Cyclic nucleotide regulation of store-operated Ca2+ influx in airway smooth muscle

Sarcoplasmic reticulum (SR) Ca2+ release and plasma membrane Ca2+ influx are key to intracellular Ca2+ ([Ca2+]i) regulation in airway smooth muscle (ASM). SR Ca2+ depletion triggers influx via store-operated Ca2+ channels (SOCC) for SR replenishment. Several clinically relevant bronchodilators mediate their effect via cyclic nucleotides (cAMP, cGMP). We examined the effect of cyclic nucleotides on SOCC-mediated Ca2+ influx in enzymatically dissociated porcine ASM cells. SR Ca2+ was depleted by 1 microM cyclopiazonic acid in 0 extracellular Ca2+ ([Ca2+]o), nifedipine, and KCl (preventing Ca2+ influx through L-type and SOCC channels). SOCC was then activated by reintroduction of [Ca2+]o and characterized by several techniques. We examined cAMP effects on SOCC by activating SOCC in the presence of 1 microM isoproterenol or 100 microM dibutryl cAMP (cell-permeant cAMP analog), whereas we examined cGMP effects using 1 microM (Z)-1-[N-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate (DETA-NO nitric oxide donor) or 100 microM 8-bromoguanosine 3',5'-cyclic monophosphate (cell-permeant cGMP analog). The role of protein kinases A and G was examined by preexposure to 100 nM KT-5720 and 500 nM KT-5823, respectively. SOCC-mediated Ca2+ influx was dependent on the extent of SR Ca2+ depletion, sensitive to Ni2+ and La3+, but not inhibitors of voltage-gated influx channels. cAMP as well as cGMP potently inhibited Ca2+ influx, predominantly via their respective protein kinases. Additionally, cAMP cross-activation of protein kinase G contributed to SOCC inhibition. These data demonstrate that a Ni2+/La3+-sensitive Ca2+ influx in ASM triggered by SR Ca2+ depletion is inhibited by cAMP and cGMP via a protein kinase mechanism. Such inhibition may play a role in the bronchodilatory response of ASM to clinically relevant drugs (e.g., beta-agonists vs. nitric oxide).     

Pulse stimulation with odors or IBMX/forskolin potentiates responses in isolated olfactory neurons

Many odor responses are mediated by the adenosine 3',5'-cyclic monophosphate (cAMP) pathway in which the cAMP-gated current is amplified by Ca2+-dependent Cl- current. In olfactory neurons, prolonged exposure to odors decreases the odor response and is an adaptive effect. Several studies suggest that odor adaptation is linked to elevated intracellular Ca2+. In the present study, using the perforated configuration of the patch clamp technique, we found that repetitive odor stimulation elicits a potentiation of the subsequent responses in olfactory neurons. This potentiation is mimicked by stimulating the cAMP pathway and does not appear to be related to phosphorylation of ion channels since protein kinase inhibitors could not block it. Our data suggest that local increases in [Ca2+]i via activation of the cAMP pathway mediate the pulse-elicited potentiation. In the first odor application, entry of Ca2+ through cyclic nucleotide-gated channels appears to be buffered. Repetitive stimulation allows local increases in [Ca2+]i, recruiting more Ca2+-dependent Cl- channels with each subsequent odor pulse.     

Involvement of calcium and protein kinase C in the activation of the Na+/H+ exchanger in cultured bovine aortic endothelial cells stimulated by extracellular ATP

We have studied the activation of the Na+/H+ exchanger which leads to the intracellular alkalinization in cultured bovine aortic endothelial cells stimulated by extracellular ATP. The alkalinization induced by ATP was largely dependent on extracellular Ca2+ and the rate of alkalinization was decreased by about 60% in the absence of extracellular Ca2+. ATP caused a rapid and transient increase and a subsequent sustained increase of the intracellular Ca2+ concentration ([Ca2+]i) in the Ca2+ buffer, while only the rapid and transient increase of [Ca2+]i was observed in the absence of extracellular Ca2+. The Ca2+-depleted cells prepared by incubation in Ca2+-free buffer containing 0.1 mM EGTA showed only a slight increase of [Ca2+]i with no alkalinization on stimulation by ATP. The alkalinization was inhibited by 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7), an inhibitor of protein kinase C, but not by another isoquinoline analogue (HA 1004), which has a less inhibitory effect on the kinase. Phorbol 12-myristate 13-acetate also induced the alkalinization by the activation of the Na+/H+ exchanger. Neither dibutyryl cyclic AMP nor dibutyryl cyclic GMP affected the alkalinization induced by ATP. Treatment of the cells by pertussis and cholera toxins had no effect on the alkalinization. The results suggest that the increase in [Ca2+]i is essential for the ATP-induced activation of the Na+/H+ exchanger in cultured bovine aortic endothelial cells and a protein kinase C-dependent pathway is involved in the activation.     

