Effects of MeCh, thapsigargin, and La3+ on plasmalemmal and intracellular Ca2+ transport in lacrimal acinar cells

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.

Characterization of the calcium signaling system in the submandibular cell line SMG-C6

Establishment of salivary cell lines retaining normal morphological and physiological characteristics is important in the investigation of salivary cell function. A submandibular gland cell line, SMG-C6, has recently been established. In the present study, we characterized the phosphoinositide (PI)-Ca2+ signaling system in this cell line. Inositol 1,4,5-trisphosphate(1,4,5-IP3) formation, as well as Ca2+ storage, release, and influx in response to muscarinic, alpha1-adrenergic, P2Y-nucleotide, and cytokine receptor agonists were determined. Ca2+ release from intracellular stores was strongly stimulated by acetylcholine (ACh) and ATP, but not by norepinephrine (NA), epidermal growth factor (EGF), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNFalpha). Consistently, 1, 4,5-IP3 formation was dramatically stimulated by ACh and ATP. ACh-stimulated cytosolic free Ca2+ concentration [Ca2+]i increase was inhibited by ryanodine, suggesting that the Ca2+-induced Ca2+ release mechanism is involved in the ACh-elicited Ca2+ release process. Furthermore, ACh and ATP partially discharged the IP3-sensitive Ca2+ store, and a subsequent exposure to thapsigargin (TG) induced further [Ca2+]i increase. However, exposure to TG depleted the store and a subsequent stimulation with ACh or ATP did not induce further [Ca2+]i increase, suggesting that ACh and ATP discharge the same storage site sensitive to TG. As in freshly isolated submandibular acinar cells, exposure to ionomycin and monensin following ACh or TG induced further [Ca2+]i increase, suggesting that IP3-insensitive stores exist in SMG-C6 cells. Ca2+ influx was activated by ACh, ATP, or TG, and was significantly inhibited by La3+, suggesting the involvement of store-operated Ca2+ entry (SOCE) pathway. These results indicate that in SMG-C6 cells: (i) Ca2+ release is triggered by muscarinic and P2Y-nucleotide receptor agonists through formation of IP3; (ii) both the IP3-sensitive and -insensitive Ca2+ stores are present; and (iii) Ca2+ influx is mediated by the store-operated Ca2+ entry pathway. We conclude that Ca2+ regulation in SMG-C6 cells is similar to that in freshly isolated SMG acinar cells; therefore, this cell line represents an excellent SMG cell model in terms of intracellular Ca2+ signaling.

An investigation of interactions between the immune system and stimulus-secretion coupling in mouse submandibular acinar cells. A possible mechanism to account for reduced salivary flow rates associated with the onset of Sjögren's syndrome

OBJECTIVES

To determine whether chronic exposure to lymphocyte-derived cytokines could inhibit the fluid secretory mechanism in salivary gland acinar cells and so account for the loss of gland function seen in the early stages of Sjögren's syndrome.

METHODS

Mouse submandibular acinar cells maintained in primary culture were exposed to a profile of cytokines produced by concanavalin A-activated splenic lymphocytes in vitro for periods up to 72 h. Agonist-evoked changes in intracellular Ca(2+) were determined microfluorimetrically in both control and cytokine-treated cells.

RESULTS

Acinar cells maintained in primary culture in the presence of cytokines for up to 72 h were able to mobilize intracellular calcium in response to stimulus by acetylcholine in an identical fashion to those maintained in primary culture in the absence of cytokines. Acute application of the conditioned medium produced by the activated lymphocytes had an antisecretory effect on acetylcholine-evoked Ca(2+) mobilization, which was found to be mediated by cholinesterase rather than by cytokines.

CONCLUSION

Neither chronic nor acute exposure to the profile of cytokines released by concanavalin A-activated splenic lymphocytes interfered in any way with the second messenger cascade and fluid and electrolyte secretion in acinar cells. Our data suggest an alternative hypothesis, in which elevated levels of cholinesterase can metabolize acetylcholine released within the salivary glands and thus prevent fluid secretion.

Regulation of plasma membrane Ca2+-ATPase by small GTPases and phosphoinositides in human platelets

We have investigated the restoration of [Ca(2+)](i) in human platelets following the discharge of the intracellular Ca(2+) stores. We found that the plasma membrane Ca(2+)-ATPase is the main mechanism involved in Ca(2+) extrusion in human platelets. Treatment of platelets with the farnesylcysteine analogs, farnesylthioacetic acid and N-acetyl-S-geranylgeranyl-l-cysteine, inhibitors of activation of Ras proteins, accelerated the rate of decay of [Ca(2+)](i) to basal levels after activation with thapsigargin combined with a low concentration of ionomycin, indicating that Ras proteins are involved in the negative regulation of Ca(2+) extrusion. Rho A, which is involved in actin polymerization, was not responsible for this effect. Consistent with this, the actin polymerization inhibitors, cytochalasin D and latrunculin A, did not alter the recovery of [Ca(2+)](i). Activation of human platelets with thapsigargin and ionomycin stimulated the tyrosine phosphorylation of the plasma membrane Ca(2+)-ATPase, a mechanism that was inhibited by farnesylcysteine analogs, suggesting that Ras proteins could regulate Ca(2+) extrusion by mediating tyrosine phosphorylation of the plasma membrane Ca(2+)-ATPase. Treatment of platelets with LY294002, a specific inhibitor of phosphatidylinositol 3- and phosphatidylinositol 4-kinase, resulted in a reduction in the rate of recovery of [Ca(2+)](i) to basal levels, suggesting that the products of these kinases are involved in stimulating Ca(2+) extrusion in human platelets.

Voltage-independent calcium entry in hypoxic pulmonary vasoconstriction of intrapulmonary arteries of the rat

It has been proposed that hypoxic pulmonary vasoconstriction (HPV) is mediated via K+ channel inhibition and Ca2+ influx through voltage-gated channels. HPV depends strongly on the degree of preconstriction, and we therefore examined the effect of Ca2+ channel blockade on tension and intracellular [Ca2+] ([Ca2+]i) during HPV in rat intrapulmonary arteries (IPAs), whilst maintaining preconstriction constant. We also investigated the role of intracellular Ca2+ stores. HPV demonstrated a transient constriction (phase I) superimposed on a sustained constriction (phase II). Nifedipine (1 microM) partially inhibited phase I, but did not affect phase II. In arteries exposed to 80 mM K+ and nifedipine or diltiazem the rises in tension and [Ca2+]i were blunted during phase I, but were unaffected during phase II. At low concentrations (< 3 microM), La3+ almost abolished the phase I constriction and rise in [Ca2+]i, but had no effect on phase II, or constriction in response to 80 mM K+. Phase II was inhibited by higher concentrations of La3+ (IC50 approximately 50 microM). IPA treated with thapsigargin (1 microM) in Ca2+-free solution to deplete Ca2+ stores showed sustained constriction upon re-exposure to Ca2+ and an increase in the rate of Mn2+ influx, suggesting capacitative Ca2+ entry. The concentration dependency of the block of constriction by La3+ was similar to that for phase I of HPV. Pretreatment of IPA with 30 microM CPA reduced phase I by > 80 %, but had no significant effect on phase II. We conclude that depolarization-mediated Ca2+ influx plays at best a minor role in the transient phase I constriction of HPV, and is not involved in the sustained phase II constriction. Instead, phase I appears to be mainly dependent on capacitative Ca2+ entry related to release of thapsigargin-sensitive Ca2+ stores, whereas phase II is supported by Ca2+ entry via a separate voltage-independent pathway.

Effect of dephostatin on intracellular free calcium concentration and amylase secretion in isolated rat pancreatic acinar cells

This study investigates the effects of dephostatin, a new tyrosine phosphatase inhibitor, on intracellular free calcium concentration ([Ca2+]i) and amylase secretion in collagenase dispersed rat pancreatic acinar cells. Dephostatin evoked a sustained elevation in [Ca2+]i by mobilizing calcium from intracellular calcium stores in either the absence of extracellular calcium or the presence of lanthanium chloride (LaCl3). Pretreatment of acinar cells with dephostatin prevented cholecystokinin-octapeptide (CCK-8)-induced signal of [Ca2+]i and inhibited the oscillatory pattern initiated by aluminium fluoride (AlF4), whereas co-incubation with CCK-8 enhances the plateau phase of calcium response to CCK-8 without modifying the transient calcium spike. The effects of dephostatin on calcium mobilization were reversed by the presence of the sulfhydryl reducing agent, dithiothreitol. Stimulation of acinar cells with thapsigargin in the absence of extracellular Ca2+ resulted in a transient rise in [Ca2+]i. Application of dephostatin in the continuous presence of thapsigargin caused a small but sustained elevation in [Ca2+]i. These results suggest that dephostatin can mobilize Ca2+ from both a thapsigargin-sensitive and thapsigargin-insensitive intracellular stores in pancreatic acinar cells. In addition, dephostatin can stimulate the release of amylase from pancreatic acinar cells and moreover, reduce the secretory response to CCK-8. The results indicate that dephostatin can release calcium from intracellular calcium pools and consequently induces amylase secretion in pancreatic acinar cells. These effects are likely due to the oxidizing effects of this compound.

