Role of cyclic GMP in the control of capacitative Ca2+ entry in rat pancreatic acinar cells

The functions of store-operated calcium channels

Store-operated calcium channels provide calcium signals to the cytoplasm of a wide variety of cell types. The basic components of this signaling mechanism include a mechanism for discharging Ca²⁺ stores (commonly but not exclusively phospholipase C and inositol 1,4,5-trisphosphate), a sensor in the endoplasmic reticulum that also serves as an activator of the plasma membrane channel (STIM1 and STIM2), and the store-operated channel (Orai1, 2 or 3). The advent of mice genetically altered to reduce store-operated calcium entry globally or in specific cell types has provided important tools to understand the functions of these widely encountered channels in specific and clinically important physiological systems. This review briefly discusses the history and cellular properties of store-operated calcium channels, and summarizes selected studies of their physiological functions in specific physiological or pathological contexts. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

The role of nitric oxide in the physiology and pathophysiology of the exocrine pancreas

SIGNIFICANCE

Nitric oxide (NO), a ubiquitous gaseous signaling molecule, contributes to both pancreatic physiology and pathophysiology.

RECENT ADVANCES

The present review provides a general overview of NO synthesis, signaling, and function. Further, it specifically discusses NO metabolism and its effects in the exocrine pancreas and focuses on the role of NO in the pathogenesis of acute pancreatitis and pancreatic ischemia/reperfusion injury.

CRITICAL ISSUES

Unfortunately, the role of NO in pancreatic physiology and pathophysiology remains controversial in numerous areas. Many questions regarding the messenger molecule still remain unanswered.

FUTURE DIRECTIONS

Probably the least is known about the downstream targets of NO, which need to be identified, especially at the molecular level.

Protein kinases modulate store-operated channels in pulmonary artery smooth muscle cells

Background

This study investigates whether protein kinase G (PKG), protein kinase A (PKA) and protein kinase C (PKC) are involved in the regulatory mechanisms of store-operated channel (SOC) in pulmonary arteries.

Methods

Pulmonary artery smooth muscle cells (PASMCs) were enzymatically dissociated from rat intralobar pulmonary arteries. Whole cell, cell-attached and inside-out patch-clamp electrophysiology were used to monitor SOCs in isolated PASMCs.

Results

Initially the Ca²⁺-ATPase inhibitor cyclopiazonic acid (CPA, 10 μM) initiated a whole cell current that was reduced by the SOC blocker SKF-96365 (10 μM). Subsequent work using both cell-attached and whole cell configurations revealed that the PKG and PKA inhibitors, KT5823 (3 μM) and H-89 (10 μM), also stimulated SOC activity; this augmentation was attenuated by the SOC blockers SKF-96365 (10 μM) and Ni²⁺ (0.1 mM). Finally using the inside-out configuration, the PKC activator phorbol 12-myristate 13-acetate (PMA, 10 μM) was confirmed to modestly stimulate SOC activity although this augmentation appeared to be more substantial following the application of 10 μM inositol 1,4,5-triphosphate (Ins(1,4,5)P₃).

Conclusions

SOC activity in PASMCs was stimulated by the inhibition of PKG and PKA and the activation of PKC. Our findings suggest that the SOC could be a substrate of these protein kinases, which therefore would regulate the intracellular concentration of calcium and pulmonary arteriopathy via SOC.

ATP depletion induces translocation of STIM1 to puncta and formation of STIM1-ORAI1 clusters: translocation and re-translocation of STIM1 does not require ATP

Depletion of the endoplasmic reticulum (ER) calcium store triggers translocation of stromal interacting molecule one (STIM1) to the sub-plasmalemmal region and formation of puncta—structures in which STIM1 interacts and activates calcium channels. ATP depletion induced the formation of STIM1 puncta in PANC1, RAMA37, and HeLa cells. The sequence of events triggered by inhibition of ATP production included a rapid decline of ATP, depletion of phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) and a slow calcium leak from the ER followed by formation of STIM1 puncta. STIM1 puncta induced by ATP depletion were co-localized with clusters of ORAI1 channels. STIM1–ORAI1 clusters that developed as a result of ATP depletion were very poor mediators of Ca²⁺ influx. Re-translocation of STIM1 from puncta back to the ER was observed during total ATP depletion. We can therefore conclude that STIM1 translocation and re-translocation as well as formation of STIM1–ORAI1 clusters occur in an ATP-independent fashion and under conditions of PI(4,5)P₂ depletion.

