Receptor-mediated calcium entry in fura-2-loaded human platelets stimulated with ADP and thrombin. Dual-wavelengths studies with Mn2+

Presynaptic strontium dynamics and synaptic transmission

Strontium can replace calcium in triggering neurotransmitter release, although peak release is reduced and the duration of release is prolonged. Strontium has therefore become useful in probing release, but its mechanism of action is not well understood. Here we study the action of strontium at the granule cell to Purkinje cell synapse in mouse cerebellar slices. Presynaptic residual strontium levels were monitored with fluorescent indicators, which all responded to strontium (fura-2, calcium orange, fura-2FF, magnesium green, and mag-fura-5). When calcium was replaced by equimolar concentrations of strontium in the external bath, strontium and calcium both entered presynaptic terminals. Contaminating calcium was eliminated by including EGTA in the extracellular bath, or by loading parallel fibers with EGTA, enabling the actions of strontium to be studied in isolation. After a single stimulus, strontium reached higher peak free levels than did calcium (approximately 1.7 times greater), and decayed more slowly (half-decay time 189 ms for strontium and 32 ms for calcium). These differences in calcium and strontium dynamics are likely a consequence of greater strontium permeability through calcium channels, lower affinity of the endogenous buffer for strontium, and less efficient extrusion of strontium. Measurements of presynaptic divalent levels help to explain properties of release evoked by strontium. Parallel fiber synaptic currents triggered by strontium are smaller in amplitude and longer in duration than those triggered by calcium. In both calcium and strontium, release consists of two components, one more steeply dependent on divalent levels than the other. Strontium drives both components less effectively than does calcium, suggesting that the affinities of the sensors involved in both phases of release are lower for strontium than for calcium. Thus, the larger and slower strontium transients account for the prominent slow component of release triggered by strontium.

The activation of nitric oxide synthase by copper ion is mediated by intracellular Ca2+ mobilization in human pulmonary arterial endothelial cells

The aim of the study was to elucidate the vasodilatory mechanism due to Cu2+ by assessing nitric oxide (NO) production as determined by NOx (NO, NO2-, and NO3-) that is released from human pulmonary arterial endothelial cell (HPAEC) monolayers using a NO chemiluminescence analyzer, and also to assess Ca2+ movement using 45Ca and fura 2 in HPAEC. Cu2+ (10(-6)-10(-4) M) significantly increased NO production in a dose-dependent manner when extracellular Ca2+ was present. 45Ca influx into the adherent cells was dose-dependently enhanced by Cu(2+) (10(-6)-10(-4) M), but not by Mn(2+), Zn(2+) or Fe(2+). [Ca2+]i, measured by monitoring the fluorescence changes of fura 2, was significantly elevated in the presence of Cu2+. The increase in [Ca2+]i induced by Cu2+ was inhibited by either diethyldithiocarbamate (DDC) or the depletion of extracellular Ca2+. The dihydropyridine receptor agonist, BayK8644, significantly attenuated the Cu2+-induced increase in [Ca2+]i in a dose dependent manner and nitrendipine or nifedipine, the dihydropyridine receptor antagonists, dose-dependently inhibited a Cu2+-induced increase in [Ca2+]i. These results suggest that Cu2+ activates eNOS through the mechanism of [Ca2+]i elevation due to Ca2+ influx into HPAEC and that the Cu2+-induced [Ca2+]i elevation in HPAEC is likely due to activation of the dihydropyridine-like receptors.

Intracellular Ca2+ store-operated influx of Ca2+ through TRP-R, a rat homolog of TRP, expressed in Xenopus oocytes

To elucidate whether rat transient receptor potential (TRP-R), a rat TRP4 homolog, functions as a store-operated Ca2+ channel (SOC), we have measured the Ca2+ entry after thapsigargin treatment in Xenopus oocytes injected with mRNA for TRP-R. While non-injected oocytes elicited an SOC response, significantly larger responses were observed in the oocytes expressing TRP-R. The oocyte-native SOC response was inhibited by injection of antisense oligodeoxyribonucleotide for mammalian TRP1. When Ca2+ concentration-SOC response curve was examined, the EC50 value was much smaller in oocytes expressing TRP-R than that of non-injected oocytes. These results suggest that TRP-R functions as SOC having higher sensitivity to external Ca2+ than amphibian TRP1 channel.

ADP-induced platelet activation

Platelet activation is central to the pathogenesis of hemostasis and arterial thrombosis. Platelet aggregation plays a major role in acute coronary artery diseases, myocardial infarction, unstable angina, and stroke. ADP is the first known and an important agonist for platelet aggregation. ADP not only causes primary aggregation of platelets but is also responsible for the secondary aggregation induced by ADP and other agonists. ADP also induces platelet shape change, secretion from storage granules, influx and intracellular mobilization of Ca2+, and inhibition of stimulated adenylyl cyclase activity. The ADP-receptor protein mediating ADP-induced platelet responses has neither been purified nor cloned. Therefore, signal transduction mechanisms underlying ADP-induced platelet responses either remain uncertain or less well understood. Recent contributions from chemists, biochemists, cell biologists, pharmacologists, molecular biologists, and clinical investigators have added considerably to and enhanced our knowledge of ADP-induced platelet responses. Although considerable efforts have been directed toward identifying and cloning the ADP-receptor, these have not been completely successful or without controversy. Considerable progress has been made toward understanding the mechanisms of ADP-induced platelet responses but disagreements persist. New drugs that do not mimic ADP have been found to inhibit fairly selectively ADP-induced platelet activation ex vivo. Drugs that mimic ADP and selectively act at the platelet ADP-receptor have been designed, synthesized, and evaluated for their therapeutic efficacy to block selectively ADP-induced platelet responses. This review examines in detail the developments that have taken place to identify the ADP-receptor protein and to better understand mechanisms underlying ADP-induced platelet responses to develop strategies for designing innovative drugs that block ADP-induced platelet responses by acting selectively at the ADP-receptor and/or by selectively interfering with components of ADP-induced platelet activation mechanisms.

Feedback regulation of ATP-induced Ca2+ signaling in HL-60 cells is mediated by protein kinase A- and C-mediated changes in capacitative Ca2+ entry

Extracellular ATP increases intracellular Ca2+ ([Ca2+]i) in HL-60 cells. When cells are stimulated with supramaximal concentrations of ATP, although the initial [Ca2+]i increase is similar over a range of 30, 100, and 300 microM ATP, the rate of the return to basal [Ca2+]i level is faster in cells treated with higher concentrations of ATP. This probably results from differences in Ca2+ influx rather than Ca2+ release, since the influx of the unidirectional Ca2+ surrogates Ba2+ and Mn2+ also exhibit similar responses. Furthermore, while 300 microM ATP had an inhibitory effect on the thapsigargin-induced capacitative Ca2+ entry, 30 microM ATP potentiated the response. However, the inhibitory action of 300 microM ATP was blocked by protein kinase C (PKC) inhibitors, such as GF 109203X and chelerythrine, and the potentiating action of 30 microM ATP was blocked by protein kinase A (PKA) inhibitors H89 and Rp-cAMPS. The PKC inhibitors also slowed the decay rate of the Ca2+ response induced by 300 microM ATP, and the PKA inhibitors increased it when induced by 30 microM ATP. In the measurements of PKA and PKC activity, 30 microM ATP activates only PKA, while 300 microM ATP activates both kinases. Taken together, these data suggest that the changes in the ATP-induced Ca2+ response result from differential modulation of ATP-induced capacitative Ca2+ entry by PKC and PKA in HL-60 cells.

Thrombin-dependent calcium signalling in single human erythroleukaemia cells

1. A combination of single cell fluorescence and patch clamp techniques were used to study the mechanisms underlying thrombin-evoked Ca2+ signals in human erythroleukaemia (HEL) cells, a leukaemic cell line of platelet-megakaryocyte lineage. 2. Thrombin caused a transient increase in intracellular Ca2+ ([Ca2+]i), consisting of both release of Ca2+ from intracellular stores and influx of extracellular Ca2+. Mn2+ quench studies indicated that the thrombin-evoked divalent cation-permeable pathway was activated during, but not prior to, release from internal stores. 3. Thapsigargin (1 microM) irreversibly released internal Ca2+ from the same store as that released by thrombin and continuously activated a Ca(2+)-influx mechanism. The amplitude of the thrombin- and thapsigargin-induced Ca2+ influx displayed a marked single cell heterogeneity which showed no correlation with the size of the store Ca2+ transient. 4. In whole-cell patch clamp recordings, both thrombin and thapsigargin evoked an inwardly rectifying Ca2+ current which developed with little or no increase in current noise, showed no reversal in the voltage range -110 to +60 mV and was blocked by 1 mM Zn2+. The apparent divalent cation permeability sequence of this pathway was Ca2+ > > Ba2+ > Mn2+, Mg2+. The thapsigargin-evoked current density at -100 mV varied between 0.42 and 2.1 pA pF-1 in different cells. Thrombin failed to activate additional Ca2+ current if it was added after the thapsigargin-induced inward current had fully developed. 5. These studies indicate that thrombin activates Ca2+ influx in HEL cells entirely via a Ca(2+)-store-release-activated Ca2+ current (Icrac) rather than via receptor-operated or second messenger-dependent Ca2+ channels. The level of expression of Icrac appears to be a major factor in determining the duration of the thrombin-evoked [Ca2+]i response and therefore represents a means by which cells can exert control over [Ca2+]i-dependent events.

