A quantitative investigation into the dependence of Ca2+ mobilisation on changes in inositol 1,4,5-trisphosphate levels in the stimulated neutrophil

Measurement of phospholipid metabolism in intact neutrophils

Phospholipid-metabolizing enzymes are important participants in neutrophil signal transduction pathways. The methods discussed herein describe assays for assessing the activities of phospholipase A2 (PLA2), phospholipase C (PLC), phospholipase D (PLD), and phosphoinositide 3-OH-kinase in intact neutrophils. PLA2 activity is measured as the release of radiolabeled arachidonic acid. PLC activity is measured as the accumulation of inositol 1,4,5-trisphosphate (IP3), a water-soluble product, using a commercially available radioreceptor assay kit. PLD activity is measured as the appearance of its radiolabeled products, phosphatidic acid and phosphatidylethanol. PI3-K activity is measured as the appearance of its radiolabeled product, phosphatidylinositol-3,4,5-trisphosphate.

Protein kinase C promotes restoration of calcium homeostasis to platelet activating factor-stimulated human neutrophils by inhibition of phospholipase C

BACKGROUND

The role of protein kinase C (PKC) in regulating the activity of phospholipase C (PLC) in neutrophils activated with the chemoattractant, platelet-activating factor (PAF, 20 and 200 nM), was probed in the current study using the selective PKC inhibitors, GF10903X (0.5 - 1 muM) and staurosporine (400 nM).

METHODS

Alterations in cytosolic Ca2+, Ca2+ influx, inositol triphosphate (IP3), and leukotriene B4 production were measured using spectrofluorimetric, radiometric and competitive binding radioreceptor and immunoassay procedures, respectively.

RESULTS

Activation of the cells with PAF was accompanied by an abrupt increase in cytosolic Ca2+ followed by a gradual decline towards basal levels. Pretreatment of neutrophils with the PKC inhibitors significantly increased IP3 production with associated enhanced Ca2+ release from storage vesicles, prolongation of the peak cytosolic Ca2+ transients, delayed clearance and exaggerated reuptake of the cation, and markedly increased synthesis of LTB4. The alterations in Ca2+ fluxes observed with the PKC inhibitors were significantly attenuated by U73122, a PLC inhibitor, as well as by cyclic AMP-mediated upregulation of the Ca2+-resequestering endomembrane ATPase.Taken together, these observations are compatible with a mechanism whereby PKC negatively modulates the activity of PLC, with consequent suppression of IP3 production and down-regulation of Ca2+ mediated pro-inflammatory responses of PAF-activated neutrophils.

CONCLUSION

Although generally considered to initiate and/or amplify intracellular signalling cascades which activate and sustain the pro-inflammatory activities of neutrophils and other cell types, the findings of the current study have identified a potentially important physiological, anti-inflammatory function for PKC, at least in neutrophils.

Measurement of phospholipid metabolism in intact neutrophils

Phospholipid metabolizing enzymes are important participants in neutrophil signal transduction pathways. The methods discussed herein describe assays for assessing the activities of phospholipase (PL)A2, PLC, PLD, and phosphoinositide 3-OH-kinase (PI3-K) in intact neutrophils. PLA2 activity is measured as the release of radiolabed arachidonic acid. PLC activity is measured as the accumulation of inositol 1,4,5-trisphosphate (IP3), a water-soluble product, using a commercially available radioreceptor assay kit. PLD activity is measured as the appearance of its radiolabeled products, phosphatidic acid and phosphatidylethanol. PI3-K activity is measured as the appearance of its radiolabeled product, phosphatidylinositol-3,4,5-trisphosphate.

Interaction between catalytically inactive calpain and calpastatin. Evidence for its occurrence in stimulated cells

Conformational changes in the calpain molecule following interaction with natural ligands can be monitored by the binding of a specific monoclonal antibody directed against the catalytic domain of the protease. None of these conformational states showed catalytic activity and probably represent intermediate forms preceding the active enzyme state. In its native inactive conformation, calpain shows very low affinity for this monoclonal antibody, whereas, on binding to the ligands Ca(2+), substrate or calpastatin, the affinity increases up to 10-fold, with calpastatin being the most effective. This methodology was also used to show that calpain undergoes similar conformational changes in intact cells exposed to stimuli that induce either a rise in intracellular [Ca(2+)] or extensive diffusion of calpastatin into the cytosol without affecting Ca(2+) homeostasis. The fact that the changes in the calpain state are also observed under the latter conditions indicates that calpastatin availability in the cytosol is the triggering event for calpain-calpastatin interaction, which is presumably involved in the control of the extent of calpain activation through translocation to specific sites of action.