Capacitative calcium entry in smooth muscle cells from preglomerular vessels

Calcium entry via voltage-gated L-type channels is responsible for at least half of the increase in cytosolic calcium ([Ca(2+)](i)) in afferent arterioles following agonist stimulation. We sought the presence of capacitative calcium entry in fresh vascular smooth muscle cells (VSMC) derived from rat preglomerular vessels. [Ca(2+)](i) was measured using fura-2 ratiometric fluorescence. Vasopressin V1 receptor agonist (V1R) (10(-7) M) increased [Ca(2+)](i) by approximately 100 nM. A calcium channel blocker (CCB), nifedipine or verapamil (10(-7) M), inhibited the response by approximately 50%. V1R in the presence of CCB increased [Ca(2+)](i) from 106 to 176 nM, confirming that calcium mobilization and/or entry may occur independent of voltage-gated channels. In nominally Ca(2+)-free buffer, V1R increased [Ca(2+)](i) from 94 to 129 nM, denoting mobilization; addition of CaCl(2) (1 mM) further elevated [Ca(2+)](i) to 176 nM, indicating a secondary phase of Ca(2+) entry. Similar responses were obtained when CCB was present in calcium-free buffer or when EGTA was present. In nominally Ca(2+)-free medium, the sarcoplasmic reticulum Ca(2+)-ATPase inhibitors (SRCAI), thapsigargin and cyclopiazonic acid (CPA), increased [Ca(2+)](i) from 97 to 128 and 143 nM, respectively, and to 214 and 220 nM, respectively, when 1 mM extracellular Ca(2+) was added. In the presence of verapamil, the results with CPA acid were nearly identical. In Ca(2+)-free buffer, the stimulatory effect of V1R or SRCAI on the Ca(2+)/fura signal was quenched by the addition of Mn(2+) (1 mM), demonstrating divalent cation entry. These studies provide evidence for capacitative (store- operated) calcium entry in VSMC freshly isolated from rat preglomerular arterioles.

Oxidizing effects of vanadate on calcium mobilization and amylase release in rat pancreatic acinar cells

The effects of vanadate were examined by monitoring intracellular free calcium concentration ([Ca2+]i) and amylase secretion in collagenase-dispersed rat pancreatic acinar cells. Vanadate increased [Ca2+]i by mobilizing calcium from agonist-releasable intracellular calcium stores, since this increase was observed in the absence of extracellular calcium and vanadate failed to increase [Ca2+]i after treatment with thapsigargin in calcium-free medium. Moreover, pretreatment of acinar cells with vanadate prevented the cholecystokinin octapeptide (CCK-8)-induced signal of [Ca2+]i, whereas co-incubation with CCK-8 potentiated the plateau phase of calcium response to CCK-8 without modifying the transient calcium spike. The effects of vanadate on calcium mobilization were reversed by the presence of the sulfhydryl reducing agent dithiothreitol. Vanadate also activated the calcium influx, since an additional enhancement of calcium influx induced by thapsigargin-evoked intracellular store depletion was observed and vanadate reversed the inhibitory effect of lanthanum (an inhibitor of calcium entry) into acinar cells. In addition, vanadate evoked a concentration-dependent release of amylase from pancreatic acinar cells and moreover, reduced the secretory response to CCK-8. We conclude that, in pancreatic acinar cells, vanadate releases calcium from the agonist-releasable intracellular calcium pool and consequently induces amylase secretion. These effects are likely due to the oxidizing effects of this compound.

Mouse trp2, the homologue of the human trpc2 pseudogene, encodes mTrp2, a store depletion-activated capacitative Ca2+ entry channel

Capacitative Ca2+ entry (CCE) is Ca2+ entering after stimulation of inositol 1,4,5-trisphosphate (IP3) formation and initiation of Ca2+ store depletion. One hallmark of CCE is that it can also be triggered merely by store depletion, as occurs after inhibition of internal Ca2+ pumps with thapsigargin. Evidence has accumulated in support of a role of transient receptor potential (Trp) proteins as structural subunits of a class of Ca2+-permeable cation channels activated by agonists that stimulate IP3 formation-very likely through a direct interaction between the IP3 receptor and a Trp subunit of the Ca2+ entry channel. The role of Trp's in Ca2+ entry triggered by store depletion alone is less clear. Only a few of the cloned Trp's appear to enhance this type of Ca2+ entry, and when they do, the effect requires special conditions to be observed, which native CCE does not. Here we report the full-length cDNA of mouse trp2, the homologue of the human trp2 pseudogene. Mouse Trp2 is shown to be readily activated not only after stimulation with an agonist but also by store depletion in the absence of an agonist. In contrast to other Trp proteins, Trp2-mediated Ca2+ entry activated by store depletion is seen under the same conditions that reveal endogenous store depletion-activated Ca2+ entry, i.e., classical CCE. The findings support the general hypothesis that Trp proteins are subunits of store- and receptor-operated Ca2+ channels.

Bradykinin elevates cytosolic Ca2+ concentration in smooth muscle cells isolated from rat duodenum

The effect of bradykinin on the cytosolic Ca2+ concentration were measured in single, Fura-2 loaded, smooth muscle cells isolated from rat duodenum. All cells responded with a Ca2+ signal when exposed to bradykinin. The bradykinin response consisted of an initial Ca2+ spike followed by a plateau. Pre-treatment of single muscle cells with either the phospholipase C blocker U-73122 or thapsigargin, which is a potent inhibitor of the endoplasmic reticulum Ca2+-ATPase, inhibited the response to bradykinin. Pre-treatment of the cells with EGTA or La3+ to inhibit the Ca2+ influx, abolished the response induced by bradykinin. We conclude that bradykinin applied to single smooth muscle cells from rat duodenum, increases cytosolic Ca2+ by emptying intracellular Ca2+ stores, and by contribution from extracellular Ca2+. In contrast to bradykinin-induced response in isolated rat duodenum (a relaxation followed by a contraction), we did not observe a biphasic effect of bradykinin on cytosolic Ca2+ in single muscle cells. Bradykinin may thus cause relaxation of duodenal smooth muscle indirectly through an effect on neighbouring cells as dilatation is brought about by this agent in blood vessels.

Bradykinin-induced phosphoinositide hydrolysis and Ca2+ mobilization in canine cultured tracheal epithelial cells

1. Experiments were designed to differentiate the mechanisms and subtype of kinin receptors mediating the changes in intracellular Ca2+ concentration ([Ca2+]i) induced by bradykinin (BK) in canine cultured tracheal epithelial cells (TECs). 2. BK and Lys-BK caused an initial transient peak of [Ca2+]i in a concentration-dependent manner, with half-maximal stimulation (pEC50) obtained at 7.70 and 7.23, respectively. 3. Kinin B2 antagonists Hoe 140 (10 nM) and [D-Arg0, Hyp3, Thi5,8, D-Phe7]-BK (1 microM) had high affinity in antagonizing BK-induced Ca2+ response with pKB values of 8.90 and 6.99, respectively. 4. Pretreatment of TECs with pertussis toxin (100 ng ml(-1)) or cholera toxin (10 microg ml(-1)) for 24 h did not affect the BK-induced IP accumulation and [Ca2+]i changes in TECs. 5. Removal of Ca2+ by the addition of EGTA or application of Ca2+-channel blockers, verapamil, diltiazem, and Ni2+, inhibited the BK-induced IP accumulation and Ca2+ mobilization, indicating that Ca2+ influx was required for the BK-induced responses. 6. Addition of thapsigargin (TG), which is known to deplete intracellular Ca2+ stores, transiently increased [Ca2+]i in Ca2+-free buffer and subsequently induced Ca2+ influx when Ca2+ was re-added to this buffer. Pretreatment of TECs with TG completely abolished BK-induced initial transient [Ca2+]i, but had slight effect on BK-induced Ca2+ influx. 7. Pretreatment of TECs with SKF96365 and U73122 inhibited the BK-induced Ca2+ influx and Ca2+ release, consistent with the inhibition of receptor-gated Ca2+-channels and phospholipase C in TECs, respectively. 8. These results demonstrate that BK directly stimulates kinin B2 receptors and subsequently phospholipase C-mediated IP accumulation and Ca2+ mobilization via a pertussis toxin-insensitive G protein in canine TECs. These results also suggest that BK-induced Ca2+ influx into the cells is not due to depletion of these Ca2+ stores, as prior depletion of these pools by TG has no effect on the BK-induced Ca2+ influx that is dependent on extracellular Ca2+ in TECs.