Direct regulation of smooth muscle contractile elements by second messengers

The effects of adenosine 3',5'-cyclic monophosphate (cAMP), guanosine 3',5'-cyclic monophosphate (cGMP) and phorbol 12,13 dibutyrate (PDBu) on the Ca2+ sensitivity of the contractile elements in the rat mesenteric artery were investigated, using a method of permeabilizing smooth muscle with Staphylococcal alpha-toxin. Both cAMP and cGMP relaxed the permeabilized rat mesenteric artery at the intracellular Ca2+ concentrations [( Ca2+]i) held constant with Ca2+ EGTA buffer and Ca2+ ionophore, ionomycin. In addition, forskolin and sodium nitroprusside which activate adenylate and guanylate cyclases, respectively, also induced relaxation at a fixed [Ca2+]i. In contrast PDBu which stimulates protein kinase C caused an increase in force at a constant [Ca2+]i which could be partially reversed by cAMP or cGMP. These results indicate that second messengers exert direct control over smooth muscle Ca2+ sensitivity of the contractile elements, which is of physiologic and pharmacologic importance.     

Differential Involvement of the Arachidonic Acid Cascade on the α1-Adrenergic Potentiation of Vasoactive Intestinal Peptide-Versus β-Adrenergic-Stimulated Cyclic AMP and Cyclic GMP Accumulation in Rat Pinealocytes

In the rat pineal gland, alpha 1-adrenergic agonists, which stimulate arachidonic acid release, also potentiate vasoactive intestinal peptide (VIP)- or beta-adrenergic-stimulated cyclic AMP (cAMP) and cyclic GMP (cGMP) accumulation. In this study, the possible involvement of the arachidonic acid pathway in the potentiation mechanism was examined in dispersed rat pinealocytes using two inhibitors of the arachidonic acid cascade, indomethacin and nordihydroguaiaretic acid. These two inhibitors appeared to have differential effects on the alpha 1-adrenergic potentiation of VIP- or beta-adrenergic-stimulated cAMP and cGMP responses. Whereas nordihydroguaiaretic acid was effective in suppressing both the alpha 1-adrenergic potentiation of VIP- or beta-adrenergic-stimulated cAMP and cGMP responses, indomethacin inhibited selectively the VIP-mediated cAMP and cGMP responses. The role of arachidonic acid metabolites was further determined using several prostaglandins--A2, I2, E2, and F2 alpha--and leukotrienes--B4, C4, and D4. Of the seven compounds tested, prostaglandins E2 and F2 alpha stimulated basal cAMP but not cGMP accumulation. The prostaglandin E2- and F2 alpha-stimulated cAMP responses were additive to those stimulated by VIP or beta-adrenergic receptors. The other five compounds had no effects on basal or VIP- or beta-adrenergic-stimulated cAMP or cGMP accumulation. Taken together, these findings indicate that the arachidonic acid cascade is likely involved in the alpha 1-adrenergic potentiation of VIP- or beta-adrenergic-stimulated cAMP and cGMP accumulation. However, the specific arachidonic acid metabolite involved in the potentiation mechanisms of VIP- versus beta-adrenergic-stimulated cyclic nucleotide responses may be different.     

Cyclic GMP regulates free cytosolic calcium in the pancreatic acinar cell

The present studies were performed in order to measure the effects of cyclic GMP (cGMP) on the regulation of free cytosolic calcium [( Ca2+]i) in the pancreatic acinar cell. In guinea pig dispersed pancreatic acini the findings demonstrated that the Ca2+ ionophore, Br A23187, caused a sustained increase in [Ca2+]i in the presence of 3 mM CaCl2 in the media and a transient 20 fold rise in cellular cGMP followed by a sustained 3-4 fold rise in cellular cGMP. Increasing cellular cGMP with nitroprusside, hydroxylamine or dibutyryl cGMP had no effect on resting [Ca2+]i. However, these agents attenuated the increase in [Ca2+]i resulting from Br A23187-induced Ca2+ influx. Nitroprusside also attenuated the carbachol-induced sustained rise in [Ca2+]i that resulted from Ca2+ influx. The nitroprusside effect on carbachol-stimulated acini occurred without decreasing Ca2+ influx across the plasma membrane or alteration in the mobilization of Ca2+ from the intracellular agonist-sensitive pool. Inhibition of the increase in cellular cGMP caused by Br A23187 by the guanylate cyclase inhibitor, 6-anilino-5,8-quinolinedione (LY83583), resulted in augmentation of the increase in [Ca2+]i. This augmentation was reversed with dibutyryl cGMP. These results indicated that cGMP regulated [Ca2+]i in the pancreatic acinar cell. The mechanism involves the removal of Ca2+ from the cytoplasm.     