Taming the neutrophil: calcium clearance and influx mechanisms as novel targets for pharmacological control

Neutrophils are relatively insensitive to the anti-inflammatory actions of conventional chemotherapeutic agents, including corticosteroids, emphasizing the requirement for novel pharmacological strategies to control the potentially harmful proinflammatory activities of these cells. In the case of commonly-occurring inflammatory diseases of the airways, the neutrophil is the primary mediator of inflammation in conditions such as chronic obstructive pulmonary disease, cystic fibrosis, acute respiratory distress syndrome, bronchiectasis and non-eosinophilic bronchial asthma. Recent insights into the mechanisms utilized by neutrophils to restore Ca(2+) homeostasis following activation with Ca(2+)-mobilizing, proinflammatory stimuli have facilitated the identification of novel targets for anti-inflammatory chemotherapy in these cells. The most amenable of these from a chemotherapeutic perspective, is the cyclic AMP-dependent protein kinase-modulated endomembrane Ca(2+)-ATPase which promotes clearance of the cation from the cytosol of activated neutrophils. Second generation type 4 phosphodiesterase inhibitors and adenosine receptor agonists operative at the level of subtype A2A adenosine receptors, which are currently undergoing clinical and preclinical assessment respectively, hold promise as pharmacologic modulators during the restoration of Ca(2+) homeostasis. If this promise is realized, it may result in novel chemotherapeutic strategies for the control of hyperacute and chronic inflammatory conditions in which neutrophils are primary offenders. Alternative, potential future targets include the Na(+), Ca(2+)-exchanger and store-operated Ca(2+) channels, which cooperate in the refilling of intracellular Ca(2+) stores.

Inositol 1,4,5-triphosphate-mediated shuttling between intracellular stores and the cytosol contributes to the sustained elevation in cytosolic calcium in FMLP-activated human neutrophils

The current study was designed to probe Ca2+ shuttling between intracellular stores and the cytosol as a potential mechanism contributing to the prolongation of elevated Ca2+ transients in N-formyl-L-methionyl-L-leucyl-L-phenylalanine (FMLP)-activated human neutrophils. Cytosolic Ca2+ concentrations and transmembrane fluxes of the cation were measured using spectrofluorimetric and radiometric procedures, respectively, while inositol 1,4,5-triphosphate (IP3) was measured using a radioreceptor assay. The Ca2+-chelating agent, ethylene glycol-bis (beta-aminoethyl ether) N,N,N'N'-tetraacetic acid (EGTA; 10mM), was used to exclude store-operated influx of Ca2+ into neutrophils, while the IP3 receptor antagonist, 2-aminoethoxydiphenyl borate (2-APB, 100 microM), added to the cells 10s after FMLP (0.01 and 1 microM), at which time the increases in IP3 and cytosolic Ca2+ were maximal, was used to eliminate both sustained release from stores and influx of Ca2+. Addition of FMLP at 0.01 or 1 microM resulted in equivalent peak increases in cytosolic Ca2+, while the increase in IP3 was greater and the rate of clearance of Ca2+ from the cytosol slower, in cells activated with 1 microM FMLP. Treatment of the cells with either EGTA or 2-APB following addition of 1 microM FMLP, completely (EGTA) or almost completely (2-APB) abolished the influx of Ca2+ and accelerated the rate of clearance of the cation from the cytosol. Post-peak cytosolic Ca2+ concentrations were lower, and the Ca2+ content of the stores higher, in cells treated with 2-APB. The involvement of IP3 was confirmed by similar findings in cells treated with U-73122 (1 microM), a selective inhibitor of phospholipase C. Taken together, these observations are compatible with IP3-mediated Ca2+ shuttling in neutrophils activated with FMLP.

