Activation of muscarinic receptors in PC12 cells. Correlation between cytosolic Ca2+ rise and phosphoinositide hydrolysis.

The intracellular signals generated by carbachol activation of the muscarinic receptor [release of inositol phosphates as a consequence of phosphoinositide hydrolysis and rise of the cytosolic Ca2+ concentration ([Ca2+]i, measured by quin2)] were studied in intact PC12 pheochromocytoma cells that had been differentiated by treatment with nerve growth factor. When measured in parallel samples of the same cell preparation 30 s after receptor activation, the release of inositol trisphosphate and of its possible metabolites, inositol bis- and mono-phosphate, and the [Ca2+]i rise were found to occur with almost superimposable carbachol concentration curves. At the same time carbachol caused a decrease in the radioactivity of preloaded phosphatidylinositol 4,5-bisphosphate, the precursor of inositol trisphosphate. Neither the inositol phosphate nor the [Ca2+]i signal was modified by preincubation of the cells with either purified Bordetella pertussis toxin or forskolin, the direct activator of adenylate cyclase. Both signals were partially inhibited by dibutyryl cyclic AMP, especially when the nucleotide analogue was applied in combination with the phosphodiesterase inhibitors RO 201724 and theophylline. The latter drug alone profoundly inhibited the carbachol-induced [Ca2+]i rise, with only minimal effect on phosphoinositide hydrolysis. Because of the diverging results obtained with forskolin on the one hand, dibutyryl cyclic AMP and phosphodiesterase inhibitors on the other, the effects of the latter drugs are considered to be pharmacological, independent of the intracellular cyclic AMP concentration. Two further drugs tested, mepacrine and MY5445, inhibited phosphoinositide hydrolysis at the same time as the 45Ca2+ influx stimulated by carbachol. Taken together, our results concur with previous evidence obtained with permeabilized cells and cell fractions to indicate phosphatidylinositol 4,5-bisphosphate hydrolysis and [Ca2+]i rise as two successive events in the intracellular transduction cascade initiated by receptor activation. The strict correlation between the carbachol concentration curves for inositol trisphosphate generation and [Ca2+]i rise, and the inhibition by theophylline of the Ca2$ signal without major effects on inositol phosphate generation, satisfy important requirements of the abovementioned interpretation.     

Neutrophil activation by inflammatory microcrystals of monosodium urate monohydrate utilizes pertussis toxin-insensitive and -sensitive pathways

The activation of leukocytes by particulates is a critical event in certain inflammatory syndromes, including acute gout associated with microcrystals of monosodium urate monohydrate. In this study we have evaluated mechanisms of human neutrophil activation by urate crystals. Both N-formyl-nor-leu-leu-phe-nor-leu-tyr-lys and uncoated urate crystals (0.5 to 5 mg/ml) but not urate crystals coated with human low density lipoprotein (which suppresses crystal-induced neutrophil responsiveness), stimulated pertussis toxin (PT)-sensitive GTPase activity in purified preparations of human neutrophil membranes. Hydroxyapatite crystals (up to 5 mg/ml) were inactive. Pretreatment of neutrophil membranes with cholera toxin also inhibited crystal-induced and formylated peptide-induced GTPase activity. Pretreatment of whole neutrophils with PT resulted in nearly complete inhibition of formylated peptide-induced cytosolic calcium mobilization, release of superoxide and release of the azurophil granule constituent alpha-mannosidase. In contrast, identical pretreatment of whole neutrophils with PT only partially suppressed urate crystal-induced alpha-mannosidase and superoxide release and failed to inhibit crystal phagocytosis and increases in cytosolic free calcium. Mechanisms of neutrophil activation by monosodium urate crystals appear to be heterogeneous in comparison with activation by formylated peptides and there is no absolute requirement for PT-sensitive membrane G proteins in neutrophil responsiveness to urate crystals.     

Inhibition by L-arginine of the endothelin-mediated increase in cytosolic calcium in human neutrophils

The effect of endothelin (ET) on the cytosolic-free calcium [(Ca2+]i) changes in polymorphonuclear leukocytes (PMN) from normal humans and Wistar rats was investigated. ET induced a dose-related [Ca2+]i peak. This [Ca2+]i transient was blunted by TMB-8 (10(-5)M) and by Ca(2+)-free EGTA medium, therefore suggesting a role of both intracellular Ca2+ release and Ca2+ influx in the generation of the [Ca2+]i peak. Preincubation of PMN with the nitric oxide (NO)-donor L-arginine (L-Arg) markedly blocked the ET-induced [Ca2+]i transient in an enantiomerically-specific manner. A similar blunting effect of L-Arg on the fMLP (10(-7)M)-induced [Ca2+]i transient was detected. The L-Arg antagonist, NG-monomethyl-L-arginine (L-NMMA), reverted the L-Arg blocking effect on both ET- and fMLP-induced [Ca2+]i transients. These data suggest that ET has a potential role in activating Ca2+ mobilization in PMN, an effect that can be inhibited by L-Arg.     