Oligomycin inhibits store-operated channels by a mechanism independent of its effects on mitochondrial ATP

Inhibitors of mitochondrial oxidative metabolism have been proposed to interfere with Ca2+ influx mediated by store-operated channels (SOC), secondary to their effects on ATP production. We assessed SOC activity by 45Ca2+ influx and fluorimetric measurements of free Ca2+ or Mn2+ quench in thapsigargin-treated Chinese hamster ovary cells and Jurkat T-cells, and additionally by electrophysiological measurements of the Ca2+-release-activated Ca2+ current (Icrac) in Jurkat T-cells. Various mitochondrial antagonists were confirmed to inhibit SOC. However, the following evidence supported the proposal that oligomycin, in particular, exerts an inhibitory effect on SOC in addition to its known actions on mitochondria and Na+-pump activity: (i) the concentrations of oligomycin required to inhibit SOC-mediated Ca2+ influx or Icrac (half-inhibitory concentration approximately 2 microM) were nearly 50-fold higher than the concentrations that blocked mitochondrial ATP production; (ii) the rank order of potency of oligomycins A, B and C for decreasing SOC-mediated Ca2+ influx or Icrac differed from that known for inhibition of mitochondrial function; (iii) oligomycin blocked Icrac under voltage clamp and with intracellular Na+ and K+ concentrations fixed by dialysis from the patch pipette, arguing that the effect was not secondary to membrane polarization or pump activity; and (iv) fixing the cytosolic ATP concentration by dialysis from the patch pipette attenuated rotenone- but not oligomycin-mediated inhibition of Icrac. Oligomycin also blocked volume-activated Cl- currents, a profile common to some other known blockers of SOC that are not known mitochondrial inhibitors. These findings raise the possibility that oligomycin interacts directly with SOC, and thus may extend the known pharmacological profile for this type of Ca2+-influx pathway.

Metformin relaxes rat tail artery by repolarization and resultant decreases in Ca2+ influx and intracellular [Ca2+]

BACKGROUND

Metformin treatment of type II diabetes is frequently associated with decreases in blood pressure, an effect that could result from a direct action of metformin on arterial smooth muscle.

OBJECTIVE

To determine the mechanisms responsible for arterial smooth muscle relaxation induced by acute application of metformin and to evaluate the effect of insulin pretreatment on intracellular [Ca2+] ([Ca2+]i) and contraction in an intact artery.

METHODS

We stimulated intact deendothelialized rat tail artery with phenylephrine, relaxed the tissue by adding increasing concentrations of metformin, and measured the membrane potential (Em), Mn2+ influx, Fura 2-estimated [Ca2+]i, and isometric force. We also evaluated the effect of insulin pretreatment on aequorin-estimated [Ca2+]i in deendothelialized swine carotid artery.

RESULTS

In rat tail artery we found that a high concentration of metformin-induced repolarization associated with proportional decreases in Mn2+ influx, Fura 2-estimated [Ca2+]i, and isometric force. Incubation of swine carotid artery in 100 mU/ml insulin for 30 min or overnight (16-22 h) did not significantly alter histamine or high-K+-induced increases in [Ca2+]i or contraction.

CONCLUSION

These data suggest that acute administration of high concentrations of metformin induces rat tail artery relaxation primarily by repolarization. Additionally, we found that insulin was not vasoactive in the swine carotid artery. It is possible that insulin may alter [Ca2+]i handling in other arteries, in other species, or only in cultured smooth muscle.

Inositol-phosphoglycan inhibits calcium oscillations in hepatocytes by reducing calcium entry

Inositol-phosphoglycan (IPG) is a putative mediator of insulin action that has been shown to affect numerous biochemical processes. IPG, prepared from liver membranes, promptly inhibited phenylephrine- or vasopressin-induced [Ca2+]i oscillations when perfused over Fura-2-dextran injected rat hepatocytes. An antibody to IPG suppressed the inhibitory effect of insulin on the [Ca2+]i oscillations. Measurement of the rate of quench of cytoplasmic Fura-2 by extracellular Mn2+ showed that Ca2+ entry occurred continuously in the unstimulated cell and was not affected by phenylephrine or vasopressin. IPG, specifically, almost completely abolished the Mn2+ quench rate. Elevated extracellular [Ca2+] reversed the inhibitory effect of IPG on [Ca2+]i oscillations. We conclude that IPG inhibits the hepatocyte Ca2+ oscillatory by reducing the continuous Ca2+ influx that is required to sustain oscillations in [Ca2+]i.

Induction of cytosolic Ca2+ elevation mediated by Mas-7 occurs through membrane pore formation

Mas-7, a mastoparan derivative, induces elevation of intracellular free Ca2+ concentration ([Ca2+]i) along two independent pathways. The minor contribution occurs via phospholipase C activation and is negatively regulated by treatment with phorbol 12-myristate 13-acetate, a protein kinase C activator. The major contribution involves plasma membrane pores allowing not only Ca2+, Mn2+, and Na+ to enter but also the uptake of ethidium bromide (314 Da) and lucifer yellow (457 Da), but not fura-2 (831 Da), Evans blue (961 Da), and fluorescein-conjugate phalloidin (1,175 Da). Mas-7-induced current, as measured in planar lipid bilayers, reveals that Mas-7-induced pores have two slope conductances, 290 and 94 pS, and that the pores are nonselective for cations. The results also indicate that Mas-7 can produce pores by direct interaction with the plasma membrane without the involvement of membrane proteins and cytosolic factors. Besides in human neuroblastoma cells, similar Mas-7 effects were also observed in other cell lines such as HL-60, 1321N1 human astrocytoma, and bovine chromaffin cells. The data suggest that the Mas-7-induced [Ca2+]i elevation is the combined result of Ca2+ release from stores via phosphoinositide turnover and prolonged Ca2+ influx through membrane pores.

Effect of a prolonged glutamate challenge on plasmalemmal calcium permeability in mammalian central neurones. Mn2+ as a tool to study calcium influx pathways

The rate of Mn(2+)-induced fluorescence quenching (RFQ) was used as a relative measure of plasma membrane Ca2+ permeability (PCa) in fura-2-loaded cultured hippocampal neurons and cerebellar granule cells during and after protracted (15-30 min) glutamate (GLU) treatment. Some limitations of this method were evaluated using a kinetic model of a competitive binding of Mn2+ and Ca2+ to fura-2 in the cell. In parallel experiment a contribution of Ca2+ influx to the cytoplasmic Ca2+ ([Ca2+]i) was repeatedly examined during and following a prolonged GLU challenge by short-duration "low-Ca2+ trials" (50 microM EGTA) and by measurements of 45Ca2+ uptake. Experiments failed to reveal a putative persistent increase in PCa that earlier was thought to underlie Ca2+ overload of the neuron caused by its toxic GLU treatment. By contrast, a sustained increase of [Ca2+]i was found to be associated with a progressive decrease in PCa and Ca2+ influx both in the period of GLU application and after its termination. These findings give new evidence in favour of the hypothesis that the GLU-induced Ca2+ overload of the neuron mainly from an impairment of its Ca2+ extrusion systems.

A voltage-operable current is involved in Ca2+ entry in human lymphocytes whereas ICRAC has no apparent role

Presently, it is thought that a non-voltage-gated current is responsible for activation-induced Ca2+ entry in nonelectrically excitable cells such as lymphocytes. However, it has also been proposed that the pathway instead involves a second messenger-regulated Ca2+ channel that is voltage operable, where "voltage operable" is defined as an intrinsic property of the channel protein(s) rather than a requirement of normal gating. To evaluate the contribution of these currents to activation-induced Ca2+ influx, each was examined with respect to its ability to account for Ca2+ influx as reported by Ca(2+)-sensitive dyes. We identified a set of reagents, nordihydroguaiaretic acid and various calmodulin inhibitors, that inhibits Ca2+ entry and blocks the voltage-operable current but leaves the non-voltage-gated current unaltered. Further-more, nordihydroguaiaretic acid inhibited Ca(2+)-dependent proliferation of mitogen-activated human peripheral blood mononuclear cells or Jurkat T cells and specifically blocked Ca(2+)-dependent interleukin 2 production by Jurkat T cells to a degree similar to the immunosuppressant drug cyclosporin A. We also identified compounds, amiloride and Mn2+, that block the non-voltage-gated current but have no effect on either the voltage-operable current or Ca2+ entry. Correspondingly, amiloride had no effect on Ca(2+)-dependent proliferation of Jurkat cells. These observations imply that blockade of the non-voltage-gated current does not block either Ca2+ entry or Ca(2+)-dependent lymphocyte proliferation, whereas blockade of the voltage-operable current does. The data suggest that the voltage-operable current may be a mediator of activation-induced Ca2+ entry in lymphocytes.