Role of Ca2+ feedback on single cell inositol 1,4,5-trisphosphate oscillations mediated by G-protein-coupled receptors

The dynamics of inositol 1,4,5-trisphosphate (Ins (1,4,5)P3) production during periods of G-protein-coupled receptor-mediated Ca2+ oscillations have been investigated using the pleckstrin homology (PH) domain of phospholipase C (PLC) delta1 tagged with enhanced green fluorescent protein (eGFP-PHPLCdelta1). Activation of noradrenergic alpha1B and muscarinic M3 receptors recombinantly expressed in the same Chinese hamster ovary cell indicates that Ca2+ responses to these G-protein-coupled receptors are stimulus strength-dependent. Thus, activation of alpha1B receptors produced transient base-line Ca2+ oscillations, sinusoidal Ca2+ oscillations, and then a steady-state plateau level of Ca2+ as the level of agonist stimulation increased. Activation of M3 receptors, which have a higher coupling efficiency than alpha1B receptors, produced a sustained increase in intracellular Ca2+ even at low levels of agonist stimulation. Confocal imaging of eGFP-PHPLCdelta1 visualized periodic increases in Ins(1,4,5)P3 production underlying the base-line Ca2+ oscillations. Ins(1,4,5)P3 oscillations were blocked by thapsigargin but not by protein kinase C down-regulation. The net effect of increasing intracellular Ca2+ was stimulatory to Ins(1,4,5)P3 production, and dual imaging experiments indicated that receptor-mediated Ins(1,4,5)P3 production was sensitive to changes in intracellular Ca2+ between basal and approximately 200 nM. Together, these data suggest that alpha1B receptor-mediated Ins(1,4,5)P3 oscillations result from a positive feedback effect of Ca2+ onto phospholipase C. The mechanisms underlying alpha1B receptor-mediated Ca2+ responses are therefore different from those for the metabotropic glutamate receptor 5a, where Ins(1,4,5)P3 oscillations are the primary driving force for oscillatory Ca2+ responses (Nash, M. S., Young, K. W., Challiss, R. A. J., and Nahorski, S. R. (2001) Nature 413, 381-382). For alpha1B receptors the Ca2+-dependent Ins(1,4,5)P3 production may serve to augment the existing regenerative Ca2+-induced Ca2+-release process; however, the sensitivity to Ca2+ feedback is such that only transient base-line Ca2+ spikes may be capable of causing Ins(1,4,5)P3 oscillations.

Intracellular sphingosine 1-phosphate production: a novel pathway for Ca2+ release

Sphingolipids such as sphingosine 1-phosphate (SPP) and sphingosylphosphorylcholine have long been recognized to possess Ca2+ mobilizing activity, yet to date little is known about their mechanism of action, or indeed their significance as Ca2+ mobilizing intracellular messengers. The recent discovery of extracellular receptors for the sphingolipids has further complicated the interpretation of many experiments in this field. This paper reviews the current literature in which molecular and pharmacological approaches have begun to uncover the signalling components associated with intracellular SPP production and Ca2+ mobilization. The functional significance of this novel Ca2+ release pathway is also discussed.

Cellular responses to FMLP challenging: a mini-review

FMLP (N-formyl-methionyl-leucyl-phenylalanine) and other N-formylpeptides are powerful "activators" of polymorphonuclear and mononuclear phagocytes, but they are also active on other cell types. Present knowledge about formylpeptide receptors and the relevant tools for their imaging and the study of their dynamics are briefly discussed. The main responses elicited by FMLP in granulocytes are cell polarisation, the generation of reactive oxygen species, the production of arachidonic acid metabolites, and the release of lysosomal enzymes. The transduction cascades involved and the agents able to modulate these responses are reviewed. Homologous desensitization and heterologous desensitization of the FMLP-receptor following ligation of other chemokine receptors are also outlined. Finally, the receptor expression and the pharmacological and toxic actions of FMLP upon other tissues and organs, and its actions on the developing embryo, are illustrated.

Complex relationship between Ins(1,4,5)P3 accumulation and Ca2+ -signalling in a human neuroblastoma reveled by cellular differentiation