Effects of Ca2+ on phosphoinositide breakdown in exocrine pancreas.

Recent studies have established that inositol 1,4,5-trisphosphate [I(1,4,5)P3] provides the link between receptor-regulated polyphosphoinositide hydrolysis and mobilization of intracellular Ca2+. Here, we report the effects of Ca2+ on inositol trisphosphate (IP3) formation from phosphatidylinositol bisphosphate (PIP2) catalysed by phospholipase C in intact and electrically permeabilized rat pancreatic acinar cells. In permeabilized cells, the Ca2+-mobilizing agonist caerulein stimulated [3H]IP3 formation when the free [Ca2+] was buffered at 140 nM, the cytosolic free [Ca2+] of unstimulated pancreatic acinar cells. When the free [Ca2+] was reduced to less than 10 nM, caerulein did not stimulate [3H]IP3 formation. Ca2+ in the physiological range stimulated [3H]IP3 formation and reduced the amount of [3H]PIP2 in permeabilized cells. The effects of Ca2+ and the receptor agonist caerulein were additive, but we have not established whether this reflects independent effects on the same or different enzymes. The effect of Ca2+ on [3H]IP3 formation by permeabilized cells was unaffected by inhibitors of the cyclo-oxygenase and lipoxygenase pathways of arachidonic acid metabolism; nor were the effects of Ca2+ mimicked by addition of arachidonic acid. These results suggest that the effects of Ca2+ on phospholipase C activity are not a secondary consequence of Ca2+ activation of phospholipase A2. Changes in free [Ca2+] (less than 10 nM-1.2 mM) did not affect the metabolism of exogenous [3H]I(1,4,5)P3 by permeabilized cells. In permeabilized cells, breakdown of exogenous [3H]IP3 to [3H]IP2 (inositol bisphosphate), and formation of [3H]IP3 in response to receptor agonists were equally inhibited by 2,3-bisphosphoglyceric acid. This suggests that the [3H]IP2 formed in response to receptor agonists is entirely derived from [3H]IP3. In intact cells, [3H]IP3 formation was stimulated when ionomycin was used to increase the cytosolic free [Ca2+]. However, a maximal concentration of caerulein elicited ten times as much IP3 formation as did the highest physiologically relevant [Ca2+]. We conclude that the major effect of receptor agonists on IP3 formation does not require an elevation of cytosolic free [Ca2+], although the increase in free [Ca2+] that normally follows IP3 formation may itself have a small stimulatory effect on phospholipase C.     

Two functionally distinct cholecystokinin receptors show different modes of action on Ca2+ mobilization and phospholipid hydrolysis in isolated rat pancreatic acini. Studies using a new cholecystokinin analog, JMV-180.