Thapsigargin potentiates histamine-stimulated HCl secretion in gastric parietal cells but does not mimic cholinergic responses.

The role of calcium in control of HCl secretion by the gastric parietal cell was examined using a recently available intracellular calcium-releasing agent, thapsigargin, which has been shown, in some cell types, to induce sustained elevation of intracellular calcium ([Ca2+]i), an action that appears to be independent of inositol lipid breakdown and protein kinase C activation and to be mediated, at least partially, by selective inhibition of endoplasmic reticulum Ca2(+)-ATPase. Using the calcium-sensitive fluorescent probe, fura-2, in combination with digitized video image analysis of single cells as well as standard fluorimetric techniques, we found that thapsigargin induced sustained elevation of [Ca2+]i in single parietal cells and in parietal cells populations. Chelation of medium calcium led to a transient rise and fall in [Ca2+]i, indicating that the sustained elevation in [Ca2+]i in response to thapsigargin was due to both intracellular calcium release and influx. Although thapsigargin appeared to affect the same calcium pool(s) regulated by the cholinergic agonist, carbachol, and the pattern of thapsigargin-induced increases in [Ca2+]i were similar to the plateau phase of the cholinergic response, thapsigargin did not induce acid secretory responses of the same magnitude as those initiated by carbachol (28 vs 600% of basal). The protein kinase C activator, 12-O-tetradecanoyl phorbol-13-acetate (TPA) potentiated the secretory response to thapsigargin but this combined response also did not attain the same magnitude as the maximal cholinergic response. In the presence but not the absence of medium calcium, thapsigargin potentiated acid secretory responses to histamine, which elevate both cyclic AMP (cAMP) and [Ca2+]i in parietal cells, as well as forskolin and cAMP analogues but had no effect on submaximal and an inhibitory effect on maximal cholinergic stimulation. Furthermore, thapsigargin did not fully mimic potentiating interactions between histamine and carbachol, either in magnitude or in the pattern of temporal response. Assuming that the action of thapsigargin is specific for intracellular calcium release mechanisms, these data suggest that 1) sustained influx of calcium is necessary but not sufficient for cholinergic activation of parietal cell HCl secretion and for potentiating interactions between cAMP-dependent agonists and carbachol; 2) mechanisms in addition to elevated [Ca2+]i and protein kinase C activation may be involved in cholinergic regulation; and 3) increases in [Ca2+]i in response to histamine are not directly involved in the mechanism of histamine-stimulated secretion.     

Cyclic AMP Analogues Potentiate k-Opiate and Attenuate α2-Adrenoceptor Agonist Effects on Intrasynaptosomal Free Calcium

The intrasynaptosomal free calcium concentration ([Ca2+]i) was measured in quin2-loaded synaptosomes prepared from rat cerebral cortex. Membrane-permeant cyclic adenosine-3',5'-monophosphate (cAMP) analogues [8-bromo-cyclic adenosine-3',5'-monophosphate (8-Br-cAMP) and dibutyryl-cyclic adenosine-3',5'-monophosphate (db-cAMP)] increased [Ca2+]i in a dose-dependent manner; The maximal increases were approximately 50% for 8-Br-cAMP and 35% for db-cAMP and occurred at approximately 10 microM with both analogues. Clonidine (1 microM) alone reduced [Ca2+]i by 26.5%; db-cAMP and 8-Br-cAMP attenuated this reduction to 14.2 and 8.2%, respectively. In contrast, the reduction (19.9%) in [Ca2+]i induced by the preferential kappa-opiate agonist dynorphin A(1-13) was not attenuated by the cAMP analogues; in fact, db-cAMP and 8-Br-cAMP potentiated the effect of dynorphin A(1-13) (1 microM), producing decreases in [Ca2+]i of 33.6 and 29.6%, respectively. We conclude that although alpha 2-adrenergic and kappa-opiate receptors both reduce [Ca2+]i, the alpha 2-adrenoceptor-mediated response and the kappa-opiate receptor-mediated response involve different effector mechanisms. It appears that presynaptic alpha 2-adrenoceptor agonist effects are linked to reductions in adenylate cyclase activity and cAMP production and a resultant increase in Ca2+ sequestration, Ca2+-channel blockade, or both. On the other hand, the kappa-opiate-mediated effects possibly involve an increase in cAMP production and a blockade of Ca2+ entry.     