Spatial Organization of Calcium Signaling Involved in Cell Volume Control in the Fucus Rhizoid

Subprotoplasts prepared from different regions of rhizoid and thallus cells of Fucus zygotes displayed mechanosensitive plasma membrane channels in cell-attached patch-clamp experiments by using laser microsurgery. In excised patches, this channel was found to be voltage gated, carrying K+ outward and Ca2+ inward, with a relative permeability of Ca2+/K+ of 0.35 to 0.5, and an increased open probability at membrane potentials more positive than -80 mV. No significant difference was found in the density of this channel type from different regions of rhizoid or thallus cells. Hypoosmotic treatment of intact zygotes induced dramatic transient elevations of cytoplasmic Ca2+, initiating at the rhizoid apex and propagating in a wavelike manner to subapical regions. Localized initiation of the Ca2+ transient correlated with greater osmotic swelling at the rhizoid apex compared with other regions of the zygote. Ca2+ transients exhibited a refractory period between successive hypoosmotic shocks, during which additional transients could not be elicited and the ability to osmoregulate was impaired. Buffering the Ca2+ transients with microinjected Br2BAPTA similarly reduced the ability of rhizoid cells to osmoregulate. Ca2+ influx was associated with the initiation of the Ca2+ transient in apical regions, whereas intracellular sources contributed to its propagation. Thus, localized signal transduction is patterned by interactions of the cell wall, plasma membrane, and intracellular Ca2+ stores.

Effects of external K+ on depletion-induced Ca2+ entry in rat ileal smooth muscle

The effects of K+ on Ca2+ influx after transient depletion of Ca2+ stores with carbachol and long-lasting depletion with thapsigarin or ryanodine were examined in fura-2-loaded rat ileal smooth muscle. After transient depletion of Ca2+ stores, application of Ca2+ caused a rise in [Ca2+]i and a contraction, both of which were increased with increasing K+ applied simultaneously in the absence of methoxyverapamil, but were decreased in its presence. In tissues, long-lasting depletion of Ca2+ stores treated with thapsigarin or ryanodine, [Ca2+]i and tension were dose dependently increased by the application of Ca2+ regardless of the absence or presence of methoxyverapamil. These responses were inhibited by K+ replacement of Na+ in a dose-dependent manner and the inhibitory action of K+ was attenuated by increasing extracellular Ca2+. The influx of Mn2+ was much greater in the tissues pretreated with thapsigarin or ryanodine than in untreated tissues. The enhanced Mn2+ influx was inhibited by the replacement of Na+ with K+. These results provide further evidence for the presence of a Ca2+ entry mechanism evoked by the depletion of Ca2+ stores in rat ileal smooth muscle, and suggest that there are two types of Ca2+ entry pathways to refill Ca2+ stores, one sensitive and the other insensitive to Ca2+ channel blockers. Ca2+ entry through the latter pathway is inhibited by increasing external K+, perhaps due to a reduction of the electrochemical gradient for Ca2+ across the plasma membrane.

Synergistic inhibition of thrombin-induced platelet aggregation by the novel nitric oxide-donor GEA 3175 and adenosine

1. The influence of the novel nitric oxide-donor GEA 3175 on thrombin- and ionomycin-stimulated human platelets was investigated. The effect of GEA 3175 was compared with that of adenosine, an activator of platelet adenylyl cyclase. 2. GEA 3175 inhibited thrombin-induced secretion of ATP but did not affect aggregation; similar results were obtained with adenosine. 3. Thrombin-stimulated rises in the cytosolic free Ca2+ concentration, [Ca2+]i, were dose-dependently inhibited by GEA 3175 and adenosine. GEA 3175 and adenosine maximally reduced the initial rise in [Ca2+]i by 41% and 35%, respectively. 4. Simultaneous exposure to GEA 3175 and adenosine nearly abolished both the functional responses (i.e. aggregation and degranulation) and the rises in [Ca2+]i in thrombin-stimulated platelets. 5. Aggregation and increases in [Ca2+]i triggered in platelets by the Ca(2+)-ionophore ionomycin were only marginally affected by a combination of GEA 3175 and adenosine. 6. GEA 3175 potently increased the guanosine 3':5'-cyclic monophosphate (cyclic GMP) content in platelets but did not affect adenosine 3':5'-cyclic monophosphate (cyclic AMP) levels. Adenosine did not increase either the cyclic AMP or the cyclic GMP levels in platelets. However, adenosine and GEA 3175 combined significantly elevated the platelet cyclic AMP content. 7. The results show that simultaneous exposure to GEA 3175 and adenosine promotes potent anti-aggregatory properties in platelets in vitro. The findings suggest that blockage of the cytosolic Ca(2+)-signal, which is probably mediated by an amplified cyclic nucleotide response, is an important event during the synergistic inhibition of thrombin-induced aggregation.

Oscillations of cytosolic free calcium in bombesin-stimulated HIT-T15 cells

The mechanism underlying the generation of cytosolic free Ca2+ ([Ca2+]i) oscillations by bombesin, a receptor agonist activating phospholipase C, in insulin secreting HIT-T15 cells was investigated. At 25 microM, 61% of cells displayed [Ca2+]i oscillations with variable patterns. The bombesin-induced [Ca2+]i oscillations could last more than 1 h and glucose was required for maintaining these [Ca2+]i fluctuations. Bombesin-evoked [Ca2+]i oscillations were dependent on extracellular Ca2+ entry and were attenuated by membrane hyperpolarization or by L-type Ca2+ channel blockers. These [Ca2+]i oscillations were apparently not associated with fluctuations in plasma membrane Ca2+ permeability as monitored by the Mn2+ quenching technique. 2,5-di-(tert-butyl)-1,4-benzohydroquinone (tBuBHQ) and 4-chloro-m-cresol, which interfere with intracellular Ca2+ stores, respectively, by inhibiting Ca(2+)-ATPase of endoplasmic reticulum and by affecting Ca(2+)-induced Ca2+ release, disrupted bombesin-induced [Ca2+]i oscillations. 4-chloro-m-cresol raised [Ca2+]i by mobilizing an intracellular Ca2+ pool, an effect not altered by ryanodine. Caffeine exerted complex actions on [Ca2+]i. It raised [Ca2+]i by promoting Ca2+ entry while inhibiting bombesin-elicited [Ca2+]i oscillations. Our results suggest that in bombesin-elicited [Ca2+]i oscillations in HIT-T15 cells: (i) the oscillations originate primarily from intracellular Ca2+ stores; and (ii) the Ca2+ influx required for maintaining the oscillations is in part membrane potential-sensitive and not coordinated with [Ca2+]i oscillations. The interplay between intracellular Ca2+ stores and voltage-sensitive and voltage-insensitive extracellular Ca2+ entry determines the [Ca2+]i oscillations evoked by bombesin.

U-73122, an aminosteroid phospholipase C inhibitor, may also block Ca2+ influx through phospholipase C-independent mechanism in neutrophil activation

1-[6-[[17 beta-3-Methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1 H-pyrrole-2,5-dione (U-73122) has been proven to be a useful tool in investigation of phospholipase C (PLC)-coupled signal transduction during cell activation. In the present studies, the inhibition by U-73122 of cytosolic free Ca2+ concentration ([Ca2+]i) of neutrophils was investigated. U-73122 suppressed the [Ca2+]i elevation of neutrophils suspended in Ca(2+)-containing medium challenged by N-formyl-Met-Leu-Phe (fMLP), cyclopiazonic acid (CPA) and ionomycin. The concentrations of U-73122 required for inhibition of CPA- and ionomycin-induced changes with IC50 values 4.06 +/- 0.27 microM and 4.04 +/- 0.44 microM, respectively, is almost 10-times that required for inhibition of the fMLP-induced response (IC50 value 0.62 +/- 0.04 microM). U-73122 also reduced the intracellular Ca2+ mobilization of neutrophils suspended in Ca(2+)-free medium stimulated by fMLP and CPA, but not by ionomycin, with IC50 values 0.52 +/- 0.02 microM and 6.82 +/- 0.74 microM, respectively. 1-[6-[[17 beta-3-Methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-2,5-pyrr olidinedione (U-73343), a close analog of U-73122 that does not inhibit PLC activity, suppressed the [Ca2+]i elevation of neutrophils challenged by fMLP in Ca(2+)-containing medium, but not in Ca(2+)-free medium, with IC50 value 22.30 +/- 1.61 microM. In Mn(2+)-quench studies, U-73122 suppressed the Mn2+ influx in CPA-activated neutrophils (IC50 value was 7.16 +/- 0.28 microM) as well as in resting neutrophils (IC50 value was 6.72 +/- 0.30 microM). U-73343 also suppressed the Mn2+ influx in resting neutrophils in a concentration-dependent manner. These results suggest that the inhibitory effect of U-73122 on [Ca2+]i of activated neutrophils is attributed partly to the suppression of Ca2+ release from the intracellular Ca2+ stores through PLC inhibition, and partly to the blockade, especially at higher concentrations, of Ca2+ entry from the extracellular space through PLC-independent processes.