A new hepatapeptide cholecystokinin (CCK) analog, JMV-180 (Boc-Tyr(SO3-)-Nle-Gly-Trp-Nle-Asp-2-phenylethylester), acts as an agonist at high affinity CCK receptors on rat pancreatic acini to stimulate amylase release but unlike cholecystokinin octapeptide (CCK8) does not act on low affinity CCK receptors to inhibit amylase release (Galas, M. D., Lignon, M. F., Rodriguez, M., Mendre, C., Fulcrand, P., Laur, J., and Martinez, J. (1988) Am. J. Physiol. 254, G176-G188). To investigate the biochemical mechanisms initiated by CCK acting on each class of CCK receptor, the effects of JMV-180 and CCK8 on amylase release, Ca2+ mobilization, and phospholipid hydrolysis were studied in isolated rat pancreatic acini. When acini were loaded with the intracellular Ca2+ chelator BAPTA, amylase release stimulated by both JMV-180 and CCK8 was reduced. Measurement of 45Ca2+ efflux and cytosolic free calcium concentration ([Ca2+]i) by the fluorescence of fura-2-loaded acini in a stirred cuvette showed that JMV-180 induced a concentration-dependent increase but with a maximal response only two-thirds that induced by CCK8. When [Ca2+]i of individual fura-2-loaded acinar cells was measured by microspectrofluorometry, all concentrations of JMV-180 (1 nM-10 microM) induced repetitive transient [Ca2+]i spikes (Ca2+ oscillations). By contrast, stimulation with a high concentration of CCK8 (1 nM) caused a large increase in [CA2+]i followed by a small sustained elevation of [Ca2+]i. The measurement of inositol trisphosphate (IP3) production by both [3H]inositol labeling and 1,4,5-IP3 radioreceptor assay showed that JMV-180 had only minimal effects at 10 microM in contrast to the large increase induced by high concentrations of CCK8 (more than 1 nM). JMV-180 blocked the effect of a high concentration of CCK8 on both [Ca2+]i and 1,4,5-IP3 productions but did not affect the response to carbamylcholine. JMV-180 caused a delayed monophasic stimulation of 1,2-diacylglycerol (DAG) sustained to 60 min without the early increase in DAG observed in response to CCK8. Furthermore, JMV-180 stimulated the release of [3H]choline metabolites, primarily phosphorylated choline, from [3H]choline-labeled acini at low concentrations and to the same extent as CCK8. Since JMV-180 interacts not only with high affinity CCK receptors as an agonist but also with low affinity CCK receptors as a functional antagonist, the present results indicate that the occupancy of high affinity state receptors by CCK induces Ca2+ oscillations, DAG formation from phosphatidylcholine hydrolysis, and amylase release with minimal phosphatidylinositol 4,5-bisphosphate hydrolysis.(ABSTRACT TRUNCATED AT 400 WORDS)     

Secretin induces rapid increases in inositol trisphosphate, cytosolic Ca2+ and diacylglycerol as well as cyclic AMP in rat pancreatic acini.

Previous studies have shown that the dose-response relationship for secretin-stimulated cyclic AMP accumulation is different from that for secretin-stimulated enzyme secretion in the rat exocrine pancreas. Here we show that secretin concentrations of 10(-10) M and higher stimulated a rise in cyclic AMP levels, with maximum effect on cyclic AMP accumulation being achieved already with 10(-8) M-secretin. However, at this concentration of secretin, enzyme secretion rates were approximately half-maximal. Unexpectedly, at concentrations of secretin greater than 10(-8) M there was evidence suggestive of phosphatidylinositol bisphosphate hydrolysis with rapid increases in inositol trisphosphate, cytosolic free calcium and diacylglycerol content of rat pancreatic acini. Furthermore, there was a dose-response relationship among secretin concentration (in the range 10(-8) M-2 X 10(-6) M), increases in inositol trisphosphate and increases in cytosolic free calcium ([Ca2+]i). Contrary to what has been previously believed, these results clearly indicate that in rat pancreatic acini secretin not only stimulates cyclic AMP accumulation but also raises inositol trisphosphate, [Ca2+]i and diacylglycerol. Thus, two second messenger systems may play a role in the regulation of secretin-induced amylase release.     

Breakdown of phosphatidylinositol 4,5-bisphosphate in a T-cell leukaemia line stimulated by phytohaemagglutinin is not dependent on Ca2+ mobilization.

Addition of phytohaemagglutinin (PHA) to the [32P]Pi-prelabelled JURKAT cells, a human T-cell leukaemia line, resulted in a decrease of [32P]phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] to about 35% of the control value. The decrease was almost complete within 30s after the PHA addition. This decrease was followed by an increase in the 32P-labelling of phosphatidic acid (maximally 2.8-fold at 2 min). The stimulation of myo-[2-3H]inositol-prelabelled JURKAT cells by PHA induced an accumulation of [2-3H]inositol trisphosphate in the presence of 5 mM-LiCl. The result indicates hydrolysis of PtdIns (4,5)P2 by a phospholipase C. The PHA stimulation of JURKAT cells induced about 6-fold increase in the cytosolic free Ca2+ concentration, [Ca2+]i, which was reported by Quin-2, a fluorescent Ca2+ indicator. Studies with partially Ca2+-depleted JURKAT cells, with the Ca2+ ionophore A23187, and with 8-(diethylamino)-octyl-3,4,5-trimethoxybenzoate indicate that the breakdown of PtdIns(4,5)P2 is not mediated through changes of [Ca2+]i. These results therefore indicate that the PHA-induced breakdown of PtdIns(4,5)P2 in JURKAT cells is not dependent on the Ca2+ mobilization.     