Comparison of the role of protein kinase C in platelet functional responses induced by three different mechanisms, PAF, ionomycin and arachidonic acid.

The role of protein kinase C (PKC) in modulating platelet activation has been examined in platelets pre-incubated with either the PKC activator 12-O-tetradecanoylphorbol 13-acetate (TPA) or the non-specific protein kinase inhibitor, staurosporine. In order to determine where in the signal transduction pathway PKC is exerting its effect platelets were activated either with a receptor-operated stimulus platelet activating factor (PAF) or by direct elevation of [Ca2+]i (ionomycin) or with arachidonic acid which is converted into thromboxane B2 (TxB2). In PAF-stimulated platelets activation of PKC inhibited both [Ca2+]i elevation and TxB2 generation but had no effect on 5-hydroxytryptamine (5-HT) release whilst staurosporine increased the duration of [Ca2+]i elevation and potentiated TxB2 generation but inhibited 5-HT release. In ionomycin-stimulated platelets modulation of PKC had no effect on [Ca2+]i elevation but in contrast to PAF-stimulated platelets PKC activation caused potentiation of TxB2 generation and 5-HT release whilst inhibition of PKC caused inhibition of TxB2 generation and 5-HT release. Modulation of PKC did not affect arachidonic acid-induced TxB2 generation. These findings suggest that in receptor activated platelets endogenously activated PKC is exerting a negative feedback role, however, when [Ca2+]i elevation is not modified by PKC activation or inhibition (such as in ionomycin stimulated platelets) the relationship between the state of PKC activation and subsequent platelet functional responses corresponds more closely. The findings from this study suggest a different relationship between PKC and TxB2 generation than between PKC and dense granule release in PAF-stimulated platelets.     

Alkalinization-Induced Changes in Intracellular Calcium in Rat Spinal Cord Neurons

It is well-known that pH changes can influence a lot of cellular processes. In this work, we have specifically studied the influence of alkalinization, which can be developed in spinal cord neurons during hyperventilation (respiratory alkalosis) and chronic renal failure (metabolic alkalosis) on calcium homeostasis. Application of Tyrode solution with increased pH (pH = 8.8) to secondary sensory neurons isolated from rat spinal dorsal horn induced elevation of intracellular free calcium concentration in the cytosol ([Ca2+]i) if applied after membrane depolarization. Repetitive application of alkaline solution led to disappearance of such elevations. Depletion of endoplasmic reticulum (ER) calcium stores by 30 mM caffeine almost completely blocked the effect of elevated extracellular pH. If caffeine-induced [Ca2+]i transients were evoked during alkalinization, their amplitudes were decreased by 41%. Preapplication of 500 nM ionomycin resulted in disappearance of alkalinization-induced [Ca2+]i transients, whereas prolonged applications (for 20 min) of 200 nM thapsigargin, a blocker of Ca2+ ATPase of the endoplasmic reticulum, resulted in disappearance of the rapid phase of the [Ca2+]i transients induced by alkalinization. Preapplication of the mitochondrial protonophore CCCP (10 microM) also induced changes in the alkalinization-induced calcium response--it lost its peak and was transformed into an irregular wave terminating in several seconds. The data obtained indicate that alkalinization induces an increase of [Ca2+]i level in the investigated neurons via a combined action of both intracellular Ca2+-accumulating structures--the endoplasmic reticulum and mitochondria. This suggestion was supported by morphological data that both structures in these neurons are tightly connected and may interact during release of accumulated calcium ions.     

Cyclic AMP inhibition of fMet-Leu-Phe-dependent metabolic responses in human neutrophils is not due to its effects on cytosolic Ca2+.