1alpha,25-dihydroxy-vitamin-D3-induced store-operated Ca2+ influx in skeletal muscle cells. Modulation by phospholipase c, protein kinase c, and tyrosine kinases

In skeletal muscle cells the steroid hormone 1alpha, 25-dihydroxy-vitamin-D3 (1,25(OH)2D3) nongenomically promotes Ca2+ release from intracellular stores and cation influx through both L-type and store-operated Ca2+ (SOC) channels. In the present work we evaluated the regulation and kinetics of the 1, 25(OH)2D3-stimulated SOC influx in chick muscle cells. Stimulation with 10(-9) M 1,25(OH)2D3 in Ca2+-free medium resulted in a rapid (40-60 s) but transient [Ca2+]i rise, which correlated with sterol-dependent inositol 1,4,5-trisphosphate production. The SOC influx stimulated by the hormone was insensitive to both L-type channel antagonists and polyphosphoinositide-specific phospholipase C (PPI-PLC) inhibitors but was fully inhibitable by La3+ and Ni2+. PPI-PLC blockade prior to 1,25(OH)2D3 stimulation suppressed both the [Ca2+]i transient and the SOC influx. 1,25(OH)2D3-induced SOC entry was markedly increased after 3 min of treatment (30% above basal) and then rapidly reached a steady-state level. The sterol-stimulated SOC influx was prevented by protein kinase C and tyrosine kinase inhibitors but unaffected by blockade of the protein kinase A pathway. None of these inhibitors altered the thapsigargin-induced SOC entry, suggesting the operation of a signaling mechanism different from that for sterol-dependent SOC influx. The present results indicate that 1,25(OH)2D3-induced activation of PPI-PLC is upstream to Ca2+ influx through SOC channels and point for a role of both protein kinase C and tyrosine kinases but not protein kinase A in the regulation of the sterol-dependent SOCE pathway.

Differential modulation of the phases of a Ca2+ spike by the store Ca2+-ATPase in human umbilical vein endothelial cells

1. Histamine-stimulated cytosolic free Ca2+ ([Ca2+]i) oscillations in human umbilical vein endothelial cells (HUVECs) comprise repetitive spikes generated by pulsatile release from stores. We have investigated the roles of the store Ca2+-ATPases in regulating both the upstroke and downstroke of a Ca2+ spike. 2. The sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) inhibitor cyclopiazonic acid (CPA) dramatically affected oscillations whereas inhibition of the plasma membrane Ca2+-ATPase (PMCA) with La3+ had little effect. This and other evidence suggested that the downstroke of a spike is predominantly mediated by SERCA. 3. Artificial [Ca2+]i spiking generated by repetitive pulsatile application of 0.3 microM histamine in Ca2+-free medium did not cause net loss of Ca2+ from the cell whereas repetitive pulsatile application of 1 and 10 microM histamine did, with the higher concentration being more effective. We conclude that there is an inverse relationship between stimulus intensity and relative SERCA activity. 4. For a Ca2+ transient, the initiation of release was suppressed by SERCA during either the lag phase or the interspike period (ISP) since: (i) the ISP was shortened by low CPA concentrations, (ii) higher concentrations of CPA stimulated an explosive Ca2+ release when applied during the ISP but not when applied in the absence of agonist, and (iii) CPA synchronized the initial Ca2+ response to a low histamine dose (even recruiting silent, histamine-unresponsive cells). 5. Two aspects of the regenerative upstroke of a spike were differently affected by SERCA inhibition: Ca2+ wave velocity was entirely unaffected by CPA whereas the local rate of rise was increased. 6. The [Ca2+]i at which a Ca2+ spike terminated depended on SERCA since CPA dose dependently enhanced the peak [Ca2+]i. 7. We conclude that SERCA plays a powerful and dynamic role in regulating [Ca2+]i oscillations in HUVECs. SERCA differentially modulates the phases of Ca2+ release in addition to bringing about the falling phase of a Ca2+ spike.

Dissociation of intracellular Ca2+ release and Ca2+ entry response to 5-hydroxytryptamine in cultured canine tracheal smooth muscle cells

The relationship between the agonist-sensitive Ca2+ pool and those discharged by the Ca2+ -ATPase inhibitor thapsigargin (TG) were investigated in canine tracheal smooth muscle cells (TSMCs). In fura-2-loaded TSMCs, 5-hydroxytryptamine (5-HT) stimulated a rapid increase in intracellular Ca2+ ([Ca2+]i), followed by a sustained plateau phase that was dependent on extracellular Ca2+. In such cells, TG produced a concentration-dependent increase in [Ca2+]i, which remained elevated over basal level for several minutes and was substantially attenuated in the absence of extracellular Ca2+. Application of 5-HT after TG demonstrated that the TG-sensitive compartment partly overlapped the 5-HT-sensitive stores. Pre-treatment of TSMCs with TG significantly inhibited the increase in [Ca2+]i induced by 5-HT in a time-dependent manner. Similar results were obtained with two other Ca2+ -ATPase inhibitors, cyclopiazonic acid and 2,5-di-t-butylhydroquinone. Although these inhibitors had no effect on phosphoinositide hydrolysis, Ca2+ -influx was stimulated by these agents. These results suggest that depletion of the agonist-sensitive Ca2+ stores is sufficient for activation of Ca2+ influx. Some characteristics of the Ca2+ -influx activated by depletion of internal Ca2+ stores were compared with those of the agonist-activated pathway. 5-HT-stimulated Ca2+ influx was inhibited by La3+, membrane depolarisation, and the novel Ca2+ -influx blocker 1-¿beta-[3-(4-methoxyphenyl) propoxy]-4-methoxyphenethyl¿-1H-imidazole hydrochloride (SKF96365). Likewise, activation of Ca2+ influx by TG also was blocked by La3+, membrane depolarisation, and SKF96365. These results suggest that (1) in the absence of PI hydrolysis, depletion of the agonist-sensitive internal Ca2+ stores in TSMCs is sufficient for activation of Ca2+ influx, and (2) the agonist-activated Ca2+ influx pathway and the influx pathway activated by depletion of the inositol 1,4,5-trisphosphate-sensitive Ca2+ pool are indistinguishable.

Histamine causes Ca2+ entry via both a store-operated and a store-independent pathway in bovine adrenal chromaffin cells

The characteristics and properties of the increase in cytosolic [Ca2+] that occurs in bovine adrenal medullary chromaffin cells on exposure to histamine have been investigated. Specifically, these experiments were conducted to determine how much external Ca2+ enters the cell through a (capacitative) Ca2+ entry pathway activated as a consequence of intracellular Ca2+ store mobilization, relative to that which enters independently of store depletion via other channels activated by histamine. In Fura-2 loaded cells continued exposure to histamine (10 microM) caused a rapid but transient increase in cytosolic [Ca2+] followed by a lower plateau that was sustained as long as external Ca2+ was present. In the absence of external Ca2+, only the initial brief transient was observed. In cells previously treated with thapsigargin (100 nM) in Ca(2+)-free medium to deplete the internal Ca2+ stores, histamine caused no increase in cytosolic [Ca2+] when external Ca2+ was absent. Re-introduction of external Ca2+ to thapsigargin-treated store-depleted cells caused a sustained increase in cytosolic [Ca2+] that was further increased (P < 0.0002) upon exposure to histamine. The histamine-evoked increase was prevented by the H1-receptor antagonist, mepyramine (2 microM). A comparison was made between store-dependent Ca2+ entry consequent upon store mobilization with histamine in Ca(2+)-free medium and plateau phase Ca2+ entry resulting from stimulation with histamine in Ca(2+)-containing medium. The latter was found to be approximately 3 times greater in magnitude than the former (P << 0.0001) at the same concentration of histamine (10 microM). It is concluded that histamine causes Ca2+ entry not only via a capacitative entry pathway secondary to internal store mobilization, but also causes substantial Ca2+ entry through other pathways.

Modulation of calcium signals by intracellular pH in isolated rat pancreatic acinar cells

1. We have investigated the interactions between intracellular pH (pH1) and the intracellular free calcium concentration ([Ca2+]i) in isolated rat pancreatic acinar cells. The fluorescent dyes fura-2 and BCECF were used to measure [Ca2+]i and pHi, respectively. 2. Sodium acetate and ammonium chloride (NH4Cl) were used to acidify and alkalinize pHi, respectively. Cytosolic acidification had no effect on [Ca2+]i in resting pancreatic acinar cells, whereas cytosolic alkalinization released Ca2+ from intracellular stores. 3. Cytosolic acidification using either acetate or a CO2-HCO3(-)-buffered medium enhanced Ca2+ signals evoked by acetylcholine (ACh) and cholecystokinin (CCK). In contrast, both NH4Cl and trimethylamine (TMA) inhibited Ca2+ signals during stimulation with either ACh or CCK. This inhibitory effect was also observed in the absence of extracellular Ca2+, and was therefore not due to changes in Ca2+ entry. 4. Calcium oscillations evoked by physiological concentrations of CCK were enhanced by cytosolic acidification and inhibited by cytosolic alkalinization. 5. In order to determine the effects of pHi upon Ca2+ handling by intracellular Ca2+ stores, intraorganellar [Ca2+] was monitored using the low affinity Ca2+ indicator mag-fura-2 in permeabilized cells. Addition of NH4Cl, which is expected to alkalinize intraorganellar pH, did not alter intraorganellar [Ca2+] in permeabilized cells, suggesting that changing intraorganellar pH does not release Ca2+ from intracellular stores. Addition of NH4Cl or acetate also did not affect the rate of Ca2+ release induced by inositol 1,4,5-trisphosphate (InsP3). 6. Modification of extraorganellar ('cytosolic') pH did not affect the rate of ATP-dependent Ca2+ uptake into stores, but did modify the rate of Ca2+ release evoked by submaximal concentrations of InsP3. The rate of Ca2+ release was increased at more alkaline extraorganellar pHs. These results would suggest that manipulation of intraorganellar pH does not affect Ca2+ handling by the intracellular stores. In contrast, extraorganellar ('cytosolic') pH does affect InsP3-induced Ca2+ release from the stores. 7. In conclusion, changes in intracellular pH in pancreatic acinar cells can profoundly alter cytosolic [Ca2+]. This may shed light on earlier observations whereby cell-permeant weak acids and bases can modulate fluid secretion in epithelia.