Extracellular ATP causes Ca2(+)-dependent and -independent insulin secretion in RINm5F cells. Phospholipase C mediates Ca2+ mobilization but not Ca2+ influx and membrane depolarization

The mechanism by which extracellular ATP stimulates insulin secretion was investigated in RINm5F cells. ATP depolarized the cells as demonstrated both by using the patch-clamp technique and a fluorescent probe. The depolarization is due to closure of ATP-sensitive K+ channels as shown directly in outside-out membrane patches. ATP also raised cytosolic Ca2+ [( Ca2+]i). At the single cell level the latency of the [Ca2+]i response was inversely related to ATP concentration. The [Ca2+]i rise is due both to inositol trisphosphate mediated Ca2+ mobilization and to Ca2+ influx. The former component, as well as inositol trisphosphate generation, were inhibited by phorbol myristate acetate which uncouples agonist receptors from phospholipase C. This manoeuvre did not block Ca2+ influx or membrane depolarization. Diazoxide, which opens ATP-sensitive K+ channels, attenuated membrane depolarization and part of the Ca2+ influx stimulated by ATP. However, the main Ca2+ influx component was unaffected by L-type channel blockers, suggesting the activation of other Ca2+ conductance pathways. ATP increased the rate of insulin secretion by more than 12-fold but the effect was transient. Prolonged exposure to EGTA dissociated the [Ca2+]i rise from ATP-induced insulin secretion, since the former was abolished and the latter only decreased by about 60%. In contrast, vasopressin-evoked insulin secretion was more sensitive to Ca2+ removal than the accompanying [Ca2+]i rise. Inhibition of phospholipase C stimulation by phorbol myristate acetate abrogated vasopressin but only reduced ATP-induced insulin secretion by 34%. These results suggest that ATP stimulates insulin release by both phospholipase C dependent and distinct mechanisms. The Ca2+)-independent component of insulin secretion points to a direct triggering of exocytosis by ATP.     

Activation of the neutrophil by calcium-mobilizing ligands. I. A chemotactic peptide and the lectin concanavalin A stimulate superoxide anion generation but elicit different calcium movements and phosphoinositide remodeling

The relevance of phosphoinositide remodeling to calcium movements and to the physiological response of superoxide anion (O2-) generation was probed in neutrophils stimulated by the chemotactic peptide fMet-Leu-Phe and the lectin concanavalin A. fMet-Leu-Phe and concanavalin A triggered O2- generation but elicited different patterns of calcium mobilization and phosphoinositide remodeling. fMet-Leu-Phe (10(-7) M) triggered a rise in cytosolic calcium by mobilization of intracellular calcium (fura-2) and increased calcium permeability (45Ca uptake), while concanavalin A (100 micrograms/ml) elicited a rise in cytosolic calcium, primarily by uptake of extracellular calcium (45Ca uptake). fMet-Leu-Phe triggered rapid breakdown of phosphatidylinositol 4,5-bisphosphate (PIP2), phosphatidylinositol 4-phosphate and phosphatidylinositol, and generation of inositol 1,4,5-trisphosphate (IP3). In contrast concanavalin A triggered breakdown of phosphatidylinositol, but not PIP2, nor was there a significant increase in IP3. However, both fMet-Leu-Phe and concanavalin A triggered a rapid biphasic increase in levels of labeled diacylglycerol (in [3H]arachidonate or [14C]glycerol prelabeled cells) and a 3-fold increase in [32P] phosphatidic acid. These results are concordant with a role for PIP2 breakdown and generation of IP3 specifically in intracellular calcium mobilization but not for other aspects of the signaling pathway for O2- generation. Calcium permeability changes were associated with elevated diacylglycerol and [32P]phosphatidic acid, although a cause and effect relationship is not apparent. Ligands such as concanavalin A enhance cytosolic calcium and trigger generation of O2- without significant PIP2 remodeling; elevated diacylglycerol and cytosolic calcium are the common events associated with ligand-induced O2- generation.     

Extracellular ATP itself elicits superoxide generation in guinea pig peritoneal macrophages.

Extracellular ATP itself elicited the generation of superoxide (O2-) in guinea pig peritoneal macrophages associated with an increase in cytosolic calcium ([Ca2+]i). The ATP-induced O2- generation was completely inhibited by pretreatment with pertussis toxin (PT) accompanied by the suppression of [Ca2+]i mobilization. Pre-exposure to a small amount of phorbol myristate acetate (PMA) primed the ATP-induced generation of O2- without a change of [Ca2+]i. The results suggest that ATP-induced O2- generation is mediated by [Ca2+]i mobilization and by PT-sensitive G protein.     