Recombinant insulin-like growth factor-1 modulates aggregation in human platelets via extracellular calcium

Insulin like growth factor-1 (IGF-1) potentiated aggregation of human platelets induced by thrombin-, collagen- and ADP in a dose-dependent manner over the range 30-300 nM. IGF-1 (100 nM) reduced EC50 values for thrombin, collagen and ADP-induced aggregation by 19.6%, 53.6% and 22.8% respectively. Potentiation by IGF-1 was dependent on the presence of extracellular Ca2+ and was inhibited by verapamil or nifedipine. Further, IGF-1 enhanced the elevation in free intraplatelet Ca2+ induced by the platelet agonists collagen and thrombin.

Cytosolic calcium concentration in resting and stimulated endothelium of excised intact rat aorta

1. Optical fibres were used to excite and record fluorescence from the lumenal face of rat aorta or tail artery loaded with fura-2. 2. Acetylcholine (ACh) evoked an endothelium-dependent rise in the fura-2 340/380 nm excitation ratio in both vessels. High [K+] or phenylephrine evoked an endothelium-independent rise in ratio in tail artery but failed to increase the ratio in aorta. These observations indicate that fura-2 fluorescence and therefore cytosolic calcium concentration ([Ca2+]i) may be selectively recorded from the endothelium of intact rat aorta. 3. In aortic endothelium, resting [Ca2+]i was 95 +/- 8 nM (n = 44). ACh evoked a monophasic rise in [Ca2+]i which was temporally coincident with a membrane hyperpolarization. 4. ATP in most (22/35) preparations evoked a rise in [Ca2+]i which declined towards resting and was followed by a secondary rise. The biphasic [Ca2+]i responses were accompanied by biphasic electrical responses of initial hyperpolarization followed by depolarization above the resting potential and subsequent restoration towards rest. In the presence of high [K+] or the K+ ionophore valinomycin, ATP did not evoke changes in membrane potential and only monophasic rises in [Ca2+]i were observed. In some (7/35) preparations, ATP evoked oscillations in [Ca2+]i, with membrane potential oscillating in antiphase. 5. These data suggest interplay between [Ca2+]i and membrane potential in the generation of agonist-evoked responses in native endothelium in situ. The observed oscillations in [Ca2+]i imply spatio-temporal synchronization of Ca2+ signalling in large groups of endothelial cells in intact rat aorta.

Ca2+ entry through the store-mediated pathway directly activates only the K+ current but the subsequent Ca2+ release from the store activates both K+ and Cl- currents in submandibular gland acinar cells of the rat

The store-mediated Ca2+ entry was detected in single and cluster of rat submandibular acinar cells by measuring the Ca2+ activated ionic membrane currents. In the cells where intracellular Ca2+ was partly depleted by stimulation with submaximal concentration of acetylcholine (ACh) under a Ca2(+)-free extracellular condition, an employment of external Ca2+ in the absence of ACh caused a sustained increase of the K+ current without affecting the Cl- current. A renewed ACh challenge without external Ca2+ caused repetitive spikes of both K+ and Cl- currents due to the Ca2+ release. SK & F 96365 inhibited the generation of the sustained K+ current and refilling of the Ca2+ store following the Ca2+ readmission. It is suggested that the Ca2+ enters the cell through the store-mediated pathway new the K+ channels and is taken up by the store. Thus, only Ca2+ released from the store can activate both the K+ and Cl- currents.

Selective inhibition of thrombin receptor-mediated Ca2+ entry by protein kinase C beta

Thrombin initiates many physiological processes in platelets and other megakaryocyte-lineage cells by interacting with surface receptors and generating rises in cytoplasmic Ca2+; these rises result from both Ca2+ release from intracellular stores and receptor-mediated Ca2+ entry. Regulators that limit Ca2+ entry after its initiation by thrombin have not been identified. In this study, prevention of expression of a single protein kinase C isoenzyme (PKC beta) by antisense cDNA overexpressed in HEL cells, a human megakaryoblastic cell line that expresses thrombin receptors, promotes thrombin receptor-mediated Ca2+ entry without altering thrombin-induced intracellular release of Ca2+. The cytoplasmic Ca2+ rise initiated by endoperoxide analogs was not affected by inhibiting PKC beta. Overexpression of a cDNA encoding wild-type PKC beta mutated to prevent recognition by the antisense cDNA abolished the enhancement of Ca2+ influx following thrombin. Thus, PKC beta appears to be a specific negative regulator of thrombin receptor-mediated Ca2+ entry.

Two classes of agonist-sensitive Ca2+ stores in platelets, as identified by their differential sensitivity to 2,5-di-(tert-butyl)-1,4-benzohydroquinone and thapsigargin

In the absence of extracellular Ca2+, extensive Ca2+ release from the platelet intracellular stores [monitored as an increase of intracellular Ca2+ concentration ([Ca2+]i)] is produced by the combined action of the endomembrane Ca(2+)-ATPase inhibitor thapsigargin and 2 nM ionomycin. The titration of Ca2+ unloading with thapsigargin (plus ionomycin) shows that a substantial fraction of the store-associated Ca2+ is released by 8-10 nM thapsigargin, but that 100-200 nM thapsigargin is required for the complete release. The store depletion obtained in similar conditions with a different endomembrane Ca(2+)-ATPase inhibitor, 2,5-di-(tert-butyl)-1,4-benzohydroquinone (TBHQ), is always incomplete. It is completed by thrombin or by 10 nM thapsigargin. We conclude that two different types of Ca2+ pumps exist in platelets, one sensitive to TBHQ and to high thapsigargin, the other insensitive to TBHQ and sensitive to low thapsigargin. They are distributed separately in discrete subpopulations of the agonist-sensitive stores. The influx of external Ca2+ is maximal when both types of stores are Ca(2+)-depleted, either by high thapsigargin or by the combined action of low thapsigargin and TBHQ.

Platelet activation

This review article describes the different receptors, second-messengers and mechanisms involved in platelet activation. Several platelet agonists have well-defined receptors at the platelet membrane of which a number are single polypeptides with 7 hydrophobic transmembrane domains. These receptors are connected, via GTP regulatory proteins, with cytoplasmic second-messenger-generating enzymes. Phospholipase C and adenylate cyclase are the two best-known enzymes, generating inositol triphosphate (IP3) and diacyl glycerol from phosphatidylinositol biphosphate and cyclic AMP from ATP respectively. The intraplatelet free calcium level, which is critical for the activation status of the platelet, is increased by IP3 and is lowered in the presence of rising cyclic AMP concentrations. Shape-change occurs with small increases in intraplatelet calcium, while aggregation and secretion of granules take place at higher calcium, levels. The role of myosin and actin filaments and of transmembrane glycoproteins is further discussed.

Stimulus-Induced Oscillations in Guard Cell Cytosolic Free Calcium

Ca2+ is implicated as a second messenger in the response of stomata to a range of stimuli. However, the mechanism by which stimulus-induced increases in guard cell cytosolic free Ca2+ ([Ca2+]i) are transduced into different physiological responses remains to be explained. Oscillations in [Ca2+]i may provide one way in which this can occur. We used photometric and imaging techniques to examine this hypothesis in guard cells of Commelina communis. External Ca2+ ([Ca2+]e), which causes an increase in [Ca2+]i, was used as a closing stimulus. The total increase in [Ca2+]i was directly related to the concentration of [Ca2+]e, both of which correlated closely with the degree of stomatal closure. Increases were oscillatory in nature, with the pattern of the oscillations dependent on the concentration of [Ca2+]e. At 0.1 mM, [Ca2+]e induced symmetrical oscillations. In contrast, 1.0 mM [Ca2+]e induced asymmetric oscillations. Oscillations were stimulus dependent and modulated by changing [Ca2+]e. Experiments using Ca2+ channel blockers and Mn2+-quenching studies suggested a role for Ca2+ influx during the oscillatory behavior without excluding the possible involvement of Ca2+ release from intracellular stores. These data suggest a mechanism for encoding the information required to distinguish between a number of different Ca2+-mobilizing stimuli in guard cells, using stimulus-specific patterns of oscillations in [Ca2+]i.