Receptor-activated Ca2+ influx via human Trp3 stably expressed in human embryonic kidney (HEK)293 cells. Evidence for a non-capacitative Ca2+ entry

Ca2+ release from its internal stores as a result of activation of phospholipase C is accompanied by Ca2+ influx from the extracellular space. Ca2+ influx channels may be formed of proteins homologous to Drosophila Trp. At least six non-allelic Trp genes are present in the mouse genome. Full-length human, bovine, mouse, and rat cDNAs for Trp1, 3, 4, 6 have been cloned. Expression of these genes in various mammalian cells has provided evidence that Trp proteins form plasma membrane Ca2+-permeant channels that can be activated by an agonist that activates phospholipase C, by inositol 1,4, 5-trisphosphate, and/or store depletion. We have stably expressed human Trp3 (hTrp3) in human embryonic kidney (HEK)293 cells. Measurement of intracellular Ca2+ concentrations in Fura2-loaded cells showed that cell lines expressing hTrp3 have significantly higher basal and agonist-stimulated influxes of Ca2+, Mn2+, Ba2+, and Sr2+ than control cells. The increase in Ca2+ entry attributable to the expression of hTrp3 obtained upon store depletion by thapsigargin was much lower than that obtained by stimulation with agonists acting via a Gq-coupled receptor. Addition of agonists to thapsigargin-treated Trp3 cells resulted in a further increase in the entry of divalent cations. The increased cation entry in Trp3 cells was blocked by high concentrations of SKF 96365, verapamil, La3+, Ni2+, and Gd3+. The Trp3-mediated Ca2+ influx activated by agonists was inhibited by a phospholipase C inhibitor, U73122. We propose that expression of hTrp3 in these cells forms a non-selective cation channel that opens after the activation of phospholipase C but not after store depletion. In addition, a subpopulation of the expressed hTrp3 may form heteromultimeric channels with endogenous proteins that are sensitive to store depletion.

Phosphatidylcholine-specific phospholipase C regulates thapsigargin-induced calcium influx in human lymphocytes

The involvement of phosphatidylcholine-specific phospholipase C (PC-PLC) and D (PC-PLD) in the regulation of the thapsigargin-induced Ca2+ increase was investigated. Pretreatment of human lymphocytes with the PC-PLC inhibitors D609 or U73122 enhanced the thapsigargin-induced Ca2+ influx. By contrast, no effect was observed in the presence of phospholipase D inhibitor butanol. Addition of exogenous PC-PLC but not PC-PLD to lymphocytes prestimulated with thapsigargin led to a decrease of intracellular Ca2+. In addition, thapsigargin was shown to release diacylglycerol (DAG) from cellular phosphatidylcholine pools. The thapsigargin-induced DAG formation was inhibited by U73122 and D609 but not by butanol. Moreover, no formation of the PC-PLD activity marker phosphatidylbutanol was detected. Thapsigargin-induced DAG formation was dependent on the Ca2+ entry, as it was abolished in the absence of extracellular Ca2+ or in the presence of Ni2+. Further investigations demonstrated that the inhibition of the cellular DAG target, protein kinase C (PKC), enhanced thapsigargin-induced Ca2+ increase, whereas direct PKC activation had an inhibitory effect. Taken together, our results reveal the involvement of PC-PLC in the regulation of the thapsigargin-induced Ca2+ increase and point to the existence of a physiologic feedback mechanism activated by Ca2+ influx and acting via consecutive activation of PC-PLC and PKC to limit the rise of intracellular Ca2+.

Expression and characterization of a trpl homolog from rat

Mammalian homologues of the Drosophila trp/trpl-gene-family code for "Ca(2+)-store-operated" channels. Here we describe the cloning and expression of a trp/trpl homologous gene from rat brain. The clone is named Rtrp3 because of its high homology to the recently described Htrp3 (Zhu et al (1996) Cell 85:661-671). Expression of Rtrp3 in the mammalian COS-1 cell line leads to 100% increase of capacitative Ca2+ entry as compared to the controls. This capacitative calcium entry can be completely blocked with La3+ (10 microM). We conclude that Rtrp3, a new member of the growing family of trp/trpl homologues in mammalians, is involved in "Ca(2+)-store-operated" Ca2+ entry into the cell.

Effects on the hepatocyte [Ca2+]i oscillator of inhibition of the plasma membrane Ca2+ pump by carboxyeosin or glucagon-(19-29)

Single rat hepatocytes, microinjected with the Ca(2+)-sensitive photoprotein aequorin, respond to agonists acting through the phosphoinositide signalling pathway by the generation of oscillations in cytosolic free Ca2+ concentration ([Ca2+]i). The duration of [Ca2+]i transients generated is characteristic of the receptor species activated; the variability results in differences in the rate of fall of [Ca2+]i from its peak. It is conceivable that the plasma membrane Ca(2+)-ATPase (PM Ca2+ pump) may have an important role in the mechanism underlying agonist specificity. It has recently been shown that an esterified form of carboxyeosin, an inhibitor of the red cell PM Ca2+ pump, is suitable for use in whole cell studies. Glucagon-(19-29) (mini-glucagon) inhibits the Ca2+ pump in liver plasma membranes, mediated by Gs. We show here that carboxyeosin and mini-glucagon inhibit Ca2+ efflux from populations of intact rat hepatocytes. We show that carboxyeosin and mini-glucagon enhance the frequency of oscillations induced by Ca(2+)-mobilizing agonists in single hepatocytes, but do not affect the duration of individual transients. Furthermore, we demonstrate that inhibition of the hepatocyte PM Ca2+ pump enables the continued generation of [Ca2+]i oscillations for a prolonged period following the removal of extracellular Ca2+.

Intracellular alkalinization mobilizes calcium from agonist-sensitive pools in rat lacrimal acinar cells

1. We have investigated interactions between intracellular pH (pHi) and the intracellular free calcium concentration ([Ca2+]i) in collagenase-isolated rat lacrimal acinar cells. The fluorescent dyes fura-2 and 2',7'-bis(carboxyethyl)-5-carboxyfluorescein (BCECF) were used to measure [Ca2+]i and pHi, respectively. 2. Application of the weak base NH4Cl alkalinized the cytosol and caused a dose-dependent increase in [Ca2+]i. Trimethylamine (TMA) also alkalinized the cytosol and increased [Ca2+]i. The increase in [Ca2+]i evoked by NH4Cl or TMA was much smaller than that evoked by the secretory agonist acetylcholine (ACh). 3. Application of NH4Cl also increased [Ca2+]i in cells bathed in Ca(2+)-free medium, indicating that NH4Cl released Ca2+ from an intracellular pool. 4. Ammonium chloride had no effect on [Ca2+]i in cells bathed in Ca(2+)-free medium if agonist-sensitive intracellular Ca2+ pools had been depleted with either ACh or the microsomal Ca(2+)-ATPase inhibitor 2,5-di(tert-butyl)hydroquinone. Treatment of cells with NH4Cl in Ca(2+)-free medium reduced the amount of Ca2+ released by ACh. These results suggest that NH4Cl released Ca2+ from the same intracellular pool released by ACh. 5. Calcium release from the agonist-sensitive pool was also triggered when the cytosol was alkalinized by removing the weak acid acetate. 6. Ammonium chloride caused a modest increase in inositol phosphate production, suggesting that NH4Cl may have released stored Ca2+ via an increase in the intracellular inositol 1,4,5-trisphosphate concentration. 7. The increase in [Ca2+]i evoked by NH4Cl was not sustained even in the presence of extracellular Ca2+. In contrast, when a low dose of ACh was used to evoke intracellular Ca2+ release of similar magnitude, sustained Ca2+ entry was observed. 8. Alkalinizing the cytosol appeared to partially inhibit Ca2+ entry triggered by thapsigargin or by ACh. 9. We suggest that alkalinizing the cytoplasm in unstimulated lacrimal acinar cells can release Ca2+ from the intracellular agonist-sensitive Ca2+ pool. However, releasing stored Ca2+ via alkalinization does not appear to trigger significant Ca2+ entry, perhaps because intracellular alkalinization inhibits either the Ca2+ entry pathway or the mechanism which couples the entry pathway to store depletion.