Thapsigargin reveals evidence for fMLP-insensitive calcium pools in human leukocytes.

To investigate the relationship between different intracellular Ca2+ pools, cytosolic free calcium ([Ca2+]i) was surveyed by means of a Fura-2 fluorescence ratio method on single isolated human leukocytes. Both monocytes and neutrophilic granulocytes (PMN) displayed long lasting spontaneous [Ca2+]i transient changes (1-2 min). In PMN stimulated with the bacterial peptide fMLP we observed transients with shorter duration (10-30 s) and smaller amplitude often superimposed on the long lasting transients. The time course of changes in [Ca2+]i was recorded in a large number (149) of single leukocytes prestimulated for 5 min with fMLP and then challenged with thapsigargin (a blocker of Ca2+ uptake in intracellular pools). Statistical analysis of [Ca2+]i responses revealed that fMLP-sensitive pools contributed to the long lasting [Ca2+]i transients seen in both leukocyte types. However, the existence of fMLP-insensitive calcium pools may explain the superimposed transients seen in PMN. Thapsigargin was also added together with EGTA (to impede contribution from extracellular Ca2+) to 198 fMLP prestimulated and 153 unstimulated PMN. Based on Ca2+ registrations in these cells and a mathematical model (supposing two separate first order responses) the amount of Ca2+ stored in the various pools and their release kinetics were estimated. The results indicate that fMLP-insensitive calcium pools exist in PMN but not in monocytes. Since the digital imaging technique also depicts cellular motility, an additional finding was that the leukocyte's ability to sequestrate the Ca2+ from the cytosol seemed important to locomotion.     

Protein kinase C activation in B cells by indolactam inhibits anti-Ig-mediated phosphatidylinositol bisphosphate hydrolysis but not B cell proliferation

In order to examine the role of phosphatidylinositol bisphosphate (PIP2) hydrolysis in B cell activation, we studied the effect of various classes of protein kinase C (PKC) activators on anti-Ig-mediated B cell stimulation. Anti-Ig-stimulated PIP2 hydrolysis, elevations in [Ca2+]i, and induction of DNA synthesis were inhibited by PMA (a phorbol ester) as previously reported. In contrast, indolactam (an alkaloid PKC activator) inhibited PIP2 hydrolysis and elevations in [Ca2+]i, but stimulated rather than inhibited cellular proliferation. In order to examine whether the binding avidity of the PKC activators to PKC played a role in determining their activity to stimulate or inhibit B cell activation, we studied two other PKC activators, bryostatin and mezerein. Again, both inhibited anti-Ig mediated PIP2 hydrolysis and elevations in [Ca2+]i, whereas only the former inhibited induction of DNA synthesis. These data suggest that a) high levels of PIP2 hydrolysis and elevations in [Ca2+]i are not essential for anti-Ig-mediated induction of B cell DNA synthesis and b) activation of PKC may induce both stimulatory and inhibitory pathways of B cell activation, and whether stimulation or inhibition of cell activation is observed may depend on the combined intensity of these two signals.     

Platelet-activating factor induces an increase in intracellular calcium and expression of regulatory genes in human B lymphoblastoid cells.

Platelet-activating factor (PAF) has recently been demonstrated to be metabolized by B lymphocytes and to cause enhancement of Ig synthesis by Ig-secreting B lymphoblastoid cell lines. We have now examined some of the early activation events triggered by PAF binding to three Ig-secreting B cell lines, LA350 (IgM secreting), HSCE- (IgG secreting), and U266 (IgE secreting). After addition of 10(-7) to 10(-11) M PAF, but not equimolar concentrations of the inactive metabolite lyso-PAF, all three cell lines demonstrated rapid dose-dependent increases in free cytosolic Ca2+ concentrations ([Ca2+]i). The increases in [Ca2+]i resulted from both the release of Ca2+ from internal stores as well as transmembrane Ca2+ uptake. Addition of PAF triggered the rapid hydrolysis of phosphatidylinositol bisphosphate and accumulation of inositol phosphates. PAF also increased expression of the cell cycle-active genes c-fos and EGR2 in a dose-dependent fashion. The stimulated increases in [Ca2+]i and phosphatidylinositol bisphosphate hydrolysis and the increases in gene expression were all inhibited by the specific PAF receptor antagonist Web 2086. The LA350 cell line (which expresses surface IgM) was also shown to increase [Ca2+]i after addition of anti-IgM antibodies. Sequential addition of PAF or anti-IgM antibody in either order failed to reveal any evidence for heterologous desensitization. Furthermore, the PAF receptor antagonist did not affect anti-IgM induced changes in [Ca2+]i. These data provide evidence for the presence of functional PAF receptors on B lymphoblastoid cells and indicate a potential role for PAF in the regulation of B cell activation.     