Pharmacological profile of store-operated Ca(2+) entry in intrapulmonary artery smooth muscle cells

Store-operated Ca(2+) entry (SOCE) plays an important role in the contraction and proliferation of pulmonary artery smooth muscle cells (PASMCs). The aim of this study was to characterise the pharmacological properties of the SOCE pathway in freshly isolated PASMCs from rat lung and to determine whether this Ca(2+) entry pathway is sensitive to nitric oxide donor drugs. Following depletion of Ca(2+) from the sarcoplasmic reticulum, by treating cells with thapsigargin, re-addition of Ca(2+) produced an increase in cytosolic fluo-4 fluorescence that was sustained for the period that extracellular Ca(2+) was present. Thapsigargin also increased the rate of quench of fura-2 fluorescence, confirming that SOCE was activated. The SOCE pathway was not affected by nifedipine or verapamil; however, it was inhibited by the divalent cations Ni(2+) (10 microM) and Cd(2+) (10 microM) by 47+/-5% and 49+/-5% respectively. SOCE was also inhibited 42+/-5% by 2-aminoethoxydiphenyl borate (2-APB; 75 microM) and 58+/-4% by Gd(3+) (10 microM), although La(3+) (100 microM) had little effect. None of the NO donors examined, including sodium nitroprusside, glyceryl trinitrate, and 2-(N,N-diethylamino)-diazenolate-2-oxide had any effect on SOCE. Thus, the pulmonary vasorelaxation produced by NO does not involve direct inhibition of SOCE in PASMCs. Western blot and immunocytochemistry using antibodies directed against specific TRPC subunits detected the presence of TRPC1, 3, and 6 in pulmonary artery and the pharmacological profile of SOCE in PASMCs favours a role for TRPC1 in mediating the underlying channels that are activated by store depletion.

Calcium-dependent release of NO from intracellular S-nitrosothiols

The paper describes a novel cellular mechanism for rapid calcium-dependent nitric oxide (NO) release. This release occurs due to NO liberation from S-nitrosothiols. We have analysed the changes of NO concentration in acutely isolated pancreatic acinar cells. Supramaximal acetylcholine (ACh) stimulation induced a Ca(2+)-dependent increase in the fluorescence in the majority of cells loaded with the NO probe DAF-FM via a patch pipette. The ACh-induced NO signals were insensitive to inhibitors of calmodulin and protein kinase C but were inhibited by calpain antagonists. The initial part of the NO signals induced by 10 muM ACh showed little sensitivity to inhibition of NO synthase (NOS); however, cell pretreatment with NO donors (increasing cellular S-nitrosothiol contents) substantially enhanced the initial component of NO responses. Pancreatic acinar cells were able to generate fast calcium-dependent NO responses when stimulated with physiological or supramaximal doses of secretagogues. Importantly, the source of this NO is the already available S-nitrosothiol store rather than de novo synthesis by NOS. A similar mechanism of NO release was found in dorsal root ganglia neurons.

Free radicals and the pancreatic acinar cells: role in physiology and pathology

Reactive oxygen and nitrogen species (ROS and RNS) play an important role in signal transduction and cell injury processes. Nitric oxide synthase (NOS)-the key enzyme producing nitric oxide (NO)-is found in neuronal structures, vascular endothelium and, possibly, in acinar and ductal epithelial cells in the pancreas. NO is known to regulate cell homeostasis, and its effects on the acinar cells are reviewed here. ROS are implicated in the early events within the acinar cells, leading to the development of acute pancreatitis. The available data on ROS/RNS involvement in the apoptotic and necrotic death of pancreatic acinar cells will be discussed.

Store-Operated Calcium Channels

In electrically nonexcitable cells, Ca2+influx is essential for regulating a host of kinetically distinct processes involving exocytosis, enzyme control, gene regulation, cell growth and proliferation, and apoptosis. The major Ca2+entry pathway in these cells is the store-operated one, in which the emptying of intracellular Ca2+stores activates Ca2+influx (store-operated Ca2+entry, or capacitative Ca2+entry). Several biophysically distinct store-operated currents have been reported, but the best characterized is the Ca2+release-activated Ca2+current, ICRAC. Although it was initially considered to function only in nonexcitable cells, growing evidence now points towards a central role for ICRAC-like currents in excitable cells too. In spite of intense research, the signal that relays the store Ca2+content to CRAC channels in the plasma membrane, as well as the molecular identity of the Ca2+sensor within the stores, remains elusive. Resolution of these issues would be greatly helped by the identification of the CRAC channel gene. In some systems, evidence suggests that store-operated channels might be related to TRP homologs, although no consensus has yet been reached. Better understood are mechanisms that inactivate store-operated entry and hence control the overall duration of Ca2+entry. Recent work has revealed a central role for mitochondria in the regulation of ICRAC, and this is particularly prominent under physiological conditions. ICRACtherefore represents a dynamic interplay between endoplasmic reticulum, mitochondria, and plasma membrane. In this review, we describe the key electrophysiological features of ICRACand other store-operated Ca2+currents and how they are regulated, and we consider recent advances that have shed insight into the molecular mechanisms involved in this ubiquitous and vital Ca2+entry pathway.