Use of La3+ to distinguish activity of the plasmalemmal Ca2+ pump from Na+/Ca2+ exchange in arterial myocytes

La3+ was tested for its ability to distinguish external Na+ (Nao)-independent Ca2+ efflux via the plasma membrane (PM) Ca2+ pump from Nao-dependent Ca2+ efflux via Na+/Ca2+ exchange. Fura-2 loaded cultured rat aortic myocytes were used with digital imaging to measure the cytosolic free Ca2+ concentration ([Ca2+]cyt) and to monitor La3+ entry. At a La3+ concentration ([La3+]o) of 0.25 mM, but not at lower concentrations, La3+ entered the cells; 0.01 mM verapamil blocked this entry. Transient increases in [Ca2+]cyt were evoked by unloading the sarcoplasmic reticulum with cyclopiazonic acid (CPA)+caffeine (CAF) in Na,Ca-free medium (to inhibit Ca2+ extrusion via Na+/Ca2+ exchange and Ca2+ influx). La3+ (0.03-0.25 mM with verapamil) augmented the Ca2+ transients and slowed Nao-independent [Ca2+]cyt recovery in a dose-dependent manner (IC50 approximately 0.01 mM La3+). This La(3+)-sensitive recovery was apparently mediated by the PM Ca2+ pump. The effects of La3+ were reversible: [Ca2+]cyt returned promptly toward base line when La3+ was washed out in Na,Ca-free medium containing CPA + CAF. Reintroduction of extracellular Na+ during [Ca2+]cyt recovery ([La3+]o = 0.06-0.25 mM) significantly speeded recovery, indicating that the Na+/Ca2+ exchanger was not inhibited by [La3+]o < or = 0.25 mM. The La(3+)-sensitive (Nao-independent) and Nao-dependent [Ca2+]cyt recovery rates were comparable. In Na(+)-loaded cells, < or = 0.25 mM La3+ also did not affect Na+/Ca2+ exchange mediated Ca2+ influx. In medium containing Na+ and Ca2+, 0.125 mM La3+ abolished the serotonin (5-HT) evoked plateau responses that resulted from Ca2+ entry via Ca2+ channels. In Na,Ca-free medium, but not Ca-free medium, however, La3+ converted 5-HT evoked Ca2+ transients into sustained responses. We conclude that low [La3+]o (0.06-0.25 mM) inhibits the PM Ca2+ pump, but spares Na+/Ca2+ exchanger mediated Ca2+ influx and efflux in arterial myocytes.

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.

Depletion of intracellular calcium stores activates a calcium conducting nonselective cation current in mouse pancreatic acinar cells

Receptor-mediated Ca2+ release from inositol (1,4,5)-trisphosphate (IP3)-sensitive Ca2+ stores causes "capacitative calcium entry" in many cell types (Putney, J. W., Jr. (1986) Cell Calcium 7, 1-12; Putney, J. W., Jr. (1990) Cell Calcium 11, 611-624). We used patch-clamp and fluorescence techniques in isolated mouse pancreatic acinar cells to identify ion currents and cytosolic calcium concentrations under conditions in which intracellular Ca2+ stores were emptied. We found that depletion of Ca2+ stores activated a calcium-release-activated nonselective cation current (ICRANC) which did not discriminate between monovalent cations. ICRANC possessed a significant conductance for Ca2+ and Ba2+. It was not inhibited by La3+, Gd3+, Co2+, or Cd2+ but was completely abolished by flufenamic acid or genistein. In whole cell and cell-attached recordings, a 40-45 pS nonselective cation channel was identified which was activated by Ca2+ store depletion. Calcium entry as detected by single cell fluorescence measurements with fluo-3 or fura-2, showed the same pharmacological properties as ICRANC. We conclude that in mouse pancreatic acinar cells 40-45 pS nonselective cation channels serve as a pathway for capacitative Ca2+ entry. This entry pathway differs from the previously described ICRAC (Hoth, M., and Penner, R. (1992) Nature 355, 353-356) in its ion-selectivity, pharmacological profile, and single-channel conductance.

Extrusion mechanisms of intracellular Ca2+ in human aortic endothelial cells

Although participation of the plasma membrane Ca2+ pump in Ca2+ efflux has been generally accepted, the existence of Na(2+)-Ca2+ exchange in endothelial cells is still controversial. In the present experiments, the role of Na(+)-Ca2+ exchange and Ca(2+)-ATPase were examined in cultured human aortic endothelial cells loaded with fura. In Ca(2+)-free solution, the declining phase of [Ca2+]i in response to histamine was clearly slowed with low Na+ solution or Ni2+. Vanadate also slightly slowed the declining phase. The declining phase was much more clearly slowed with La3+. The response to thapsigargin, a specific endoplasmic reticulum Ca2+ ATPase inhibitor, was much more clearly prolonged by those interventions. These results strongly imply the presence of Na(2+)-Ca2+ exchange and Ca2+ ATPase in the plasma membrane, and suggest that not only Ca2+ uptake into the internal stores but also Na(+)-Ca2+ exchange and plasma membrane Ca2+ ATPase have a physiological role in reducing [Ca2+]i elevated by receptor agonists and endoplasmic reticulum Ca(2+)-ATPase inhibitors in cultured human aortic endothelial cells.

Regulation of intracellular calcium is closely linked to glucose metabolism in J774 macrophages

The effects of 2-deoxy-D-glucose (2dGlc) and glucose deprivation were investigated in the J774 murine macrophage-like cell line. 2dGlc addition or glucose deprivation for 4 min led to an inhibition in the transient increase in cytoplasmic free Ca2+ ([Ca2+]i) that otherwise occurs in response to three different agonists: IgG, ATP and platelet activating factor. This inhibition was preceded by a partial release of Ca2+ from intracellular, thapsigargin-sensitive stores. In contrast, the transition from 5 to 30 mM glucose caused a decrease in [Ca2+]i and a corresponding increase in thapsigargin-sensitive sequestered Ca2+. The effects of an alternate glycolytic inhibitor, NaF, and a mitochondrial inhibitor, rotenone, were also tested. These inhibitors caused neither a release of Ca2+ from intracellular stores nor an inhibition in any of the agonist responses. The capacitative influx of extracellular Ca2+ following depletion of intracellular stores was also found to be selectively inhibited by the prior addition of 2dGlc or with glucose deprivation. In addition, when an elevated plateau of [Ca2+]i was established by the irreversible depletion of intracellular Ca2+ stores, the addition of 2dGlc caused a decrease in the on-going capacitative entry of Ca2+.

The effect of L-type calcium channel modulators on the mobilization of intracellular calcium stores in guinea-pig intestinal smooth muscle

1. The action of Ca2+ channel modulators has been examined on the intracellular Ca2+ signal in the longitudinal smooth muscle cells of the guinea-pig intestine after exposure to histamine and to agents known to affect intracellular Ca2+ stores. Isometric contraction has been measured simultaneously with front-surface fluorometry of fura 2-loaded preparations. 2. Histamine (10 microM) evoked a phasic and tonic increase in [Ca2+]i and contraction which were both sensitive to the Ca2+ channel blockers, nimodipine and D600. 3. Caffeine (10 mM) evoked in rapid increase in [Ca2+]i which was sustained as long as the preparation was exposed to the drug, whereas the contractile response was only phasic. In the presence of nimodipine 1 microM, the phasic contraction was absent although the fura 2-Ca2+ signal amounted to 32% of the control. 4. Ryanodine (10 microM) evoked a slow increase in [Ca2+]i and a contraction, both of which were reversed after exposure to nimodipine (1 microM) or D600 (10 microM). In the presence of diazoxide (500 microM), a hyperpolarizing agent, the ryanodine-evoked increase in [Ca2+]i and in muscle tone were inhibited. 5. Thapsigargin (1 microM) also produced an increase in [Ca2+]i and a contraction both of which were blocked by nimodipine (1 microM). 6. In Ca2+-free solution, histamine 10 microM evoked non-reproducible phasic Ca2+ signal and contraction. This response was recovered after refilling in Ca2+ containing solution. The recovery was blocked by nimodipine, D600 or diazoxide and was facilitated by the Ca2+ channel activator, Bay K 8644. When the refilling medium was supplemented with thapsigargin, the recovered response was significantly reduced, but Bay K 8644 still had some action. 7. The present results show that blockage of L-type Ca2+ channels inhibited changes in [Ca2+]i evoked by histamine, caffeine and ryanodine which are generally attributed to Ca2+ mobilization from intracellular stores. They also show that when the tissue was exposed to nimodipine, D600 and diazoxide during the procedure of refilling after depletion of intracellular stores, the action of histamine on [Ca2+]i and contraction was blocked. Bay K 8644 had an opposite effect even when the Ca2+ pumping activity of the sarcoplasmic reticulum was reduced by thapsigargin. This indicates that refilling of intracellular Ca2+ stores depleted by histamine in guinea-pig intestine mainly occurred through L-type Ca2+ channels.