Inositol lipid hydrolysis contributes to the Ca2+ wave in the activating egg of Xenopus laevis.

We have used fluorescence ratio-imaging of fura-2 in the activating egg of Xenopus laevis to study the wave of increased intracellular free Ca2+ concentration ([Ca2+]i) while monitoring that of cortical granule exocytosis. Naturally matured eggs were dejellied, injected with fura-2, and activated by the iontophoresis of 1-30 nCoul of inositol-1,4,5-trisphosphate which triggers an immediate increase in free [Ca2+]i at the injection site. The Ca2+ rise spreads throughout the egg, reaching the opposite side in 5-8 min, and is followed by elevation of the fertilization envelope about 20-30 sec behind the [Ca2+]i wave. [Ca2+]i returns to preactivation levels within about 20 min after activation. We further studied the role of phosphatidylinositol-4,5-bisphosphate (PIP2) hydrolysis by microinjecting antibodies to PIP2 into the egg. PIP2 antibodies did not alter the propagation velocity of the wave but greatly reduced the amount of Ca2+ released in the egg cortex. These data suggest that PIP2 hydrolysis plays a role in the release of [Ca2+]i in the outer regions of the egg following activation.     

Calcium channel blockers nifedipine and diltiazem inhibit Ca2+ release from intracellular stores in neutrophils.

We have undertaken a detailed study of the mechanisms of maintenance of intracellular Ca2+ homeostasis in human polymorphonuclear neutrophils (PMN) and its implications for phagocytosis and IgG Fc receptor (FcR) signaling. When PMN were incubated in Ca(2+)-free medium, cytoplasmic calcium concentration ([Ca2+]i) was markedly depressed and intracellular stores were depleted of calcium. [Ca2+]i in these depleted cells increased within 1 min when PMN were placed in medium containing Ca2+ and then decreased to a level close to the normal basal [Ca2+]i, replenishing the intracellular Ca2+ pools. LaCl3 prevented entry of Ca2+ into Ca(2+)-depleted PMN, but the calcium channel blockers nifedipine, diltiazem, and verapamil did not. Nifedipine and diltiazem but not verapamil inhibited the movement of Ca2+ from cytosol to intracellular stores. Nifedipine and diltiazem inhibited the normal increase in [Ca2+]i from aggregated IgG binding to FcR and also prevented formyl-methionyl-leucyl-phenyl-alanine (fMLP)-induced [Ca2+]i rise. Verapamil had no effect on either an fMLP- or IgG-mediated increase in [Ca2+]i. Consistent with this, nifedipine and diltiazem inhibited fMLP-stimulated phagocytosis (which is dependent on an increase in [Ca2+]i) when PMN had repleted intracellular stores. In contrast, LaCl3 inhibited fMLP-stimulated ingestion only in PMN which had intracellular store depleted. None of these compounds had any effect on phorbol dibutyrate-stimulated ingestion (which is independent of a [Ca2+]i rise). In summary, these data show that Ca2+ is in rapid equilibrium between intracellular and extracellular compartments in PMN. Exchange of cytoplasmic Ca2+ with the extracellular space is inhibited by LaCl3, while exchange of Ca2+ between the cytosol and intracellular stores is inhibited by the dihydropyridine nifedipine and the benzothiazepine diltiazem. These data suggest that these drugs, which are known to regulate some plasma membrane Ca2+ channels in excitable cells, can also regulate Ca2+ release from intracellular stores in PMN and that this regulation may have significant effects on PMN function.     