Calcium influx, fertilisation potential and egg activation in Fucus serratus

Fertilisation in the marine alga Fucus serratus is accompanied by increased influx of Ca2+ from the external medium. The onset of this increase, monitored with the Mn2+ fluorescence quench technique, corresponded with the depolarisation phase of the fertilisation potential. External Ca2+ was necessary for the onset of the fertilisation potential and the early activation events, including cell wall exocytosis. Removal of Ca2+ from, or addition of Sr2+ to, the external medium during the fertilisation potential reduced the magnitude of the depolarisation and prolonged its duration. While fertilisation potentials could not be elicited in the presence of 0.1 mM Ca2+, addition of Ba2+ in the presence of 0.1 mM Ca2+ allowed normal fertilisation potential and egg activation. Microinjection of ryanodine or cyclic guanosine 5' -monophosphate (cGMP) did not induce cytoplasmic Ca2+ elevation or egg activation. Inositol 1,4,5-triphosphate [Ins(1,4,5)P3] produced a transient elevation of cytoplasmic Ca2+, monitored using ratio photometry, but did not cause cell wall exocytosis except at the site of microinjection. The results demonstrate an essential role for Ca2+ influx during Fucus egg activation. The relative importance of influx and intracellular Ca2+ release in Fucus egg activation is discussed.

Activation of Ca2+ influx by transforming Ha-ras

The effect of an induction of transforming Ha-ras on Ca2+ influx into NIH3T3 cells was studied employing Fura-2 quenching by Mn2+. The expression of transforming p21Ha-ras caused a significant increase in Mn2+ influx which was blocked by Cd2+, La3+, niguldipine and the Ca(2+)-channel blocker SK&F96365. This effect was specific for transforming Ha-ras and was not seen after overexpression of the Ha-ras proto-oncogene or v-mos. In addition to the enhanced Mn2+ influx, transforming p21Ha-ras elicited an increased efflux of the K(+)-congener 86Rb+ which was inhibitable by Ca(2+)-channel blockers and charybdotoxin, a selective inhibitor of high and intermediate conductance Ca(2+)-dependent K+ channels. Charybdotoxin did not reduce the increase in Mn2+ influx by ras, demonstrating that the activation of Ca(2+)-dependent K+ channels was not required for the sustained Mn2+/Ca2+ influx in the presence of transforming Ha-ras. In ras-expressing cells, the bradykinin-induced Mn2+ influx and charybdotoxin sensitive 86Rb+ efflux were markedly potentiated. The increase in the inositol- 1,4,5-trisphosphate and inositol-1,3,4,5-tetrakisphosphate levels by ras is not sufficient to explain the elevated Mn2+ influx. The mitogenic response to an expression of transforming Ha-ras was inhibited by the Ca(2+)-channel blockers not, however, by charybdotoxin. These data suggest the existence of an agonist-independent activation of a receptor- or second messenger-operated Ca2+ channel by transforming Ha-ras which is necessary for the mitogenic response to the activation of the oncogene.

Extracellular ATP inhibits agonist-induced mobilization of internal calcium in human platelets

Our previous studies have demonstrated that platelets possess ATP purinergic receptors in addition to the ADP, P2T, receptor. Occupancy of the P2 receptor by ATP inhibited agonist-induced platelet aggregation. This study demonstrated that the mechanism of inhibition may involve ATP inhibition of agonist-induced mobilization of internal calcium. Within the cardiovascular system, the ATP inhibition of calcium mobilization is unique to platelets. All other cell types in the cardiovascular system, where calcium mobilization is affected by extracellular ATP, responded with an increased mobilization as opposed to inhibition. The platelet inhibitory response to ATP was enhanced by the addition of an ATP generating system, creatine phosphate/phosphocreatine kinase. ATP and ATP analogues were found to inhibit calcium mobilization with a rank order of alpha beta-methylene ATP, beta gamma-methylene ATP approximately ATP > benzoyl ATP > 2 methylthio ATP which is a characteristic of P2x-like receptors. The inhibitory effect of ATP could be abrogated by prolonged treatment of platelets with the P2x desensitizing agent, alpha beta-methylene ATP. Also, UTP and CTP were approximately as effective inhibitors as ATP while GTP was not. ATP competition with ADP for the P2T receptor was excluded in studies with platelets derived from an aspirin-treated individual which were essentially insensitive to ADP. The agonist-induced calcium mobilization and inhibition by ATP occurred with the thromboxane A2 mimetic, U46619, collagen and thrombin; however, the kinetics of mobilization varied somewhat with the different agonists. The responses to extracellular ATP were independent of extracellular Ca2+, where 1 mM calcium or 0.3 mM EGTA was added to the reaction mixture. The inhibition of calcium mobilization coupled to inhibition of platelet aggregation by extracellular ATP may serve an important physiologic role. ATP, released from activated platelets at localized sites of vascular injury, may help to limit the size of the platelet plug-clot that, if left unregulated, could occlude the injured blood vessel.

Vasopressin-stimulated electrogenic sodium transport in A6 cells is linked to a Ca(2+)-mobilizing signal mechanism

Vasopressin is known to activate two types of cell surface receptors; V2, coupled to adenylate cyclase, and V1, linked to a Ca(2+)-dependent transduction system. We investigated whether arginine vasopressin (AVP) stimulation of electrogenic sodium transport in A6 cells, derived from Xenopus laevis, is mediated by activation of either one or both types of AVP-specific receptors. AVP caused a rapid increase in electrogenic sodium transport, reflected by the transepithelial potential difference (VT) and equivalent short circuit current (Ieq) measurements. AVP also rapidly increased intracellular Ca2+ (Ca2+i) and total inositol trisphosphate. The increase in Ieq was dependent on the rise in (Ca2+i), because 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) dose-dependently inhibited the Ieq response. There was no evidence, however, that activation of adenylate cyclase mediated AVP-stimulated Ieq; transport was not inhibited after AVP-induced activation of adenylate cyclase was abolished by 2',5'-dideoxyadenosine or when cAMP-dependent protein kinase (PKA) activity was abolished by the specific PKA inhibitor IP20. Further studies showed that although both forskolin and 8-(4-chlorophenylthio)-cAMP stimulated Ieq, this occurred by mechanisms independent of PKA activation. These results indicate that AVP-stimulated Na+ transport is mediated by a V1 receptor and a Ca(2+)-dependent mechanism.

Adenosine stimulation of Na+ transport is mediated by an A1 receptor and a [Ca2+]i-dependent mechanism

Studies were performed to determine the primary signal transduction mechanism that mediates adenosine stimulation of electrogenic sodium transport in renal epithelial cells. Experiments were performed on cultured amphibian A6 cells with an adenosine analogue that preferentially binds to the A1 receptor, cyclohexyladenosine (CHA). Sodium transport was assessed by the equivalent short circuit current (Ieq). CHA was found to stimulate Ieq via activation of an A1 receptor because (1) the threshold concentration was 1 nM compared to that of 10 microM for the specific A2 agonist CGS21680, (2) CHA inhibited vasopressin (AVP)-stimulated cAMP production by a pertussis toxin-sensitive mechanism, and (3) the action of CHA was inhibited by the A1 antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX). CHA increased intracellular Ca2+ ([Ca2+]i) and stimulated phosphoinositide turnover at concentrations that increased Ieq and in a time course that paralleled the increase in Ieq. Ion transport was stimulated by a Ca(2+)-dependent mechanism because the CHA induced increase in Ieq was inhibited by chelating [Ca2+]i with 5,5'dimethyl BAPTA in a dose-dependent manner, with a Ki of approximately 10 microM. The increase in Ieq was also dose-dependently inhibited by the specific PKC inhibitors dihydroxychlorpromazine and chelerythrine, and by trifluoperazine which inhibits PKC and calmodulin. Further studies indicated that CHA-stimulated Ieq was independent of cAMP generation because CHA did not induce an increase in cAMP accumulation parallel to the increase in Ieq in a dose-response analysis, and the adenylate cyclase inhibitor 2',5' dideoxy-adenosine (DDA) did not affect the CHA-induced increase in Ieq.(ABSTRACT TRUNCATED AT 250 WORDS)

Receptor occupancy regulates Ca2+ entry and intracellular Ca2+ redistribution in activated human platelets