1. Differentiation of SH-SY5Y neuroblastoma cells induces morphological and biochemical changes consistent with a more neuronal phenotype. These cells may therefore provide a model for studying phenomena such as signal transduction in a neuronal context whilst retaining the advantages of a homogenous cell population expressing a well characterized array of G-protein coupled receptors. 2. This study examined the effects of differentiating SH-SY5Y cells on muscarinic- and bradykinin-receptor-mediated phosphoinositide and Ca2+ signalling. Retinoic acid (10 microM, 6 days) along with a lowered serum concentration produced phenotypic changes consistent with differentiation including reduced proliferation and increased neurite outgrowth. 3. Differentiation increased the magnitude and potency of rapid Ins(1,4,5)P3 responses to a full muscarinic receptor agonist. Bradykinin receptor-mediated Ins(1,4,5)P3 signalling was also potentiated following differentiation. Determination of agonist-evoked accumulation of [3H]-inositol phosphates under lithium-block demonstrated these changes reflected enhanced phospholipase C activity which is consistent with observed increases in the expression of muscarinic and bradykinin receptors. 4. Despite the marked alterations in Ins(1,4,5)P3 signalling following differentiation, elevations of intracellular [Ca2+] were totally unaltered. Thus, in SH-SY5Y cells, the relationship between the elevations of Ins(1,4,5)P3 and intracellular [Ca2+] is agonist dependent and affected by the state of differentiation. This demonstrates that mechanisms other than the measured increase in Ins(1,4,5)P3 regulate the elevation of intracellular [Ca2+].

Determination of the inositol (1,4,5) trisphosphate requirement for histamine- and substance P-induced Ca2+ mobilisation in human U373 MG astrocytoma cells

In human U373 MG astrocytoma cells, histamine and substance P stimulated similar peak increases in intracellular free calcium concentrations ([Ca2+]i), as measured by single cell imaging of Fura-2 fluorescence. Best-fit EC50 values for the peak Ca2+ response were 1.86 microM for histamine and 0.93 nM for substance P. The histamine Ca2+ response was manifest as either a series of repetitive spikes, or, at higher concentrations, a peak followed by a lower plateau level of Ca2+. In contrast, the substance P response became more transient at higher agonist concentrations. Substance P (10 nM) stimulated a biphasic increase in levels of inositol (1,4,5) trisphosphate (Ins(1,4,5)P3) with a peak of 97 +/- 5 pmoles/mg protein at 10 s. In contrast, the Ins(1,4,5)P3 response to 100 microM histamine was only marginally above basal levels of around 12 pmoles/mg protein. However, concentrations of histamine and substance P giving similar Ins(1,4,5)P3 responses produce similar peak increases in [Ca2+]i. HPLC analysis indicated that histamine stimulated the production of [3H]-Ins(1,4,5)P3 and its metabolites, although the magnitude of response was smaller than that observed with substance P. The initial Ca2+ responses to histamine and substance P did not require the presence of extracellular Ca2+. The Ca2+ response to histamine was unaffected by treatment with ryanodine, and was shifted to areas of lower agonist concentration by thimerosal. These results demonstrate that extremely small increases in Ins(1,4,5)P3 can stimulate large increases in [Ca2+]i in U373 MG cells, and suggest a marked redundancy for Ins(1,4,5)P3 production in the Ca2+ signalling pathway.

Effects of SCA40 on human isolated bronchus and human polymorphonuclear leukocytes: comparison with rolipram, SKF94120 and levcromakalim

1. SCA40 (0.1 nM-0.1 mM) produced concentration-dependent suppression of the spontaneous tone of human isolated bronchus (-log EC50 = 6.85 +/- 0.09; n = 10) and reached a maximal relaxation similar to that of theophylline (3 mM). The potency (-log EC50 values) of SCA40 compared to other relaxants was rolipram (7.44 +/- 0.12; n = 9) > SCA40 > or = levcromakalim (6.49 +/- 0.04; n = 6) > SKF94120 (5.87 +/- 0.10; n = 9). 2. When tested against the activity of the isoenzymes of cyclic nucleotide phosphodiesterase (PDE) isolated from human bronchus, SCA40 proved highly potent against PDE III (-log IC50 = 6.47 +/- 0.16; n = 4). It was markedly less potent against PDE IV (4.82 +/- 0.18; n = 4) and PDE V (4.32 +/- 0.11; n = 4). 3. Human polymorphonuclear leukocytes (PMNs) stimulated with N-formylmethionyl-leucyl-phenylalanine (FMLP) produced a concentration-dependent superoxide anion generation and elastase release. SCA40 (1 nM-10 microM) produced a concentration-related inhibition of FMLP (30 nM approximately EC50)-induced superoxide production (-log IC50 = 5.48 +/- 0.10; n = 6) and elastase release (-log IC50 = 5.50 +/- 0.26; n = 6). Rolipram was an effective inhibitor of superoxide generation and elastase release (-log IC50 values approximately 8) while SKF94120 and levcromakalim were scarcely effective. 4. FMLP (30 nM) and thimerosal (20 microM) induced leukotriene B4 production and elevation of intracellular calcium concentration in human PMNs. The production of leukotriene B4 was inhibited by SCA40 in a concentration-related manner (-log IC50 = 5.94 +/- 0.22; n = 6) but SCA40 was less effective against the elevation of intracellular calcium. Rolipram was an effective inhibitor of leukotriene B4 synthesis (-log IC50 approximately 7) and intracellular calcium elevation (-log IC50 approximately 6) while SKF94120 and levcromakalim were scarcely effective. 5. It is concluded that SCA40 is an effective inhibitor of the inherent tone of human isolated bronchus. The bronchodilatation produced by SCA40 appears mainly related to PDE inhibition since the potency of SCA40 as a relaxant of human isolated bronchus was found to be close to its potency as inhibitor of PDE III activity isolated from human bronchus. In addition, SCA40 exhibited inhibitory effects on human PMN function stimulated by FMLP. These effects may be related to the ability of SCA40 to inhibit PDE IV from human PMNs while the contribution of PDE V inhibition is uncertain. We found no evidence of a role for levcromakalim-sensitive plasmalemmal K+-channels in human PMNs.