Distribution of nitric oxide synthase and secretory role of exogenous nitric oxide in the isolated rat pancreas

BACKGROUND

Pancreatic production and in vivo effects of nitric oxide (NO) have been shown by several studies. In order to examine the direct actions of the NO donor sodium nitroprusside (SNP), this study used in vitro specimens of the rat pancreas where the distribution of neuronal nitric oxide synthase (NOS) and the secretory effects of SNP and the cyclic GMP (cGMP) analog 8-bromo cyclic GMP (8-Br cGMP) were investigated.

METHODS

NO containing pancreatic nerves were visualized by NOS immunohistochemistry. Basal and stimulated amylase output from rat pancreatic segments was measured by an on-line fluorimetric method. Stimulation was achieved by either acetylcholine (ACh) or electrical field stimulation (EFS). Intracellular free calcium concentration ([Ca2+]i) was measured in dispersed pancreatic acinar cells.

RESULTS

NOS containing nerves were demonstrated in the vicinity of pancreatic acini and blood vessels. SNP and 8-Br cGMP inhibited both basal and EFS evoked amylase output but failed to inhibit ACh induced amylase output. Basal [Ca2+]i was decreased by both SNP and 8-Br cGMP but neither SNP nor 8-Br cGMP influenced the ACh evoked increase in [Ca2+]i.

CONCLUSION

NO is well distributed in the rat exocrine pancreas. Exogenous nitric oxide may have a dual action in the isolated rat pancreas: Inhibition of basal amylase secretion in acinar cells and inhibition of ACh release from intrinsic nerve terminals. Both effects seem to be calcium dependent and possibly mediated by cGMP.

The TRP family of cation channels: probing and advancing the concepts on receptor-activated calcium entry

Stimulation of membrane receptors linked to a phospholipase C and the subsequent production of the second messengers diacylglycerol and inositol-1,4,5-trisphosphate (InsP(3)) is a signaling pathway of fundamental importance in eukaryotic cells. Signaling downstream of these initial steps involves mobilization of Ca(2+) from intracellular stores and Ca(2+) influx through the plasma membrane. For this influx, several contrasting mechanisms may be responsible but particular relevance is attributed to the induction of Ca(2+) influx as consequence of depletion of intracellular calcium stores. This phenomenon (frequently named store-operated calcium entry, SOCE), in turn, may be brought about by various signals, including soluble cytosolic factors, interaction of proteins of the endoplasmic reticulum with ion channels in the plasma membrane, and a secretion-like coupling involving translocation of channels to the plasma membrane. Experimental approaches to analyze these mechanisms have been considerably advanced by the discovery of mammalian homologs of the Drosophila cation channel transient receptor potential (TRP). Some members of the TRP family can be expressed to Ca(2+)-permeable channels that enable SOCE; other members form channels activated independently of stores. TRP proteins may be an essential part of endogenous Ca(2+) entry channels but so far expression of most TRP cDNAs has not resulted in restitution of channels found in any mammalian cells, suggesting the requirement for further unknown subunits. A major exception is CaT1, a TRP channel demonstrated to provide Ca(2+)-selective, store-operated currents identical to those characterized in several cell types. Ongoing and future research on TRP channels will be crucial to understand the molecular basis of receptor-mediated Ca(2+) entry, with respect to the structure of the entry channels as well as to the mechanisms of its activation and regulation.

Mechanisms and physiological significance of the cholinergic control of pancreatic beta-cell function

Acetylcholine (ACh), the major parasympathetic neurotransmitter, is released by intrapancreatic nerve endings during the preabsorptive and absorptive phases of feeding. In beta-cells, ACh binds to muscarinic M(3) receptors and exerts complex effects, which culminate in an increase of glucose (nutrient)-induced insulin secretion. Activation of PLC generates diacylglycerol. Activation of PLA(2) produces arachidonic acid and lysophosphatidylcholine. These phospholipid-derived messengers, particularly diacylglycerol, activate PKC, thereby increasing the efficiency of free cytosolic Ca(2+) concentration ([Ca(2+)](c)) on exocytosis of insulin granules. IP3, also produced by PLC, causes a rapid elevation of [Ca(2+)](c) by mobilizing Ca(2+) from the endoplasmic reticulum; the resulting fall in Ca(2+) in the organelle produces a small capacitative Ca(2+) entry. ACh also depolarizes the plasma membrane of beta-cells by a Na(+)- dependent mechanism. When the plasma membrane is already depolarized by secretagogues such as glucose, this additional depolarization induces a sustained increase in [Ca(2+)](c). Surprisingly, ACh can also inhibit voltage-dependent Ca(2+) channels and stimulate Ca(2+) efflux when [Ca(2+)](c) is elevated. However, under physiological conditions, the net effect of ACh on [Ca(2+)](c) is always positive. The insulinotropic effect of ACh results from two mechanisms: one involves a rise in [Ca(2+)](c) and the other involves a marked, PKC-mediated increase in the efficiency of Ca(2+) on exocytosis. The paper also discusses the mechanisms explaining the glucose dependence of the effects of ACh on insulin release.

