SY5Y cells. The results indicated that the decrease of carbachol-stimulated phosphoinositide hydrolysis by SK & F 96365 is due to the competitive inhibition of agonist binding to the M3 muscarinic receptors.

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.

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.

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.