Capacitative calcium entry in rat Sertoli cells

Depletion of intracellular Ca2+ stores activates an influx of Ca2+ from the extracellular medium. This capacitative Ca2+ entry as originally proposed by Putney in 1986 can be studied with drugs that inhibit sarco/endoplasmic reticulum ATPase. In the present study we examined the effects of depletion of internal Ca2+ stores on Ca2+ influx in rat Sertoli cells utilizing thapsigargin and cyclopiazonic acid. Both inhibitors induced a rapid and dose-dependent rise in [Ca2+]i that was dependent on an influx of Ca2+ from the extracellular medium since it was rapidly blocked by the addition of the Ca2+ chelating agent EGTA. In the absence of external Ca2+ thapsigargin and cyclopiazonic acid still produced an increase in [Ca2+]i that was lower than that observed in Ca2+ medium and was transient since [Ca2+]i returned to basal levels by few minutes. In these experimental conditions readdition of Ca2+ induced a rapid rise in [Ca2+]i supporting a role for Ca2+ influx. Increase of plasma membrane permeability to Ca2+ induced by thapsigargin and cyclopiazonic acid were confirmed by the ability of Mn2+ to permeate through Ca2+ channels and to quench intracellular fura-2 fluorescence after challenge with these inhibitors. Mn2+ induced influx was blocked by La3+, a well known blocker of Ca2+ channels. These results demonstrate that internal Ca2+ stores depletion induce Ca2+ influx from the extracellular medium in rat Sertoli cells providing evidence for the existence of capacitative Ca2+ entry in these cells.

Amplification of the thapsigargin-evoked increase in the cytosolic free Ca2+ concentration by acetylcholine in acutely isolated mouse submandibular acinar cells

The intracellular Ca2+ concentration was measured in single, acutely isolated, mouse submandibular acinar cells loaded with fura-2 AM. All experiments were performed in the absence of extracellular Ca2+ in order to eliminate Ca2+ influx. The microsomal ATPase inhibitor, thapsigargin, was used to release Ca2+ from intracellular stores and simultaneously prevent re-uptake into the stores. Sequential application of thapsigargin (2 microM) and the Ca2+ ionophore ionomycin (500 nM) indicated that thapsigargin was able to mobilize practically all intracellular Ca2+. Furthermore, in comparison with results obtained following inhibition of the plasma membrane Ca(2+)-ATPase by La3+ (2 mM), it may be shown that slowly unloading the intracellular Ca2+ stores using thapsigargin does not normally cause a massive, cytotoxic, increase in the cytosolic Ca2+ concentration, because Ca2+ is rapidly extruded from the cell across the plasma membrane. Application of a submaximal dose of acetylcholine (500 nM) during the rising phase of the response to thapsigargin caused a 3-4-fold increase in the amplitude of the rise in the cytosolic Ca2+ concentration without any significant alteration of the time course of the response. As thapsigargin alone is capable of mobilizing all releasable Ca2+, this increase in amplitude is most likely the result of inhibition of the Ca2+ extrusion process by acetylcholine.

trp, a novel mammalian gene family essential for agonist-activated capacitative Ca2+ entry

SUMMARY

Capacitative calcium entry (CCE) describes CA2+ influx into cells that replenishes CA2+ stores emptied through the action of IP3 and other agents. It is an essential component of cellular responses to many hormones and growth factors. The molecular basis of this form of Ca2+ entry is complex and may involve more than one type of channel. Studies on visual signal transduction in Drosophila led to the hypothesis that a protein encoded in trp may be a component of CCE channels. We reported the existence of six trp-related genes in the mouse genome. Expression in L cells of small portions of these genes in antisense orientation suppressed CCE. Expression in COS cells of two full-length cDNAs encoding human trp homologs, Htrp1 and Htrp3, increased CCE. This identifies mammalian gene products that participate in CCE. We propose that trp homologs are subunits of CCE channels, not unlike those of classical voltage-gated ion channels.

Role of extracellular Ca2+ in acetylcholine-induced repetitive Ca2+ release in submandibular gland acinar cells of the rat

Acetylcholine (ACh) caused repetitive transient Cl- currents activated by intracellular Ca2+ in single rat submandibular grand acinar cells. As the concentration of ACh increased the amplitude and the frequency of the transient Cl- currents increased. These responses occurred also in the absence of extracellular Ca2+ but disappeared after several minutes. Repetitive transient Cl- currents were restored by readmission of Ca2+ to the extracellular solution. The higher the concentration of extracellular Ca2+ readmitted, the larger the amplitude of the transient Cl- currents. Ca2+ entry through a store-coupled pathway was detected by application of Ca2+ to the extracellular solution during a brief cessation of stimulation with ACh. In these experiments too, the higher the concentration of Ca2+, the larger the transient Cl- currents activated by Ca2+ released from the stores. The time course of decrease in total charge movements of repetitive transient responses to ACh with removal of extracellular Ca2+ depended on a decrease in charge movements of each transient event rather than a decrease in frequency of the repetitive events. The decrease of charge movements of each transient event was due to a decrease in its amplitude rather than its duration. The results suggest that in this cell type and amplitude-modulated mechanism is involved in repetitive Ca2+ release and that Ca2+ entry is essential to maintain the repetitive release of Ca2+. The results further suggest that the magnitude of Ca2+ entry determines the number of unitary stores filled with Ca2+ which can synchronously respond to ACh.

Effect of inositol 1,3,4,5-tetrakisphosphate on inositol trisphosphate-activated Ca2+ signaling in mouse lacrimal acinar cells

In mouse lacrimal acinar cells, microinjection of the metabolically stable analog of inositol 1,4,5-trisphosphate, inositol 2,4,5-trisphosphate ((2,4,5)IP3), stimulated both intracellular Ca2+ mobilization and Ca2+ entry. Microinjection of inositol 1,3,4,5-tetrakisphosphate ((1,3,4,5)IP4), the inositol 1,4,5-trisphosphate-3-kinase product, was ineffective at mobilizing intracellular Ca2+ or activating Ca2+ entry. In lacrimal cells previously microinjected with submaximal levels of (2,4,5)IP3, the subsequent microinjection of low to moderate concentrations of (1,3,4,5)IP4 did not result in additional release of intracellular Ca2+, nor did it potentiate the Ca2+ entry phase attributable to (2,4,5)IP3. However, as previously demonstrated (Bird, G. S. J., Rossier, M. F., Hughes, A. R., Shears, S. B., Armstrong, D. L., and Putney, J. W., Jr. (1991) Nature 352, 162-165), additional injections of (2,4,5)IP3 induced further mobilization of intracellular Ca2+ and increased the elevated and sustained Ca2+ entry phase. Introduction of high concentrations of (1,3,4,5)IP4 appeared to inhibit or block the (2,4,5)IP3-induced Ca2+ entry phase. These results were consistent with the observed effect of (1,3,4,5)IP4 in permeabilized lacrimal cells, where (1,3,4,5)IP4 did not release cellular 45Ca2+ but at high concentrations inhibited the ability of submaximal concentrations of (2,4,5)IP3 to release 45Ca2+. Likewise, injection of a high concentration of (1,3,4,5)IP4 prior to injection of (2,4,5)IP3 blocked both release and influx of Ca2+. The inhibitory action of (1,3,4,5)IP4 on Ca2+ signaling observed in intact cells occurred at concentrations that might be obtained in agonist-stimulated cells. However, in permeabilized cells, (1,3,4,5)IP4 inhibited Ca2+ mobilization at concentrations exceeding those likely to occur in agonist-stimulated cells. These results suggest that physiologically relevant levels of (1,3,4,5)IP4 in the cell cytoplasm do not release Ca2+, nor do they potentiate inositol trisphosphate-induced Ca2+ entry across the plasma membrane. Rather, the possibility is raised that (1,3,4,5)IP4 or one of its metabolites could function as a negative feedback on Ca2+ mobilization by inhibiting inositol 1,4,5-trisphosphate-induced Ca2+ release.