Inhibition of capacitative calcium entry is not obligatory for relaxation of the mouse anococcygeus by the NO/cyclic GMP signalling pathway

1. The object of this study was to determine whether inhibition of capacitative calcium entry is essential for relaxation of the mouse anococcygeus via the NO/cyclic GMP signalling pathway. 2. In intact muscles, thapsigargin (Tg; 100 nM)-induced tone was relaxed by NO, sodium nitroprusside (SNP), 8-Br-cyclic GMP, and nitrergic field stimulation. The relaxations were similar in magnitude to those observed against carbachol (50 microM) tone and, with the exception of those to 8-Br-cyclic GMP, were reduced by the soluble guanylyl cyclase inhibitor 1H-[1,2,4]oxodiazolo[4,3-a]quinoxalin-1-one (ODQ, 5 microM). 3. In single smooth muscle cells, loaded with Fura-2, both carbachol and Tg produced sustained elevations in cytoplasmic calcium levels ([Ca2+]i). SNP inhibited the rise in [Ca2+]i produced by carbachol, an effect attenuated by ODQ. In contrast, neither SNP nor 8-Br-cyclic GMP reduced the elevated [Ca2+]i associated with Tg. 4. In beta-escin skinned preparations, NO had no effect on tone induced by calcium (1 microM in the presence of 100 microM GTP). Carbachol and Tg produced further increases in calcium/GTP-induced tone and, in both cases, this additional tone was relaxed by NO and 8-Br-cyclic GMP. 5. The results support the hypothesis that the NO/cyclic GMP pathway inhibits capacitative calcium entry by refilling the internal stores, since reduction in [Ca2+]i was not observed in the presence of Tg. However, as muscle relaxation was still observed, impairment of capacitative calcium entry cannot be considered obligatory for relaxation. Results from skinned tissues suggest that inhibition of calcium sensitization processes, perhaps associated with store-depletion, may be an important mechanism of NO/cyclic GMP-induced relaxation.

D609-phosphatidylcholine-specific phospholipase C inhibitor attenuates thapsigargin-induced sodium influx in human lymphocytes

Previously, we reported that the phosphatidylcholine-specific phospholipase C (PC-PLC) inhibitor tricyclodecan-9-yl xanthogenate (D609) potentiates thapsigargin-induced Ca(2+) influx in human lymphocytes. In the present study we examined the effect of D609 on the thapsigargin-induced Na(+) entry. We found that the early phase of the thapsigargin-induced increase in the intracellular Na(+) concentration (approx. 1-2 min after stimulation) was attenuated after preincubation of lymphocytes with D609. By contrast, thapsigargin-induced Na(+) influx was not affected in the presence butan-1-ol, which inhibits phosphatidylcholine-specific phospholipase D (PC-PLD). The thapsigargin-induced Na(+) influx could be mimicked by PC-PLC exogenously added to the lymphocyte suspension, whereas addition of PC-PLD had no effect. In addition, thapsigargin stimulated formation of the physiological PC-PLC products, diacylglycerol. Cell-permeable diacylglycerol analogue, dioctanoyl-glycerol (DOG), produced time- and concentration-dependent increase in the intracellular Na(+) concentration. Both thapsigargin- and DOG-induced Na(+) increases were not affected in the presence of Na(+)/H(+) antiport inhibitor, HOE609, or Na(+)/Ca(2+) antiport inhibitor, dimethylthiourea, as well as in the presence of Co(2+) and Ni(2+), which block store-operated Ca(2+) entry. By contrast, markedly reduced thapsigargin- and DOG-induced Na(+) influx were noted in the presence of flufenamic acid, which blocks the non-selective cation current (I(CRANC)). In conclusion, our results suggest that diacylglycerol released due to the PC-PLC activation contributes to the thapsigargin-induced Na(+) entry.