Fura-2-loaded human platelets were used to study Ca2+ release from intracellular compartments, as well as Ca2+ influx from the extracellular space. We investigated the response towards the endoperoxide/thromboxane-receptor agonist. U46619, and the inhibitor of the endoplasmic-reticulum Ca(2+)-ATPase, thapsigargin. U46619 dose-dependently depleted intracellular Ca2+ stores, followed by active sequestration of released Ca2+. Ca2+ influx induced by U46619 largely relies on receptor occupancy. Removing the thromboxane analogue from its receptor by using the endoperoxide/thromboxane-receptor antagonist BM 13177 largely blunted U46619-mediated Ca2+ influx. The Ca(2+)-ATPase inhibitor thapsigargin evoked a gradual rise in intracellular Ca2+, which was potentiated by a preceding activation of platelets with the receptor agonist U46619. This agonist-sensitizing effect also depends on receptor occupancy. Removing U46619 from its receptor by addition of the endoperoxide/thromboxane-receptor antagonist BM13177 suppressed the sensitizing effect completely. Furthermore, interrupting downstream receptor signalling events by raising intracellular levels of cyclic nucleotides (cyclic AMP, cyclic GMP) again suppressed the U46619-sensitizing effect on thapsigargin-induced Ca2+ release. This study indicates that the process of Ca2+ release followed by resequestration in response to a platelet agonist by its own is not sufficient to produce the sensitizing effect. Rather, a continuously occupied receptor triggering sustained downstream signalling events seems to be required for sensitization. The presence of a receptor agonist may induce an increased cycling of Ca2+ between the agonist-responsive and the thapsigargin-dischargeable compartment, leading to faster and more intense accumulation of Ca2+ in the cytosolic compartment after inhibition of the Ca(2+) ATPase. Suggestively, receptor occupancy increases the Ca(2+)-releasing potency of thapsigargin by coupling the thapsigargin-sensitive Ca(2+)-storing compartments with an agonist-responsive compartment that exhibits a high leakage rate in stimulated platelets.

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

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

Ca2+ influx in platelets: activation by thrombin and by the depletion of the stores. Effect of cyclic nucleotides

In aspirin-treated platelets the thrombin-induced increase of cytosolic Ca2+ ([Ca2+]i) associated with the release from the intracellular stores is followed by a decrease to the baseline which is largely dependent on the re-uptake into the stores. This is shown by the further increase of [Ca2+]i upon inhibition of the endomembrane Ca(2+)-ATPase with thapsigargin. The re-uptake of Ca2+ into the stores is accelerated by sodium nitroprusside (SNP) or prostacyclin (PGI2). In all cases, after store depletion with thapsigargin the influx of external Ca2+ is maximal. After a thrombin-induced cycle of Ca(2+)-release re-uptake the stores are partly full: in these conditions the addition of external Ca2+ elicits a significant increment of [Ca2+]i and a further filling of the stores. Both are strongly reduced if Ca2+ addition is preceded by SNP or PGI2. Similar results are obtained also if (by supplementing and then cheleting Ca2+) the stores are as full as in native platelets at the moment of adding Ca2+. The thrombin-activated Ca2+ influx is reversed by hirudin. A PGI2- and SNP-sensitive Mn2+ influx is observed if Mn2+ is added in place of Ca2+. It is concluded that thrombin activates a cyclic nucleotide-sensitive Ca2+ (and Mn2+) influx pathway dependent on the occupancy of the thrombin receptor and independent of the filling state of the stores. In the absence of thrombin, thapsigargin releases Ca2+ relatively rapidly from a fraction of the stores; the remaining deposits are discharged much more slowly. This may indicate that platelets contain two distinct classes of agonist-sensitive stores. The addition of external Ca2+ (or Mn2+) at short or long incubation times with thapsigargin monitors the influx of Ca2+ activated by the depletion of one or both types of stores. The depletion of each type of store activates Ca2+ (Mn2+) influx. This type of cation influx is not inhibited by the cyclic nucleotides.

G-protein- and capacitatively regulated Ca2+ entry pathways are activated by muscarinic receptor stimulation in a human submandibular ductal cell line

In the human submandibular ductal cell line (HSG) thapsigargin and carbachol stimulated Ca2+ release from the internal Ca2+ pool, resulting in the activation of capacitatively regulated Ca2+ entry (CRCE). This entry pathway was permeant to both Ca2+ and Mn2+, blocked by Ni2+ and insensitive to the muscarinic antagonist, atropine. Carbachol also stimulated an increase in cytosolic [Ca2+] in internal Ca(2+)-pool-depleted (i.e. thapsigargin-treated) cells which was dependent on the presence of external Ca2+ and blocked by Ni2+, demonstrating that it was due to Ca2+ entry. However, under the same experimental conditions, carbachol was unable to stimulate Mn2+ entry. Additionally, this latter carbachol-stimulated Ca2+ entry pathway was blocked by atropine. Pretreatment of HSG cells with AlF4-increased basal rates of Mn2+ entry due to CRCE activation, but attenuated carbachol-stimulated Ca2+ entry into thapsigargin-treated cells. The data suggest that two distinct divalent cation entry pathways are activated in muscarinic-receptor-stimulated HSG cells; a CRCE mechanism, permeable to both Mn2+ and Ca2+, and a second entry mechanism, permeable only to Ca2+. The latter does not depend on internal pool depletion, but appears to be regulated via G-protein activation.

Inhibition of Ca2+ entry by Ca2+ overloading of intracellular Ca2+ stores in human platelets

1. This study examined the effect of overloading human platelet intracellular Ca2+ stores on the rate of agonist-evoked external Ca2+ entry. To overload the Ca2+ stores (presumably dense tubules), external Na(+)-dependent Ca2+ efflux via Na(+)-Ca2+ exchange was inhibited by pretreating the cells with ouabain or Na(+)-free medium. Ca2+ regulation was then examined after exposure to thrombin, ADP and thapsigargin. Cytosolic free Ca2+ levels were monitored using the fluorescent probe fura-2 and external Ca2+ influx was assessed by the rates of extracellular Mn2+ or 45Ca2+ uptake. 2. Both ouabain and Na(+)-free pretreatments caused a slight increase in the resting cytosolic free Ca2+. 3. In 1 mM Ca(2+)-containing medium, Ca(2+)-overloaded platelets showed similar thrombin-evoked cytosolic free Ca2+ responses to those of control platelets. However, in Ca(2+)-free medium, they showed substantially greater thrombin-evoked cytosolic free Ca2+ responses than control platelets. Moreover, increased thrombin-evoked Ca2+ mobilization from Ca2+ storage sites was accompanied by a diminished rate of thrombin-evoked external Ca2+ entry. 4. Similar reductions in the rate of external Ca2+ entry were observed after treatment with ADP and thapsigargin. 5. Protein kinase C inhibitors (calphostin C and staurosporine) failed to reverse the effect of ouabain pretreatment on thrombin-induced changes in the cytosolic free Ca2+ response. 6. Inositol 1,4,5-trisphosphate profiles in ouabain-treated and non-treated platelets were not significantly different. 7. These data indicate that increased Ca2+ in the dense tubules is associated with diminished agonist-evoked external Ca2+ influx.

Ca(2+)-permeable channel associated with platelet-derived growth factor receptor in mesangial cells

We used patch-clamp methods to study the effect of platelet-derived growth factor (PDGF) on Ca2+ entry in cultured rat glomerular mesangial cells. In cell-attached patches, application of 50 ng/ml PDGF-BB inside, but not outside, the pipette frequently induced channel openings. The unitary conductance was 0.67 +/- 0.09 pS (n = 8) with 110 mM Mn2+ and 1.03 +/- 0.19 pS (n = 11) with 110 mM Ca2+ as the charge carrier. Number of channels times open probability was 0.515 +/- 0.144 (n = 14) with intrapipette PDGF and 0.037 +/- 0.022 (n = 12) without. Channel kinetics were only slightly voltage dependent. There was no effect of replacing chloride with gluconate in excised inside-out patches, showing that the channel was cation selective. The permeability (P) ratio for PMn/PNa was 1.65 and for PCa/PNa was 1.24. With the use of amphotericin B " perforated" whole cell patches, PDGF induced a small inward current (-16.1 +/- 4.33 pA; n = 11, membrane potential = -70 mV) consistent with 3,000-4,000 channels/cell. In summary, we have described a very-low-conductance Ca(2+)-permeable channel in rat mesangial cells with the following properties. 1) Activation by PDGF-BB occurs only when applied in close proximity to the channel. 2) Once activated, open probability is only slightly voltage dependent. 3) Under normal circumstances, the channel would probably appear to be cation nonselective, but with a permeability to divalent more than monovalent cations. 4) This PDGF-induced channel could provide a ligand-gated pathway for Ca2+ entry into mesangial cells that does not require membrane depolarization.