Dual effects of SK&F 96365 in human leukemic HL-60 cells. Inhibition of calcium entry and activation of a novel cation influx pathway

The effects of the receptor-mediated Ca2+ entry blocker, SK&F 96365 on thapsigargin (TSG)-induced Ca2+ entry in fura-2-loaded HL-60 cells were studied. After Ca2+ release induced by 30 nM TSG, readmission of Ca2+ resulted in a sustained Ca2+ entry, which could be partially inhibited by 1-3 microM SK&F 96365. Surprisingly, SK&F 96365 at 30-100 microM, instead of causing a stronger inhibition, actually promoted Ca2+ entry. Furthermore, at 16-100 microM, this drug released intracellular Ca2+ on its own and induced Ca2+ entry upon readmission of Ca2+. This SK&F 96365-activated Ca2+ entry pathway was insensitive to nifedipine and, interestingly, accessible to Ni2+ and La3+. However, SK&F 96365 (30 microM) almost completely blocked (basal) Mn2+ entry and only caused 4.4% of the cells to be stained with trypan blue, strongly suggesting that the SK&F 96365-activated cation entry was not due to damage nor to a very nonselective permeabilization of the plasma membrane. These data indicate that low concentrations of SK&F 96365 inhibited Ca2+ entry and higher concentrations activated a novel cation entry pathway. Because these 2 opposing effects overlapped at an intermediate concentration (16 microM), which is within the range commonly used to block Ca2+ entry, cautious use of this Ca2+ antagonist appears to be warranted.

Submicromolar La3+ concentrations block the calcium release-activated channel, and impair CD69 and CD25 expression in CD3- or thapsigargin-activated Jurkat cells

The calcium release-activated channel (CRAC) opened in Jurkat cells activated either with CD3 monoclonal antibody or the endoplasmic reticulum Ca2(+)-ATPase blocker, thapsigargin, is blocked by La3+ with an IC50 of 20 nM. Similarly, the entry of Mn2+, used as a surrogate for Ca2+, is also blocked by submicromolar La3+ concentrations. La3+ seems to play its role simply by plugging the CRAC because this ion does not penetrate the cells, as demonstrated by chelation experiments with EGTA. Blocking the Ca2+ influx in activated Jurkat cells results in a lack of expression of CD25, a chain of the interleukin-2 receptor and of CD69, a marker of T-cell activation. By contrast, the very early steps of the T-cell signalling pathway such as the release of Ca2+ from intracellular stores and the subsequent inhibition of phosphatidylserine synthesis are not affected by La3+.

The effect of extracellular polyvalent cations on bovine anterior pituitary cells. Evidence for a Ca(2+)-sensing receptor coupled to release of intracellular calcium stores

We have investigated the effects of extracellular cations ([ION]ex) on cytosolic free calcium levels ([Ca2+]i) in bovine anterior pituitary (bAP) cells, using single-cell microfluorimetry. Increasing the [Ca2+]ex from 1 mM to 20 mM caused [Ca2+]i to increase in 64 +/- 14% of bAP cells. The [Ca2+]ex-induced [Ca2+]i increase was observed when cells were maintained in the presence of the voltage-gated-calcium-channel antagonist nitrendipine, but not when cells were treated with thapsigargin. Addition of [La3+]ex (5-15 microM) decreased [Ca2+]i, whereas 30 microM-1 mM caused a [Ca2+]i rise in 60.9 +/- 8.8% of bAP cells. [La3+]ex-induced [Ca2+]i changes were abolished by treating bAP-cells with either thapsigargin or ionomycin, but not nitrendipine. [La3+]ex at 15 microM did not increase [Ca2+]i in any cells tested, but when cells were treated with thimerosal, [La3+]ex (15 microM) caused a [Ca2+]i increase in 62.5 +/- 12.2% of bAP cells. In the presence of 1 mM [Ca2+]ex, successive additions of La3+ caused successive [Ca2+]i rises, but in nominally [Ca2+]ex-free medium only the first addition of [La3+]ex caused a [Ca2+]i rise. Addition of thyroliberin (TRH) in the presence of 1 mM [Ca2+]ex, caused [Ca2+]i to increase in 70% of bAP cells; subsequent addition of [La3+]ex (1 mM) only caused [Ca2+]i increases in 75% of those cells which had already responded to TRH. However, all cells which responded to 1 mM [La3+]ex also responded subsequently to TRH. After treatment with TRH in medium that was nominally [Ca2+]ex free, addition of La3+ (0.5-1 mM) did not increase [Ca2+]i in any cells tested. The number of cells which showed [La3+]ex-induced [Ca2+]i increases decreased in culture: only 21.75 +/- 2.2% cells responded after 7-11 days. When cells were cultured for 7-11 days in the presence of tunicamycin, [La3+]ex failed to increase [Ca2+]i in any cells tested. [Mn2+]ex rapidly quenched the Fura-2 signal measured from all bAP cells, but at 10 mM it also triggered a [Ca2+]i rise in about 60% of bAP cells. The Mn(2+)-induced [Ca2+]i rise was specifically abolished in cells cultured in the presence of tunicamycin although quenching was still observed. From these data we suggest that bAP cells may express a polyvalent cation receptor coupled to the release of calcium from intracellular stores.

Inhibition of microsomal and mitochondrial Ca2+-sequestration in rat cerebellum by polychlorinated biphenyl mixtures and congeners. Structure-activity relationships

Recent studies from our laboratory indicate that polychlorinated biphenyl (PCB) congeners in vitro perturbed signal transduction mechanisms including cellular Ca2+-homeostasis and protein kinase C translocation. We have now investigated the structure-activity relationship (SAR) of three PCB mixtures, 24 PCB congeners and one dibenzofuran for their effects on microsomal and mitochondrial Ca2+-sequestration in rat cerebellum. Ca2+-sequestration by these intracellular organelles was determined using radioactive 45CaCl2. All three mixtures studied, Aroclor 1016, Aroclor 1254 and Aroclor 1260, were equally potent in inhibiting microsomal and mitochondrial Ca2+-sequestration with IC50 values of 6-8 microM. 1,2,3,7,8-Pentachlorodibenzofuran had no effect on Ca2+-sequestration by these organelles. The SAR among the congeners revealed: (1) congeners with ortho-/meta- or ortho-, para-chlorine substitutions were the most potent in inhibiting microsomal and mitochondrial Ca2+-sequestration (IC50 = 2.4-22.3 microM); (2) congeners with only para- but without ortho-substitutions were not effective in inhibiting Ca2+-sequestration by microsomes and mitochondria; (3) increased chlorination was not related to the effectiveness of these congeners. The present SAR studies indicate that the effects of most PCB congeners in vitro may be related to an interaction at specific sites having preference for low lateral substitution or lateral content (meta- or para) in the presence of ortho-substitution.

Pharmacologic characterization of refilling inositol 1,4,5-trisphosphate-sensitive Ca2+ stores in NG108-15 cells

Following mobilization with the inositol 1,4,5-trisphosphate (IP3)-generating agonist bradykinin, Ca2+ stores in neuroblastoma x glioma hybrid, NG108-15 cells require extracellular Ca2+ to refill. The process by which this store refills with Ca2+ was characterized by recording bradykinin-induced intracellular free Ca2+ concentration transients as an index of the degree of refilling of the store. Cyclopiazonic acid, a microsomal Ca2+ ATPase inhibitor, reversibly depleted intracellular Ca2+ stores in these cells, but did not recruit detectable Ca2+ influx, suggesting that these cells lack substantial capacitative Ca2+ entry. The paucity of voltage-sensitive Ca2+ channels in undifferentiated NG108-15 cells, suggested that a channel analogous to that proposed to mediate capacitative Ca2+ entry in nonexcitable cells might assist refilling IP3-sensitive Ca2+ stores in these cells. The possibility that compounds shown previously to inhibit capacitative Ca2+ entry in nonexcitable cells might inhibit the refilling of the IP3-sensitive store in NG108-15 cells was explored. The IP3-sensitive store was depleted by exposure to bradykinin, allowed to refill briefly in the presence of the test compound and then challenged again with bradykinin to evaluate the degree of refilling of the store. The imidazole derivatives, econazole (10 microM), L-651582 (10 microM) and SKF 96365 (20 microM), all completely blocked the bradykinin-induced Ca2+ response. Calmodulin antagonists, W-7 (100 microM) and trifluoperazine (10 microM), were also effective, although at concentrations well above those required to inhibit calmodulin. Because of the high concentrations required to inhibit bradykinin responses, the possibility that these agents might have additional effects was explored. Compounds were tested in a paradigm in which the store was preloaded with Ca2+ before treatment. All of these agents depleted, at least partially, the preloaded store. Econazole was the least effective of the compounds tested for releasing stores, although it was comparable to the other compounds for inhibition of refilling. Although NG108-15 cells refill intracellular Ca2+ stores by a plasmalemmal Ca2+ leak, this leak shares a pharmacology similar to the capacitative Ca2+ entry pathway described for nonexcitable cells.