Phospholipase A(2) is involved in thapsigargin-induced sodium influx in human lymphocytes

Previously, we reported that emptying of intracellular Ca(2+) pools with endoplasmatic Ca(2+)-ATP-ase inhibitor thapsigargin leads to the Na(+) influx in human lymphocytes (M. Tepel et al., 1994, J. Biol. Chem. 269, 26239-26242). In the present study we examined the mechanism underlying the thapsigargin-induced Na(+) entry. We found that the thapsigargin-induced increase in Na(+) concentration was effectively inhibited by three structurally unrelated phospholipase A(2) (PLA(2)) inhibitors, p-bromophenacyl bromide, 3-(4-octadecyl)-benzoylacrylic acid (OBAA), and bromoenol lactone (BEL). The thapsigargin-induced Na(+) influx could be mimicked by PLA(2) exogenously added to the lymphocyte suspension. In addition, thapsigargin stimulated formation of arachidonic acid (AA), the physiological PLA(2) product. AA induced Na(+) entry in a time- and concentration-dependent fashion. Both, thapsigargin-induced Na(+) influx and AA liberation were completely inhibited in the presence of tyrosine kinase inhibitor genistein but not in the absence of extracellular Ca(2+). Collectively, these data show that thapsigargin-induced Na(+) entry is associated with tyrosine kinase-dependent stimulation of PLA(2).

Endoplasmic reticulum Ca(2+)-ATPase inhibitors stimulate membrane guanylate cyclase in pancreatic acinar cells

In this study, we show that particulate guanylate cyclase (GC) is present in rat pancreatic acinar cells and is located both on plasma membrane and membranes of endoplasmic reticulum (ER). Western blot analysis indicates that the enzyme isoform GC-A is present in the acinar cell membranes. The specific inhibitors of ER Ca(2+)-ATPase thapsigargin, 2,5-di-(t-butyl)-1,4-hydroquinone (BHQ), and cyclopiazonic acid all activated particulate GC in pancreatic acini, both in membrane fractions and intact cells. These inhibitors also induced dephosphorylation of GC. Dose dependencies of Ca(2+)-ATPase inhibition and GC activation by BHQ are very similar, and those for thapsigargin partially overlap. ER Ca(2+)-ATPase and GC are coimmunoprecipitated both by antisera against membrane GC and by antisera against ER Ca(2+)-ATPase, suggesting a physical association between the two enzymes. The results suggest that thapsigargin and the other inhibitors act through ER Ca(2+)-ATPase to activate membrane GC in pancreatic acinar cells, although their direct effect on GC cannot be excluded.

Nitric oxide acts independently of cGMP to modulate capacitative Ca(2+) entry in mouse parotid acini

Carbachol- and thapsigargin-induced changes in cGMP accumulation were highly dependent on extracellular Ca(2+) in mouse parotid acini. Inhibition of nitric oxide synthase (NOS) and soluble guanylate cyclase (sGC) resulted in complete inhibition of agonist-induced cGMP levels. NOS inhibitors reduced agonist-induced Ca(2+) release and capacitative Ca(2+) entry, whereas the inhibition of sGC had no effect. The effects of NOS inhibition were not reversed by 8-bromo-cGMP. The NO donor GEA-3162 increased cGMP levels blocked by the inhibition of sGC. GEA-3162-induced increases in Ca(2+) release from ryanodine-sensitive stores and enhanced capacitative Ca(2+) entry, both of which were unaffected by inhibitors of sGC but reduced by NOS inhibitors. Results support a role for NO, independent of cGMP, in agonist-mediated Ca(2+) release and Ca(2+) entry. Data suggest that agonist-induced Ca(2+) influx activates a Ca(2+)-dependent NOS, leading to the production of NO and the release of Ca(2+) from ryanodine-sensitive stores, providing a feedback loop by which store-depleted Ca(2+) channels are activated.

Ca2+ pool emptying stimulates Ca2+ entry activated by S-nitrosylation

The entry of Ca2+ following Ca2+ pool release is a major component of Ca2+ signals; yet despite intense study, how "store-operated" entry channels are activated is unresolved. Because S-nitrosylation has become recognized as an important regulatory modification of several key channel proteins, its role in Ca2+ entry was investigated. A novel class of lipophilic NO donors activated Ca2+ entry independent of the well defined NO target, guanylate cyclase. Strikingly similar entry of Ca2+ induced by cell permeant alkylators indicated that this Ca2+ entry process was activated through thiol modification. Significantly, Ca2+ entry activated by either NO donors or alkylators was highly stimulated by Ca2+ pool depletion, which increased both the rate of Ca2+ release and the sensitivity to thiol modifiers. The results indicate that S-nitrosylation underlies activation of an important store-operated Ca2+ entry mechanism.