Cytoplasmic Ca2+ concentrations in intact Merkel cells of an isolated, functioning rat sinus hair preparation

An isolated, functioning sinus hair preparation was developed to investigate cytoplasmic Ca2+ concentrations in intact Merkel cells using microfluorimetric techniques. Intracellular Ca2+ levels were monitored by means of photon counters in small groups of Merkel cells loaded with the calcium fluorescent indicators fura-2 or fluo-3. Mechanical stimulation of Merkel cells with fine glass rods resulted in small transient increases in intracellular Ca2+ levels (by about 20%) in the group of Merkel cells around the stimulating probe. A rise in Ca2+ is presumed to be essential for the postulated synaptic transmission to the afferent nerve terminal. Depolarization with a high concentration of potassium chloride (100 mM) caused increases in intracellular Ca2+ concentrations in Merkel cells (by about 70%) only in the presence of extracellular Ca2+, indicating an influx of Ca2+ through voltage-gated channels. The Ca2+ response was abolished neither by (+)-BayK8644 nor omega-conotoxin, suggesting that the Ca2+ channels are different from the classical L- or N-type channels. Extracellular application of ATP (10 microM to 5 mM) caused dose-dependent increases in intracellular Ca2+ levels in Merkel cells of up to sevenfold from the basal level of about 100 nM. Similar responses to ATP were also measured during superfusion with Ca(2+)-free medium, suggesting intracellular stores as the main Ca2+ source. Pre-incubation of Merkel cells with the purinoceptor antagonist suramin (100 microM) for 30 min reduced the Ca2+ responses to ATP by about 50% compared with control conditions. In conclusion, the results have demonstrated that a rise in intracellular Ca2+ in Merkel cells can be evoked by mechanical stimulation, membrane depolarization and chemical stimulation by ATP. These observations strongly suggest a possible contribution of Ca2+ to the normal responsiveness of Merkel cell mechanoreceptors, in turn supporting the hypothesis that Merkel cells are involved in the mechano-electric transduction process in sinus hair type I mechanoreceptors.

Quantitative comparisons of muscarinic and bradykinin receptor-mediated Ins (1,4,5)P3 accumulation and Ca2+ signalling in human neuroblastoma cells