Oxidized glutathione decreases luminal Ca2+ content of the endothelial cell ins(1,4,5)P3-sensitive Ca2+ store

The model oxidant, t-butyl hydroperoxide (t-buOOH), inhibits Ins(1,4,5)P3-dependent Ca2+ signalling in calf pulmonary artery endothelial cells. Metabolism of t-buOOH within the cytosol is coupled to the oxidation of glutathione. In this study, we investigated whether oxidized glutathione (GSSG) is the intracellular moiety responsible for mediating the effects of t-buOOH on Ca2+ signalling. The increase in cytosolic [Ca2+] stimulated by application of 2,5-di-t-butylhydroquinone (BHQ) was used to estimate the luminal Ca2+ content of the Ins(1,4,5)P3-sensitive store in intact cells. Luminal Ca2+ content was unaffected by t-buOOH (0.4 mM, 0-3 h) unless intracellular GSSG content was concomitantly elevated. The effect was specific for increased GSSG and was not replicated by depletion of GSH. These results suggest that cytosolic GSSG, produced endogenously within the endothelial cell, decreases the luminal Ca2+ content of Ins(1,4,5)P3-sensitive Ca2+ stores. Depletion of internal Ca2+ stores by GSSG may represent a key mechanism by which some forms of oxidant stress inhibit signal transduction in vascular tissue. At the plasma membrane, t-buOOH is known to inhibit the capacitative Ca2+ influx pathway. Increased intracellular GSSG potentiated the inhibitory effect of t-buOOH on Ca2+ influx, thereby providing the first evidence that activity of the capacitative Ca2+ influx channel is sensitive to thiol reagents formed endogenously within the cell.

Hyperglycemia alters cytoplasmic Ca2+ responses to capacitative Ca2+ influx in rat aortic smooth muscle cells

Concentrations of free cytoplasmic Ca2+ in rat aortic smooth muscle (RASM) cells were monitored using the ratiometric Ca2+ indicator fura 2-acetoxymethyl ester (AM). In RASM cells cultured in 5 mM Glc, incubation with angiotensin II, ATP, or thapsigargin [a selective inhibitor of the sarcoplasmic reticulum (SR) Ca(2+)-ATPase] depleted SR Ca2+ stores and initiated a capacitative Ca2+ influx through the plasma membrane. This influx was resistant to verapamil, a selective inhibitor of L-type voltage-gated Ca2+ channels, but was sensitive to SKF-96365, an inhibitor of the receptor-operated Ca2+ entry pathway. RASM cells cultured in 25 mM Glc exhibited a significant decrease in cytoplasmic Ca2+ responses to agonist-induced Ca2+ release from SR stores and to subsequent capacitative Ca2+ entry. In addition, the cytoplasmic response to thapsigargin-induced release of Ca2+ from the SR in hyperglycemic cells peaked more sharply than in control cells and returned to baseline more rapidly. The effects of hyperglycemia were not overcome by myo-inositol supplementation.

Sr2+ can become incorporated into an agonist-sensitive, cytoplasmic Ca2+ store in a cell line derived from the equine sweat gland epithelium

We have explored the properties of a Ca(2+)-dependent cell-signalling pathway that becomes active when cultured equine sweat gland cells are stimulated with ATP. The ATP-regulated, Ca(2+)-influx pathway allowed Sr2+ to enter the cytoplasm but permitted only a minimal influx of Ba2+. Experiments in which cells were repeatedly stimulated with ATP suggested that Sr2+, but not Ba2+, could become incorporated into the agonist-sensitive, cytoplasmic Ca2+ store. Further evidence for this was provided by experiments using ionomycin, a Ca2+ ionophore which has no affinity for Sr2+.

Ca2+ increase and Ca(2+)-influx in human tracheal smooth muscle cells: role of Ca2+ pools controlled by sarco-endoplasmic reticulum Ca(2+)-ATPase 2 isoform

1. The contribution of sarco-endoplasmic reticulum Ca(2+)-ATPases (SERCA)-regulated Ca2+ stores to the increase in intracellular free calcium ([Ca2+]i) induced by bradykinin (BK) was investigated in fura-2 loaded human tracheal smooth muscle cells (TSMC). For this purpose, we used thapsigargin, a selective inhibitor of Ca(2+)-ATPases of intracellular organelles. 2. Thapsigargin (10(-9) to 10(-6) M) induced a dose-dependent increase in [Ca2+]i in the presence of external Ca2+ with an EC50 value of 7.33 +/- 1.26 nM. In Ca(2+)-free conditions, the addition of Ca2+ (1.25 mM) caused an increase in [Ca2+]i which was directly proportional to the pre-incubation time of the cells with thapsigargin. Net increases of 60 +/- 9, 150 +/- 22 and 210 +/- 27 nM were obtained after 1, 3 and 5 min, respectively. 3. In the presence of extracellular Ca2+, BK induced a typical biphasic increase in [Ca2+]i with a fast transient phase and a sustained phase. The sustained component was reversed by addition of a bradykinin B2-receptor antagonist (Hoe 140, 10(-6) M) to the buffer as well as by deprivation of Ca2+. The transient phase induced by BK, histamine and carbachol was inhibited in a time-dependent way by preincubation of the cells with thapsigargin. 4. Comparative western blotting of human TSMC membranes using anti-SERCA2 isoform-specific antibodies clearly showed the greater expression of the 100-kDa SERCA2-b isoform compared with the SERCA2-a isoform. 5. Our data show that thapsigargin-sensitive Ca2+ stores contribute significantly to the activation of human TSMC which suggests a role for these stores in the subsequent induction of Ca2+ influx. These stores appear to be controlled by the Ca2+-ATPases (SERCA2-b isoform) which could also participate in the regulation of Ca2+ influx through the plasma membrane.

Region-specific activity of the plasma membrane Ca2+ pump and delayed activation of Ca2+ entry characterize the polarized, agonist-evoked Ca2+ signals in exocrine cells

The initial release of Ca2+ from the intracellular Ca2+ stores is followed by a second phase during which the agonist-dependent Ca2+ response becomes sensitive to the extracellular Ca2+, indicating the involvement of the plasma membrane (PM) Ca2+ transport systems. The time course of activation of these transport systems, which consist of both Ca2+ extrusion and Ca2+ entry pathways, is not well established. To investigate the participation of these processes during the agonist-evoked Ca2+ response, isolated pancreatic acinar cells were exposed to maximal concentrations of an inositol 1,4,5-trisphosphate-mobilizing agonist (acetylcholine, 10 microM) in different experimental conditions. Following the increase of [Ca2+]i, there was an almost immediate activation of the PM Ca2+ extrusion system, and maximal activity was reached within less than 2s. The rate of Ca2+ extrusion was dependent on the level of [Ca2+]i, with a steep activation at values just above the resting [Ca2+]i and reached a plateau value at 700 nM Ca2+. In contrast, the PM Ca2+ entry pathway was activated with a much slower time course. There was also a delay of 3-4 s between the maximal effective depletion of the intracellular Ca2+ stores and the activation of this entry pathway. By use of digital imaging data, the PM Ca2+ transport systems were also analyzed independently in two regions of the cells, the lumenal and the basal poles. With respect to the activation of the Ca2+ entry pathways, no significant difference existed between these two regions. In contrast, the PM Ca2+ pump displayed a different pattern of activity in these regions. In the basal pole, the pump activity was more sensitive to changes of [Ca2+]i and had a higher maximal activity. Also, in the lumenal pole, the pump became saturated at values of [Ca2+]i around 700 nM, whereas at the basal pole [Ca2+]i had a biphasic effect on the pump activity, and higher [Ca2+]i inhibited the pump. It is argued that these differences in sensitivity to the levels of [Ca2+]i and the different relationship between [Ca2+]i and the rate of extrusion at the two functional poles of the pancreatic acinar cells indicate that the plasma membrane Ca2+ ATPase might play an important role in the polarization of the Ca2+ response.

Thapsigargin induces cytoplasmic free Ca2+ oscillations in mouse oocytes

The mechanisms of calcium signalling in mammalian oocytes during maturation and fertilization are controversial. In this study we measured intracellular free Ca2+ concentrations ([Ca2+]i) with the photoprotein aequorin microinjected into immature mouse oocytes. Immature mouse oocytes typically produced [Ca2+]i responses to muscarinic acetylcholine (ACh) stimulation with two types of component. The first component consisted of a broad transient rise in [Ca2+]i lasting about 1 min. The second component consisted of pulsatile oscillations which could occur before, during or after the broad transient, but typically occurred on the rising phase of the broad transient, with a duration of about 5 s. Removal of external Ca2+ ([Ca2+]o) abolished the Ca2+ responses to ACh. Exposure of oocytes to the specific microsomal Ca(2+)-ATPase inhibitors thapsigargin (TG) and cyclopiazonic acid unexpectedly produced sustained oscillations in [Ca2+]i which were sensitive to the concentration of Ca2+ in the external milieu. The frequency of these oscillations was slow, and ceased, sometimes after several cycles, when Ca2+o was removed. Raised [Ca2+]o significantly increased the frequency in cells oscillating to TG and stimulated nonoscillating cells to begin oscillating. The majority of responsive oocytes which did not produce oscillations to ACh alone (70%), did so after TG treatment. Detailed data analysis indicated that these oscillations were identical to those generated by TG alone, with a similar sensitivity to changes in [Ca2+]o. Exposure of oocytes to ryanodine did not inhibit oscillatory behaviour. These results suggest that immature mouse oocytes possess a store which is insensitive to both TG and ryanodine and is capable of supporting [Ca2+]i oscillations.