Capacitative Ca2+ entry in enteric glia induced by thapsigargin and extracellular ATP

Mobilization of intracellular Ca2+ stores is coupled to Ca2+ influx across the plasma membrane, a process termed capacitative Ca2+ entry. Capacitative Ca2+ entry was examined in cultured guinea pig enteric glia exposed to 100 microM ATP, an inositol trisphosphate-mediated Ca2+-mobilizing agonist, and to 1 microM thapsigargin, an inhibitor of microsomal Ca2+ ATPase. Both agents caused mobilization of intracellular Ca2+ stores followed by influx of extracellular Ca2+. This capacitative Ca2+ influx was inhibited by Ni2+ (88 +/- 1%) and by La3+ (87 +/- 1%) but was not affected by L- or N-type Ca2+ channel blockers. Pretreatment of glia with 100 nM phorbol 12-myristate 13-acetate for 24 h decreased capacitative Ca2+ entry by 48 +/- 2%. Chelerythrine (0.1-10 microM), a specific antagonist of protein kinase C (PKC), dose dependently inhibited capacitative Ca2+ entry. The nitric oxide synthase inhibitor NG-nitro-L-arginine (1 mM) decreased Ca2+ influx by 42 +/- 1%. Capacitative Ca2+ entry was inhibited to a similar degree by the guanylate cyclase inhibitor (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one). Capacitative Ca2+ entry occurs in enteric glial cells via lanthanum-inhibitable channels through a process regulated by PKC and nitric oxide.

Receptor-operated calcium influx mediated by protein tyrosine kinase pathways

Calcium influx from the extracellular space elicited by activation of heterotrimeric G protein-coupled and heptahelical receptors plays a critical role in transmembrane signal transduction in a wide variety of cell systems. In nonexcitable cells, the precise voltage-independent mechanism by which calcium enters the cell remains unknown. Multiple mechanisms appear to be operating in different cell types (1-3): 1. G protein-operated calcium influx, 2. Second messenger-operated calcium influx, 3. Capacitative calcium influx, and 4. Phosphorylation of calcium channels. Receptor-operated calcium channels have a fundamental role in stimulus-secretion coupling in many different cells, but these channels remain to be purified and cloned. This review proposes that receptor-operated calcium influx is mediated by protein tyrosine kinase pathways. The function of protein tyrosine kinase pathways and their interactions with other receptor-operated calcium influx mechanisms are described.

nNOS and Ca2+ influx in rat pancreatic acinar and submandibular salivary gland cells

Regulation of agonist-activated Ca2+ influx by the NOS pathway through generation of cGMP is being found in an increasing number of cell types. In the present work, we examined the role of the NOS pathway in agonist-evoked [Ca2+]i oscillations and attempted to identify the NOS isoform most likely to regulate Ca2+ influx. For this, we first show that two Ca(2+)-mobilizing agonists acting on pancreatic acinar cells, bombesin (BS) and the cholecystokinin analog CCK-JMV-180 (CCKJ), evokes different type of [Ca2+]i oscillations. The BS-evoked [Ca2+]i oscillations rapidly became acutely dependent on the presence of extracellular Ca2+, whereas the CCKJ-evoked oscillations continue for long periods of time in the absence of Ca2+ influx. This differential behavior allowed us to isolate Ca2+ influx and study its regulation while controlling for non specific effects on all other Ca2+ transporting events involved in generating [Ca2+]i oscillations. Inhibitors of selective steps in the NOS pathway inhibited agonist-induced cGMP production. The inhibitors were then used to show that scavenging NO with reduced hemoglobin, inhibition of guanylyl cyclase with 1H-[1,2,4] oxadiazolo[4,3-a] quinoxaline-1-one (ODQ) and inhibition of protein kinase G with Rp-8-pCPT-cGMPS inhibited [Ca2+]i oscillations evoked by BS but not those evoked by CCKJ. These findings were extended to duct and acinar cells of the SMG. In these cells, Ca(2+)-mobilizing agonists stimulate large Ca2+ influx, which was inhibited by all inhibitors of the NOS pathway. Western blot analysis and immunolocalization revealed that the cells did not express iNOS, eNOS was expressed only in blood vessels and capillaries whereas nNOS was expressed at high levels next to the plasma membrane of all cells. Accordingly, the nNOS inhibitor 7-nitroindazole (7-NI) inhibited BS- but not CCKJ-evoked [Ca2+]i oscillations and Ca2+ influx into SMG acinar and duct cells. Thus, together, our findings favor nNOS as the isoform activated by the Ca2+ released from internal stores to generate cGMP and regulate Ca2+ influx.