1. Muscarinic and bradykinin receptor-mediated Ins(1,4,5)P3 accumulation, Ca2+ mobilization and Ca2+ entry have been examined in human SH-SY5Y neuroblastoma cells. This has allowed both direct comparison of signalling events by two receptor types potentially linked to the same transduction pathway and an investigation of the interactions between the components of this pathway. 2. Stimulation of muscarinic receptors with carbachol produced biphasic accumulations of Ins(1,4,5)P3 consisting of a rapid peak followed by a lower sustained phase. Both phases were dose-dependent but the potency of elevation at peak was significantly less than that of the sustained phase. Bradykinin also dose-dependently stimulated Ins(1,4,5)P3 accumulation but responses were smaller and not sustained. 3. Lowering of [Ca2+]e reduced basal Ins(1,4,5)P3 levels. Peak Ins(1,4,5)P3 elevation in response to carbachol and bradykinin were lowered by an amount approximating this reduction over the entire dose-response curves. Sustained Ins(1,4,5)P3 elevation in response to carbachol showed a more marked absolute reduction. Agonist potencies were unaffected by lowering [Ca2+]e. Thus, a consistent but small amount of PLC activity during rapid activation appears to be sensitive to lowered [Ca2+]e, whilst activity during sustained stimulation is greatly facilitated by external Ca2+, probably through Ca2+ entry. 4. The temporal- and dose-dependency of carbachol-mediated Ins(1,4,5)P3 accumulations were unaffected by loading cells with fura-2, thus allowing direct comparison of Ins(1,4,5)P3 and [Ca2+]i changes monitored by fura-2. 5. Changes in [Ca2+]i by both agonists revealed temporal patterns that were similar to Ins(1,4,5)P3 accumulations. Only carbachol stimulated a marked sustained [Ca2+]i signal and this was fully dependent on external Ca2+. 6. All agonist-mediated [Ca2+]i elevations occurred with significantly greater potency than that of the respective Ins(1,4,5)P3 accumulations. Further examination of peak elevations in response to carbachol indicated that this was independent of Ca2+ entry. Thus, a major site for amplification of the potency of rapid agonist-mediated responses lies at the level of the Ins(1,4,5)P3 receptor. 7. The transient nature of Ins(1,4,5)P3 and [Ca2+]i peaks followed by either lower but sustained levels with carbachol or a return to basal levels with bradykinin suggests rapid but partial desensitization of the muscarinic receptor and complete desensitization of the bradykinin receptor. This indicates receptor-specific desensitization. Further analysis of this was provided by detecting accumulations of [3H]-inositol phosphates ([3H]-InsPs) in Li(+)-blocked, myo-[3H]-inositol labelled cells. Carbachol produced a rapid accumulation over the first minute, followed by a slower linear accumulation for at least 29 min. At this point accumulations were dose-related with a potency similar to that of sustained Ins(1,4,5)P3 accumulation.However, bradykinin produced a minor accumulation of [3H]-InsPs, maximal by 1 min. Thus,analysis of PLC activation by measurement of [3H]-InsPs over relatively long time frames will indicate the ability of agonists for predominantly sustained PLC activation, potentially driven by a partially desensitized receptor, as opposed to rapid activation by a fully sensitized receptor.8. These data provide quantitative comparisons between and within components of the receptor mediated phosphoinositide and Ca2+ signalling pathway, provide mechanistic insights into regulation of these components and characterize a model system in which heterologous interaction between two receptors linked to the same transduction pathway may be examined.

A comparison between muscarinic receptor occupancy, inositol 1,4,5-trisphosphate accumulation and Ca2+ mobilization in permeabilized SH-SY5Y neuroblastoma cells

Electrically permeabilized SH-SY5Y neuroblastoma cells have been used to examine the relationship between receptor occupation by muscarinic agonists, inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) accumulation and Ca2+ mobilization from intracellular stores. The kinetics, concentration-dependence and guanine nucleotide-sensitivity of these responses have been characterized for the agonists, carbachol, arecoline and oxotremorine. Carbachol stimulated Ins(1,4,5)P3 accumulation and Ca2+ mobilization with an EC50 value approximately 50 microM, only slightly lower than the apparent affinity of this agonist for the "free" receptor (100 microM). Arecoline and oxotremorine were partial agonists, mobilizing 45 and 21% of the Ca2+ mobilized by carbachol, and yielded EC50 values for both Ins(1,4,5)P3 and Ca2+ responses, similar to their binding affinity. Guanosine 5'-O-3 thio-triphosphate (GTP gamma S) markedly enhanced the responses elicited by all three agonists. Carbachol became significantly more potent for both Ins(1,4,5)P3 accumulation (EC50 = 4.1 microM) and Ca2+ mobilization (EC50 = 0.25 microM), revealing a separation of the dose-response relationships. GTP gamma S caused a smaller separation of the responses elicited by arecoline (Ca2+ mobilization EC50 = 0.9 microM; Ins(1,4,5)P3 accumulation EC50 = 3.6 microM), and only enhanced maximal responses for oxotremorine. These data reveal that the functional coupling of muscarinic receptors to activation of phosphoinositidase C and subsequent Ca2+ mobilization from intracellular stores is maintained after electrical permeabilization. Furthermore, this model has been used to reveal differences in the relative activities of muscarinic agonists and how they are influenced by a hydrolysis-resistant guanine nucleotide.