Role of type 1 and type 2A phosphatases in signal transduction of platelet-activating-factor-stimulated rabbit platelets

Calyculin A, the potent inhibitor of type 1 (PP1) and type 2A (PP2A) phosphatases, has been employed in order to investigate the role of endogenously activated PP1/PP2A in the signal-transduction pathway of platelet-activating-factor (PAF)-stimulated platelets. Calyculin A alone caused an increase in protein phosphorylation in unstimulated platelets, with the detection of a number of newly phosphorylated proteins, whereas in PAF-stimulated platelets phosphorylation of the major substrates of protein kinase C and myosin light-chain kinase were no longer transient, but phosphorylation was sustained. PP1/PP2A appear to play a role in Ca2+ homoeostasis, as inhibition of PP1/PP2A caused an inhibition of Ca2+ mobilization and Ca2+ influx through the plasma membrane in PAF-stimulated platelets. The effect of calyculin A on Ca2+ mobilization correlated with the observed inhibition of the production of the signal molecule Ins(1,4,5)P3. The release reaction (which is a Ca(2+)-dependent event) was also inhibited by calyculin A. The results are discussed in relation to the possible role of protein kinase C in mediating the events leading to the effects observed with calyculin A.

Indirect regulation of Ca2+ entry by cAMP-dependent and cGMP-dependent protein kinases and phospholipase C in rat platelets

The Ca2+ responses of rat platelets are dominated by the influx of extracellular Ca2+ across the plasma membrane [Heemskerk, J. W. M., Feijge, M. A. H., Rietman, E. & Hornstra, G. (1991) FEBS Lett. 284, 223], which allows the study of Ca2+ entry into these cells by measuring increases in cytosolic Ca2+ concentration, [Ca2+]i. Several pieces of evidence indicated that, as in human platelets [Sage, S. O., Reast, R., & Rink, T. J. (1990) Biochem. J. 265, 675-680; Alonso, M., Alvarez, J., Montero, M., Sanchez, A. & García-Sancho, J. (1991) Biochem. J. 280, 783-789], agonist-stimulated Ca2+ entry was linked to the mobilisation of Ca2+ from intracellular stores: there was good correlation between the potency of receptor agonists in elevating [Ca2+]i in the presence or absence of external CaCl2; agonist-induced Ca2+ entry was inhibited to a similar degree as internal mobilisation by activators of cAMP-dependent or cGMP-dependent protein kinase or by the phospholipase C inhibitor, U73122; thapsigargin (an inhibitor of endomembrane Ca(2+)-ATPases) evoked store depletion and Ca2+ entry, which were both reduced by prior activation of cAMP-dependent or cGMP-dependent protein kinase but were not affected by U73122. In platelets with depleted Ca2+ stores, the addition of CaCl2 resulted in a considerable entry of Ca2+ which was insensitive to cAMP-dependent and cGMP-dependent protein kinase activation. In control platelets with full Ca2+ stores, CaCl2 potentiated the thrombin-induced generation of myo-inositol phosphates, suggesting that Ca2+ entry potentiated phospholipase C activity. Taken together, these results indicate that Ca2+ entry in rat platelets, (a) is mostly secondary to store depletion, (b) is not directly downregulated by cAMP-dependent and cGMP-dependent protein kinase, but indirectly by inhibition of store depletion, (c) can proceed in the absence of phospholipase C activation, but is stimulated by this activity probably by increased mobilisation of Ca2+ from the stores. These results lead to the concept that a major part of receptor-mediated Ca2+ entry in rat platelets is regulated in an indirect way by factors that stimulate or inhibit the degree of Ca2+ mobilisation from the internal stores.

Voltage-independent calcium channels mediate slow oscillations of cytosolic calcium that are glucose dependent in pancreatic beta-cells

Pancreatic beta-cells and HIT-T15 cells exhibit oscillations of cytosolic calcium ([Ca2+]i) that are dependent on glucose metabolism and appear to trigger pulsatile insulin secretion. Significantly, differences in the pattern of this [Ca2+]i oscillatory activity may have important implications for our understanding of how glucose homeostasis is achieved during the feeding and fasting states. When single beta-cells are exposed to a stepwise increase in glucose concentration that mimics the transition from fasting to feeding states, fast irregular oscillations of [Ca2+]i are observed. Alternatively, when single beta-cells are equilibrated in a steady state concentration of glucose that mimics the fasting state, slow periodic oscillations of [Ca2+]i are noted. Here we report a fundamental difference in the mechanism by which glucose induces these two types of [Ca2+]i oscillatory activity. In agreement with previous studies, we substantiate a role for L-type voltage-dependent Ca2+ channels as mediators of the fast oscillations of [Ca2+]i observed after a stepwise increase in glucose concentration. In marked contrast, we report that voltage-independent calcium channels (VICCs) mediate slow oscillations of [Ca2+]i that occur when beta-cells are equilibrated in steady state concentrations of glucose. Slow [Ca2+]i oscillations are mediated by VICCs which are pharmacologically and biophysically distinguishable from voltage-dependent Ca2+ channels that mediate fast oscillations. Specifically, slow [Ca2+]i oscillations are blocked by extracellular La3+, but not by nifedipine, and are independent of changes in membrane potential. Measurement of membrane conductance also indicate an important role for VICCs, as demonstrated by a steady state inward Ca2+ current that is blocked by La3+. The steady state Ca2+ current appears to generate slow [Ca2+]i oscillations by triggering Ca(2+)-induced Ca2+ release from intracellular Ca2+ stores, a process that is mimicked by extracellular application of caffeine, a sensitizer of the ryanodine receptor/Ca2+ release channel. Depletion of intracellular Ca2+ stores with thapsigargin stimulated Mn2+ influx, suggesting the presence of Ca(2+)-release-activated Ca2+ channels. Taken together, these observations are consistent with a role for VICCs (possibly G-type channels) and/or Ca(2+)-release-activated Ca2+ channels as mediators of slow [Ca2+]i oscillations in beta-cells. We propose that slow oscillations of [Ca2+]i probably serve as important initiators of insulin secretion under conditions in which tight control of glucose homeostasis is necessary, as is the case during the fasting normoglycemic state.

Cytosolic calcium elevation in response to Fc receptor cross-linking in undifferentiated and differentiated U937 cells

We have used the calcium indicator, Fura-2, to investigate cytosolic calcium responses to cross-linking of monomeric IgG-occupied surface Fc gamma receptors (Fc gamma R), using populations of the human monocyte-like cell line, U937. The magnitude and duration of the calcium response observed, and the relative contribution to the response of internal stores and external calcium, are found to depend on the state of differentiation of these cells. Initial release of calcium from stores following Fc gamma R cross-linking is enhanced by prior treatment of U937 cells with both interferon-gamma, and, to a lesser extent, with dibutyryl cAMP. A large and prolonged entry of external calcium is observed in dibutyryl cAMP treated cells; this may be due to direct regulation of calcium channels by the low affinity receptor, Fc gamma RII (whose expression is up-regulated in these cells), since the smaller and more transient entry observed in interferon-gamma treated cells, (where the high affinity receptor, Fc gamma RI, is up-regulated) argues against a common pathway of store-mediated calcium entry.

Antiplatelet activity of tetramethylpyrazine

Tetramethylpyrazine (TMPZ) is an active ingredient of a Chinese herbal medicine. We have previously shown that TMPZ possesses antithrombotic activity in rat thrombotic models. Inhibition of platelet aggregation and promotion of disaggregation contributed to the antithrombotic activity of TMPZ. In this study, we evaluated the antiplatelet actions of TMPZ using human platelets and found that antiplatelet activity of TMPZ included the inhibition of intracellular Ca++ mobilization (mainly the release from internal stores), the enhancement of intracellular cAMP by inhibition of phosphodiesterase activity, and the reduction of GP IIb/IIIa exposure on the surface of activated platelets.

Insulin inhibits serotonin-induced Ca2+ influx in vascular smooth muscle

BACKGROUND

Insulin in physiological concentrations attenuates the agonist-induced intracellular Ca2+ ([Ca2+]i) transient and inhibits contraction in individual nonproliferated cultured canine femoral artery vascular smooth muscle cells (VSMCs). In the present study, we wished to define the effects of insulin on individual components of Ca2+ transport in vascular smooth muscle.