Emptying of intracellular Ca2+ stores stimulates Ca2+ entry in mouse pancreatic beta-cells by both direct and indirect mechanisms

1. In non-excitable cells, the depletion of intracellular Ca2+ stores triggers Ca2+ influx by a process called capacitative Ca2+ entry. In the present study, we have investigated how the emptying of these stores by thapsigargin (1 microM) influences Ca2+ influx in electrically excitable pancreatic beta-cells. The cytoplasmic Ca2+ concentration ([Ca2+]i) was monitored in clusters of mouse beta-cells or in whole islets loaded with fura-2. 2. The membrane was first held hyperpolarized by diazoxide, an opener of ATP-sensitive K+ (KATP) channels, in the presence of 4.8 mM K+. Alternating between Ca(2+)-free medium and medium containing 2.5 mM Ca2+ caused a minor rise in [Ca2+]i (approximately 14 nM) in clusters of beta-cells. A larger rise (approximately 65 nM), resistant to the blockade of voltage-dependent Ca2+ channels by D600, occurred when extracellular Ca2+ was readmitted after emptying intracellular Ca2+ stores with thapsigargin or acetylcholine. Thus there exists a small capacitative Ca2+ entry in beta-cells. 3. When the membrane potential was clamped at depolarized levels with 10, 20 or 45 mM K+ in the presence of diazoxide, [Ca2+]i increased to different plateau levels ranging between 100 and 900 nM. Thapsigargin consistently caused a further transient rise in [Ca2+]i, but had little (at 10 mM K+) or no effect on the plateau level. This confirms that the capacitative Ca2+ entry is small. 4. In clusters of cells whose membrane potential was not clamped with diazoxide, 15 mM glucose (in 4.8 mM K+) induced [Ca2+]i oscillations by promoting Ca2+ influx through voltage-dependent Ca2+ channels. The application of thapsigargin accelerated these oscillations and increased their amplitude, sometimes causing a sustained elevation of [Ca2+]i. Similar results were obtained from whole islets perifused with a medium containing > or = 6 mM glucose. The effect of thapsigargin was always much larger than expected from the capacitative Ca2+ entry, probably because of a potentiation of Ca2+ influx through voltage-dependent Ca2+ channels. 5. This potentiating effect of thapsigargin did not result from an acceleration of cell metabolism since the drug did not affect glucose-induced changes in NAD(P)H fluorescence. It is also unlikely to involve the inhibition of KATP channels because thapsigargin steadily elevated [Ca2+]i in cells in which [Ca2+]i oscillations persisted in the presence of a maximally effective concentration of tolbutamide. 6. In conclusion, the emptying of intracellular Ca2+ stores in beta-cells induces a small capacitative Ca2+ entry and activates a depolarizing current which potentiates glucose-induced Ca2+ influx through voltage-dependent Ca2+ channels.

Nitric oxide induces transient Ca2+ changes in endothelial cells independent of cGMP

Ca2+ changes induced by nitric oxide (NO.) were investigated in cultured human endothelial cells. Sodium nitroprusside (SNP) (1-100 mumol/L) and S-Nitroso-N-acetylpenicillamine (SNAP) (100 mumol/L) were used as NO. donors. The cytoplasmatic Ca2+ concentration was calculated using ratiometric FURA2 fluorescence measurements. Both NO. donors caused transient oscillatory Ca2+ changes, which were not detectable in the presence of oxyhemoglobin (50 mumol/L). Digital ratio imaging revealed initiation sites within cells where Ca2+ increases started spreading, which indicates that nonuniformly distributed targets might be involved in these reactions. Calcium was released from intracellular stores as indicated by experiments performed in Ca(2+)-free buffer. L-type Ca(2+)-channel blocker diltiazem (100 mumol/L) was not able to block these responses. NO.-induced Ca2+ release from intracellular stores caused capacitative Ca2+ entry. Both thapsigargin (1 mumol/L) and cyclopiazonic acid (10 mumol/L) inhibited the SNP response completely, whereas neither ryanodine (up to 100 mumol/L) nor dantrolene (100 mumol/L) was able to inhibit Ca2+ changes induced by SNP, indicating that primarily inositol 1,4,5-triphosphate (IP3)-dependent stores are released upon stimulation with NO.. A small inhibitory effect of ATP- and SNP-induced peak [Ca2+]i increase was measured in the presence of both caffeine (20 mmol/L) and procaine (1 mmol/L). Evidence is presented that cGMP is not involved in NO.-induced Ca2+ signals, as neither inhibitors of guanylate cyclase (methylene blue and LY 83583) nor cell permeant analogues of cGMP altered or simulated [Ca2+] changes. An inhibitor of cGMP-dependent protein kinase was also ineffective. We therefore propose that endothelial cells have specific targets proximal or at IP3 receptors to induce Ca2+ changes in endothelial cells stimulated with NO..