Interactions between inositol phosphates and cytosolic free calcium following bradykinin stimulation in cultured human skin fibroblasts

The inositol triphosphate (IP3) that results from hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) is generally accepted to be responsible for the mobilization of intracellular calcium. However, some studies suggest that low concentrations of agonists elevate cytosolic free calcium concentration ([Ca2+]i) without IP3 formation. Thus, in the present studies, a comparison of the temporal response of inositol phosphates (IP3, IP2 and IP) and [Ca2+]i to a wide range of bradykinin concentrations was used to examine the relation of these two signal transduction events in cultured human skin fibroblasts (GM3652). In addition, the effects of alterations in internal or external calcium on the response of these second messengers to bradykinin were determined. Bradykinin stimulated accumulation of inositol phosphates and a rise of [Ca2+]i in a time- and dose-dependent manner. Decreasing the bradykinin concentration from 1 microM to 0.1 microM increased the time until the IP3 peak, and when the bradykinin concentration was reduced to 0.01 microM IP3 was not detected. [Ca2+]i was examined under parallel conditions. As the bradykinin concentration was reduced from 1 microM to 0.01 microM, the time to reach the peak of [Ca2+]i increased progressively, but the magnitude of the peak was unaltered. These two second messengers were variably dependent on external calcium. Although the bradykinin-stimulated initial spike of [Ca2+]i did not depend on extracellular calcium, the subsequent sustained levels of [Ca2+]i were abolished in calcium free medium. The bradykinin-stimulated inositol phosphate formation was not dependent on the extracellular calcium nor on the elevation of [Ca2+]i that was produced with Br-A23187. These results demonstrate that bradykinin-induced IP3 formation can be independent of [Ca2+]i and of external calcium, whereas changes in [Ca2+]i are partially dependent on external calcium.     

Internal calcium release and activation of sea urchin eggs by cGMP are independent of the phosphoinositide signaling pathway.

We show that microinjecting cyclic GMP (cGMP) into unfertilized sea urchin eggs activates them by stimulating a rise in the intracellular free calcium ion concentration ([Ca2+]i). The increase in [Ca2+]i is similar in both magnitude and duration to the transient that activates the egg at fertilization. It is due to mobilization of calcium from intracellular stores but is not prevented by the inositol trisphosphate (InsP3) antagonist heparin. Furthermore, cGMP does not stimulate the eggs Na+/H+ antiport when the [Ca2+]i transient is blocked by the calcium chelator bis-(O-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA), suggesting that cGMP does not activate eggs by interacting with the their phosphoinositide signaling pathway. However, the [Ca2+]i increase and activation are prevented in eggs in which the InsP3-sensitive calcium stores have been emptied by the prior microinjection of the InsP3 analogue inositol 1,4,5-trisphosphorothioate. These data indicate that cGMP activates eggs by stimulating the release of calcium from an InsP3-sensitive calcium store via a novel, though unidentified, route independent of the InsP3 receptor.     

Role of guanine nucleotide regulatory protein in polyphosphoinositide degradation and activation of phagocytic leukocytes by chemoattractants

Leukocyte activation by chemoattractants provides an important model to study the biochemical mechanisms of stimulus-response coupling in these cells. Well-defined chemotactic factors induce readily quantifiable responses in phagocytic leukocytes. These include directed migration and the production and release of toxic substances including oxygen radicals and lysosomal enzymes. The development of radiolabeled synthetic oligopeptides with potent chemotactic activity allowed the demonstration of chemoattractant receptors on polymorphonuclear leukocytes (PMNs) as well as macrophages. In membrane preparations from these cells, these receptors exist in high- and low-affinity states which are regulated by guanosine di- and triphosphates. This suggested that chemoattractant receptors interact with guanine nucleotide regulatory proteins (N or G proteins). Although chemoattractants elicit a rapid but transient increase in intracellular cAMP levels, they neither stimulate nor inhibit membrane-bound adenylate cyclase, suggesting a novel role for N proteins in certain receptor-transduction mechanisms. Stimulation of phagocytes by chemoattractants is also associated with a rapid increase in cytosolic Ca2+ concentrations ([ Ca2+]i) which appears to result from the production of inositol 1,4,5-triphosphate (IP3) as a consequence of the diesteric cleavage of phosphatidylinositol 4,5-bisphosphate (PIP2). Treatment of phagocytes with pertussis toxin (PT), which ADP-ribosylates and thereby inactivates certain N proteins, abolishes the cells' responsiveness to chemoattractants. More direct evidence for a role of a PT-sensitive N protein in leukocyte activation was provided by the demonstration that chemoattractants stimulate the hydrolysis of PIP2 in PMN membranes only in the presence of GTP. This receptor-mediated hydrolysis of PIP2 is not observed in plasma membranes prepared from PT-treated PMNs. Therefore, these studies suggest that occupancy of chemoattractant receptors activates a PT-sensitive N protein. The activated N protein shifts the Ca2+ requirement for phospholipase C activity from supraphysiological levels to ambient cytosolic Ca2+ concentrations. Cleavage of PIP2 results in the formation of the second messenger molecules, IP3 and 1,2-diacylglycerol, which can initiate cellular activation. These messengers also seem to activate responses which feed back to attenuate receptor stimulation of phospholipase.     