METHODS AND RESULTS

Insulin (40 microU/mL) attenuated the 5-hydroxytryptamine (5-HT, serotonin; 10(-5) mol/L)-induced [Ca2+]i transient (measured by fura 2 fluorescence) in primary confluent canine femoral artery VSMCs in the presence of extracellular Ca2+. In Ca(2+)-free media, the 5-HT-induced [Ca2+]i transient was reduced by 42% and was not affected by insulin. This finding suggested that insulin inhibits 5-HT-induced Ca2+ influx but does not affect sarcolemmal Ca2+ efflux or Ca2+ release from intracellular stores. In support of those conclusions, we found that insulin inhibited the 5-HT-induced component of Mn2+ (a Ca2+ surrogate) influx (measured by fura 2 fluorescence quenching at the Ca2+ isosbestic excitation wavelength). In addition, 5-HT stimulated the rates of 45Ca2+ efflux from intact cells (a measure of sarcolemmal Ca2+ efflux) and from saponin-permeabilized cells (a measure of Ca2+ release from intracellular stores), but insulin did not affect these rates of 45Ca2+ efflux.

CONCLUSIONS

We conclude that a physiological insulin concentration attenuates the 5-HT-induced [Ca2+]i transient in confluent primary cultured canine femoral artery VSMCs by inhibiting the 5-HT-induced component of Ca2+ influx but not by affecting sarcolemmal Ca2+ efflux or Ca2+ release from intracellular stores.

Temporal relationships between Ca2+ store mobilization and Ca2+ entry in an exocrine cell

Consideration of the principal current models for agonist-induced activation of Ca2+ entry in electrically non-excitable cells suggests that it may be possible to distinguish between them on the basis of predicted differences in the temporal relationship(s) between intracellular Ca2+ release and the activation of Ca2+ entry. Measurements of changes in [Ca2+]i and Mn2+ quench in individual exocrine cells from the avian nasal gland indicate that, whereas Ins(1,4,5)P3-induced release of intracellular Ca2+ occurs within 3-5 s, the increase in Mn2+ quench is delayed by some 20-30 s. Mn2+ quench rate is similarly increased by thapsigargin, and is blocked by SK&F 96365, indicating that the increased Mn2+ quench observed genuinely reflects agonist-enhanced activity of the divalent cation entry pathway normally traversed by Ca2+. Additional experiments indicate that the observed delay is not due to inhibition of this pathway by elevated [Ca2+]i. Furthermore, the delay cannot be explained by the time required for Ins(1,3,4,5)P4 generation, which is essentially maximal within 10 s of agonist addition. It is concluded that the observed delay in the activation of the Ca2+ entry pathway is best explained by 'capacitative' models where increased entry requires the generation, and transmission to the plasma membrane, of an unknown messenger as a direct result of the depletion of intracellular Ca2+ stores.

Effect of picotamide on the calcium mobilization and phospholipase C activation in human platelets

The effect of picotamide (G137 or N,N'-bis-3-picolyl-4-methoxyisophthalamide), a dual thromboxane A2 (TxA2) synthetase inhibitor/TxA2 endoperoxide receptor antagonist, on the phospholipase C (PLC) activation and calcium mobilization in human platelets stimulated by arachidonic acid (AA) and TxA2 receptor synthetic agonist U46619, has been studied. Preincubation with picotamide (10(-4) M) for 1 and 3 min significantly reduced (p < 0.03 and p < 0.005 respectively) the calcium concentration changes induced in gel-filtered platelets (GFPs) by U46619 125 nM and 250 nM. Picotamide also reduced the calcium concentration changes induced in GFPs by AA 75 and 150 microM and by ADP 5 and 10 microM. In thrombin degranulated platelets picotamide inhibited the effect of U46619 up to 500 nM. The PLC activation, as indicated by inositol-1,3,4 P3 (Ins 1,3,4 P3) formation in response to U46619 250 nM and AA 150 microM was also inhibited by picotamide. These results may suggest a dual effect of picotamide on the receptor/effector systems through which TxA2 mediates platelet activation.

Ca2+ efflux from platelets. Control by protein kinase C and the filling state of the intracellular Ca2+ stores

Large amounts of Ca2+ (almost 20 nmol/10(8) cells) are released from platelets by exocytosis. This secretory-granule-associated Ca2+ does not contribute to the cytosolic free Ca2+ ([Ca2+]i), which is controlled by the much smaller agonist-sensitive Ca2+ pool, unless high (1 microM), but not low (0.04 microM) concentrations of ionomycin are present. Low concentrations of ionomycin release Ca2+ almost exclusively from the agonist-sensitive stores. In aspirinated platelets incubated in the presence of 0.5 mM EGTA the extensive depletion of the agonist-sensitive stores is obtained by the combined action of low ionomycin and the endomembrane Ca(2+)-ATPase inhibitor thapsigargin (which individually promote only a partial depletion). The subsequent decay of [Ca2+]i is increased by phorbol-myristate acetate, confirming that Ca2+ efflux from platelets is potentiated by the activation of protein kinase C [Pollock, W. K., Sage, S. O. & Rink, T. J. (1987) FEBS Lett. 210, 132-140]. A novel type of control of Ca2+ efflux appears to be exerted by the filling state of the stores. Treatment with low ionomycin or thapsigargin determines the release of a fraction of the stores-associated Ca2+; the subsequent decay of [Ca2+]i is slow. The decay rate of [Ca2+]i accelerates after extensive depletion of the stores following the addition of thapsigargin or ionomycin. If the depletion of the stores is induced by thrombin, added alone or in combination with thapsigargin, the increases of [Ca2+]i are the same and the subsequent decay rates are largely superimposable; however a large fraction of [Ca2+]i is reaccumulated into the stores in the absence, but not in the presence of thapsigargin, indicating that Ca2+ efflux is activated when the stores are empty. Ca2+ efflux can proceed against a concentration gradient. In 45Ca-loaded platelets, the thrombin-promoted 45Ca efflux is potentiated by thapsigargin. The protein-kinase-C-dependent and store-depletion-dependent stimulations of 45Ca efflux are additive. These observations indicate that, in addition to being activated by protein kinase C, Ca2+ efflux from platelets is activated by the depletion of the stores. The two activations appear to be additive.

Gamma interferon activates a previously undescribed Ca2+ influx in T lymphocytes from patients with multiple sclerosis

Multiple sclerosis (MS) is an immune-mediated demyelinating disease of the central nervous system. The etiology of the disease is still unknown. Activated T lymphocytes are considered essential in mediating the inflammatory process leading to demyelination in MS. They operate through a complex network of cytokines among which gamma interferon (gamma-IFN) plays a key role. Here we report that exposure to gamma-IFN of T lymphocytes from patients with MS activates, by a protein kinase C-mediated pathway, a previously undescribed gamma-IFN-activated Ca2+ influx, functionally coupled to the gamma-IFN receptor. The influx mainly expressed by CD4+ T lymphocytes, was found in 12 of 15 (80%) patients with clinically active MS and in 14 of 30 (46%) patients with stable MS. The influx was found in only 3 of 24 (12%) control patients and in none of the 15 healthy subjects studied. Our results document the appearance in MS lymphocytes of a gamma-IFN-activated, protein kinase C-dependent, Ca2+ influx that might be due to the expression of a new cation-specific plasmalemma channel. This finding suggests that at least part of gamma-IFN's contribution to the pathogenesis of MS is exerted through a Ca(2+)-dependent regulation of T lymphocyte activity.

Phosphatase inhibitors suppress Ca2+ influx induced by receptor-mediated intracellular Ca2+ store depletion in human platelets

The effects of three phosphatase inhibitors including okadaic acid, calyculin A and tautomycin were evaluated on platelet Ca2+ mobilization. Calyculin A and tautomycin at appropriate concentrations appeared to have a selective inhibitory effect on thrombin-induced Ca2+ influx, but not on [Ca2+]i release from intracellular Ca2+ storage sites. In contrast, pretreatment with okadaic acid at concentrations that effectively lowered Ca2+ influx also suppressed Ca2+ release from intracellular Ca2+ stores. In a system that specifically evaluates the effects of agents on Ca2+ influx induced by the Ca(2+)-depleted state of intracellular Ca2+ storage sites, the three phosphatase inhibitors attenuated Ca2+ influx in a dose dependent manner and showed complete inhibition at appropriate concentrations. These findings suggest that protein phosphorylation/dephosphorylation plays an important role in mediating signals to open Ca2+ channels when Ca2+ depletion in intracellular Ca2+ stores is caused by thrombin. In contrast, Ca2+ influx induced by thapsigargin, a Ca(2+)-ATPase inhibitor, was only partially suppressed by pretreatment with each of the three phosphatase inhibitors. Based on these findings, we suggest that the Ca(2+)-depleted state of intracellular Ca2+ stores by thapsigargin induces the opening of Ca2+ channels via phosphatase inhibitor-insensitive pathways. All the phosphatase inhibitors, at the highest concentrations tested in the present study, only partially inhibited Mn2+ entry induced by thrombin. These findings suggest that there are at least two types of divalent ion channels on platelet plasma membranes and that one of them, that preferentially allows Mn2+ entry, is resistant to the inhibitory effects of phosphatase inhibitors.