On the molecular basis and regulation of cellular capacitative calcium entry: roles for Trp proteins

During the last 2 years, our laboratory has worked on the elucidation of the molecular basis of capacitative calcium entry (CCE) into cells. Specifically, we tested the hypothesis that CCE channels are formed of subunits encoded in genes related to the Drosophila trp gene. The first step in this pursuit was to search for mammalian trp genes. We found not one but six mammalian genes and cloned several of their cDNAs, some in their full length. As assayed in mammalian cells, overexpression of some mammalian Trps increases CCE, while expression of partial trp cDNAs in antisense orientation can interfere with endogenous CCE. These findings provided a firm connection between CCE and mammalian Trps. This article reviews the known forms of CCE and highlights unanswered questions in our understanding of intracellular Ca2+ homeostasis and the physiological roles of CCE.

Cell type-specific modes of feedback regulation of capacitative calcium entry

The Ca2+-ATPase inhibitor, thapsigargin, activated Ca2+ entry into pancreatic acinar cells, a process known as capacitative calcium entry. In cells loaded with the calcium chelator BAPTA, the transient Ca2+ release was blunted and the rise of [Ca2+]i on readdition of Ca2+ was slowed. However, the steady-state [Ca2+]i due to Ca2+ entry was substantially augmented compared with control cells. This indicates that [Ca2+]i exerts a negative feedback on Ca2+ entry from a compartment buffered by BAPTA and separated from the bulk of cytoplasmic Ca2+. This interaction probably occurs close to the calcium channel where [Ca2+] is higher than in the bulk of the cytoplasm. In support of this interpretation, the slower Ca2+ chelator, EGTA, also blunted the release of Ca2+ and slowed the rise of the sustained [Ca2+]i phase but failed to augment steady-state [Ca2+]i. In contrast, Ca2+ entry in NIH 3T3 cells was characterized by a transient rise of [Ca2+]i that decays to near prestimulus levels. This decay in Ca2+ entry also results from negative feedback by Ca2+ because the decrease in Ca2+ entry was reversed by incubation in a Ca2+-deficient medium. However, unlike its effects in acinar cells, BAPTA neither augmented steady-state [Ca2+]i nor prevented the inactivation of entry. Rather, in BAPTA-loaded cells, [Ca2+]i failed to increase substantially suggesting that negative regulation by Ca2+ may occur at a site distinct from the cytoplasmic compartment and inaccessible to cytoplasmic BAPTA. These two distinct types of feedback behavior may indicate subtypes of store-operated calcium channels expressed in different cells or a single type of channel which is differentially regulated in a cell type-specific manner.

Pharmacological and functional properties of voltage-independent Ca2+ channels

During the last few years, considerable progress has taken place in our knowledge of the molecular and functional properties of the various voltage-independent Ca2+ channels. In addition to the ionotropic receptor-channels (ROCs), that are not discussed in the present review, these channels include the SMOCs, activated via second messengers or other transducing processes directly triggered by receptor activation; and the SOCCs, activated as a consequence of depletion of the rapidly exchanging Ca2+ stores in the cytoplasm. In parallel, a pharmacological approach to the study of these channels has been developed, based primarily on heterogeneous drugs already known for different biological effects, and subsequently recognized as voltage-independent Ca(2+)-channel blockers. From the systematic analysis of the effects of these drugs new information has emerged about SMOCs and SOCCs function. In addition, pharmacological blockade of these channels appears to have beneficial therapeutic effects in pathological conditions such as tumoral cell growth, inflammation and immunity. At the moment the field is rapidly evolving, with major developments expected in the years ahead.

cGMP is not required for capacitative Ca2+ entry in Jurkat T-lymphocytes

In this study, we have addressed the potential role of cGMP in regulating calcium entry in Jurkat T-lymphocytes. These cells display capacitative Ca(2+)-entry in response to the intracellular Ca(2+)-ATPase inhibitor, thapsigargin (TG). In the presence of extracellular Ca2+, TG stimulates a sustained elevation of intracellular cGMP levels. In the absence of extracellular Ca2+, TG induces no apparent increase in the levels of cGMP. However, experiments using Mn2+, as a surrogate for Ca2+, demonstrated that TG increased the rate of divalent cation entry in the absence of extracellular Ca2+. Treatment of Jurkat cells with the guanylyl cyclase inhibitor, LY83583 (20 microM), completely blocked cGMP formation in response to TG. However, LY83583 treated cells still exhibited a sustained, albeit partially reduced, Ca2+ response induced by TG. These data demonstrate that, in Jurkat cells, the sustained formation of cGMP is dependent on elevated intracellular Ca2+, and that elevated levels of cGMP are not necessary for the activation of capacitative Ca2+ entry.