Similar age-related changes of free intracellular calcium in lymphocytes and central neurons: effects of Alzheimer's disease

Several studies suggest that alterations of cytosolic free calcium concentration ([Ca2+]i) are involved in the pathophysiology of aging and Alzheimer's disease (AD). However, only few data are presently available giving detailed information about specific characteristics of age-related or AD-specific changes in cellular Ca(2+)-homeostasis. To allow a comprehensive evaluation of age-related changes in [Ca2+]i we performed parallel investigations in central mouse brain cells and mouse spleen lymphocytes of young and aged animals and also in human lymphocytes and granulocytes of young and aged donors and additionally of AD patients. In aged animals, basal [Ca2+]i was decreased in brain cells but increased in spleen lymphocytes. No age-related alterations in baseline [Ca2+]i was found in human lymphocytes or granulocytes. However, comparison of activation-induced rise in [Ca2+]i revealed parallel age-related changes in the different cell-types investigated. The increase in [Ca2+]i after depolarization of mouse brain cells with KCl and after stimulation of mouse lymphocytes with phytohaemagglutinin (PHA) was significantly impaired in aged animals. Moreover, activation of human lymphocytes with PHA also revealed a reduced increase in [Ca2+]i in cells of aged donors. In lymphocytes of AD-patients there was a tendency to higher basal [Ca2+]i compared to their aged matched controls, but no specific alterations in [Ca2+]i could be found after stimulation with PHA. Also no age-related or AD-specific changes were found in granulocytes after stimulation with N-formyl-methionyl-leucyl-phenylalanine (fMLP).(ABSTRACT TRUNCATED AT 250 WORDS)

The calcium signal in human neutrophils and its relation to exocytosis investigated by patch-clamp capacitance and Fura-2 measurements

Intracellular calcium ([Ca2+]i) and exocytosis of human neutrophils were investigated with patch-clamp capacitance and Fura-2 fluorescence measurements. Intracellular application of GTP gamma S induces a calcium transient and exocytosis. The onset of degranulation occurs at the time where the maximal [Ca2+]i is reached. Despite the close correlation in time, buffering [Ca2+]i at the resting level or at approximately 2 microM leaves the extent and the time course of degranulation unchanged. The decay of the calcium transient is due to diffusional equilibration between the cytosol and the pipette volume. GTP gamma S activates no cellular mechanisms for Ca2+ reuptake or extrusion. The endogenous calcium buffer capacity can be estimated to be as low as that of approximately 90 microM Fura-2. Stimulation with fMLP also induces degranulation and a calcium transient. The decay of fMLP-induced calcium transients is much faster than that of GTP gamma S-induced transients and is independent of diffusion indicating that fMLP also induces rapid reuptake or extrusion of Ca2+. Degranulation but not the calcium transient requires the presence of intracellular GTP. Different signalling pathways appear to be involved in GTP gamma S- and fMLP-stimulated calcium signals. The intracellular calcium release is not an essential signal to initiate exocytosis in neutrophils.

Tyrosine phosphorylation is involved in receptor coupling to phospholipase D but not phospholipase C in the human neutrophil

The tyrosine kinase inhibitors ST271, ST638 and erbstatin inhibited phospholipase D (PLD) activity in human neutrophils stimulated by fMet-Leu-Phe, platelet-activating factor and leukotriene B4. These compounds did not inhibit phorbol ester-stimulated PLD, indicating that they do not inhibit PLD per se, but probably act at a site between the receptor and the phospholipase. In contrast, the protein kinase C inhibitor Ro-31-8220 inhibited phorbol 12,13-dibutyrate- but not fMet-Leu-Phe-stimulated PLD activity, arguing against the involvement of protein kinase C in the receptor-mediated activation of PLD. ST271 did not inhibit Ins(1,4,5)P3 generation, but did inhibit protein tyrosine phosphorylation stimulated by fMet-Leu-Phe. The phosphotyrosine phosphatase inhibitor pervanadate increased tyrosine phosphorylation and stimulated PLD. These results suggest that tyrosine kinase activity is involved in receptor coupling to PLD but not to PtdIns(4,5)P2-specific phospholipase C in the human neutrophil.