Does beta-adrenoceptor activation stimulate Ca2+ mobilization and inositol trisphosphate formation in parotid acinar cells?

The effects of the beta-adrenoceptor agonist, isoprenaline, on Ca2+ mobilization and inositol phosphate formation in parotid acinar cells were examined. Isoprenaline (2 microM) failed to increase cytosolic [Ca2+] in acinar cells, as measured by Fura-2 fluorescence, even in the presence of a phosphodiesterase inhibitor. Likewise, neither the 8-bromo nor the dibutyryl derivatives of cAMP (both at 2 mM concentration) increased [Ca2+]i. However, in confirmation of results previously published, a higher concentration of isoprenaline (200 microM) increased cytosolic [Ca2+]i of rat parotid acinar cells, from 104 +/- 4 nM to 151 +/- 18 nM. The increase in [Ca2+]i in response to isoprenaline, while transient in the absence of extracellular Ca2+, was sustained in Ca2(+)-containing medium. This isoprenaline-stimulated Ca2+ signal was more potently antagonized by phentolamine than by propranolol, suggesting that the higher concentration of isoprenaline activated alpha-adrenoceptors. Furthermore, the Ca2+ signal generated in response to the alpha-adrenoceptor agonist, phenylephrine, also was blocked by the same concentrations of propranolol necessary to block the effects of isoprenaline, suggesting that propranolol may block alpha-adrenoceptors under certain experimental conditions. The high concentration of (-)isoprenaline (200 microM) also increased inositol (1,4,5) trisphosphate and inositol (1,3,4) trisphosphate formation 45% within 30 s. Analogous to the increase in intracellular Ca2+, the formation of inositol phosphates stimulated by isoprenaline was more potently antagonized by the alpha-adrenoceptor antagonist, phentolamine, than by the beta-adrenoceptor antagonist, propranolol, again suggesting that isoprenaline interacts with alpha-adrenoceptors on parotid cells. Thus, the effects of isoprenaline on [Ca2+]i do not appear to be mediated by cAMP.(ABSTRACT TRUNCATED AT 250 WORDS)     

Increased plasma membrane permeability to Ca2+ in anti-Ig-stimulated B lymphocytes is dependent on activation of phosphoinositide hydrolysis

The biochemical basis of Ca2+ mobilization after anti-Ig binding to B cell Ag-R has been further characterized by flow cytometric analysis of indo-1-loaded B cells. The ability to distinguish intracellular Ca2+ release from extracellular Ca2+ influx by using an extracellular calcium depletion-repletion approach has allowed us to study the relationship between the mobilization of Ca2+ from these sources. Studies involving manipulation of the Ca2+ gradient across the plasma membrane indicate that a significant portion of the Ca2+ mobilization response is preserved even when the normal inwardly directed Ca2+ gradient is reversed. In the presence of an extracellular calcium concentration ([Ca2+]o) of 10 microM, the response to anti-Ig is not blocked by the organic Ca2+ channel blockers. This response is not reduced by further depletion of [Ca2+]o by EGTA Ca2+-binding buffers. Thus, the Ca2+ response that occurs when [Ca2+]o less than or equal to 10 microM represents intracellular calcium release. Analysis of B cells stimulated with anti-Ig in low Ca2+ medium ([Ca2+]o = less than 10 microM) followed by repletion of [Ca2+]o to 1 to 5 mM reveals that a significant increase in permeability of the plasma membrane to Ca2+ develops in the stimulated cells. The resultant Ca2+ influx is nimodipine (20 microM) sensitive. Both intracellular Ca2+ release and Ca2+ influx are reduced in parallel as the concentration of anti-Ig stimulus is decreased, suggesting that Ca2+ influx may be coupled to the release of intracellular stores. Neomycin blocks anti-Ig-stimulated formation of inositol trisphosphate, which mediates release of Ca2+ from the endoplasmic reticulum. It also blocks the anti-Ig-induced release of intracellular Ca2+ stores as well as Ca2+ influx, indicating that both responses may be dependent upon phosphatidylinositol 4,5-bisphosphate hydrolysis.