Cyclic AMP powerfully inhibits the fMet-Leu-Phe-dependent respiratory burst and exocytosis of azurophilic and specific granules without affecting Ca2+ release from intracellular stores. The elevation of [Ca2+]i induced by fMet-Leu-Phe is short-lived in cyclic AMP-treated cells and similar to that of untreated cells stimulated in the absence of external Ca2+. Nevertheless, in these latter cells fMet-Leu-Phe induces metabolic activation. We therefore suggest that the inhibitory action of cyclic AMP on neutrophil responses is not due to its effects on [Ca2+]i homoeostasis.     

Mitogenic and non-mitogenic ligands trigger a calcium-dependent cytosolic acidification in human T lymphocytes

We have investigated the patterns of cytosolic pH and Ca2+ ([Ca2+]i) changes after exposure of human peripheral blood T cells to different mitogenic and non-mitogenic ligands. Using ligands that have different accessory cell requirements and varying effect on [Ca2+]i or cell proliferation, we observed that intracellular acidification occurred only with agents that increased [Ca2+]i. However, treatment of the cells with the phorbol ester, 12-O-tetradecanoylphorbol-13-acetate, results in significant cytosolic alkalinization without detectable acidification, but did not affect the proliferative responses to mitogenic ligands and was a potent co-mitogen with non-mitogenic ligands. These data indicate that initial acidification or alkalinization responses are not essential for early activation or triggering of DNA synthesis.     

Forskolin and 3-Isobutyl-l-Methylxanthine Increase Basal and Sodium Nitroprusside-Elevated Cyclic GMP Levels in Adult Guinea-Pig Cerebellar Slices

Abstract: In this study, the interaction between 3′,5′-cyclic adenosine monophosphate (cAMP) and 3′,5′-cyclic guanosine monophosphate (cGMP) in [³H]adenine-or [³H]-guanine-prelabelled adult guinea-pig cerebellar slices was investigated. Basal levels of [³H]cGMP were enhanced by forskolin, although no plateau was reached over the concentration range tested (0.1-100 μM). However, forskolin elicited a concentration-dependent, saturable potentiation of sodium nitroprusside (SNP)-stimulated [³H]cGMP accumulation (forskolin EC₅₀ value of 0.98 β 0.23 μM; 10 μM forskolin produced a 1.8 β 0.3-fold potentiation of the SNP response at 2.5 min). The forskolin potentiation was observed at all concentrations of SNP tested (0.001-10 mM). forskolin also elicited a large stimulation of [³H]-cAMP in [³H]adenine-prelabelled guinea-pig cerebellar slices; however, 1,9-dideoxyforskolin failed to elicit either a [³H]cAMP response or a potentiation of the SNP-induced [³H]cGMP response at concentrations up to 100 μM. Pretreatment with oxyhaemoglobin (50 μM) inhibited the response to SNP (1 mM) and forskolin (10 μM), as well as the response evoked by the combination of SNP and forskolih. A^G-Nitro-l -arginine (100 μM) inhibited the response to forskolin alone, but did not change the response to SNP or the potentiation induced by forskolin on SNP-induced [³H]cGMP levels. The protein kinase inhibitors 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H7; 100 μM), staurosporine (10 μM), polymyxin B (100 μM), and Ro 31-8220 (10 μM) had no effect on the [³H]cGMP response to either SNP or the combination of SNP plus forskolin. N⁶,2′-Dibutyryl cAMP, at concentrations up to 10 mM, was also without effect on [³H]cGMP levels induced by SNP. 3-lso-butyl-1-methylxanthine reproduced the effect of forskolin on SNP-induced [³H]cGMP levels, but a less-than-additive effect was observed when the response to SNP was studied in the presence of forskolin and 3-isobutyl-1-methylxanthine. Taken together, these results infer that crosstalk between cyclic nucleotides takes place in guinea-pig cerebellar slices, and that cAMP may regulate cGMP-mediated responses in this tissue.     

A Novel Effect of Cyclic AMP on Capacitative Ca2+ Entry in Cultured Rat Cerebellar Astrocytes

One of the most important intracellular Ca2+ regulatory mechanisms in nonexcitable cells, "capacitative Ca2+ entry" (CCE), has not been adequately studied in astrocytes. We therefore investigated whether CCE exists in cultured rat cerebellar astrocytes and studied the roles of cyclic AMP (cAMP) and protein kinase C (PKC) in CCE. We found that (1) at least two different intracellular Ca2+ stores, the endoplasmic reticulum and mitochondria, are present in cerebellar astrocytes; (2) CCE does exist in these cells and can be inhibited by Ni2+, miconazole, and SKF 96365; (3) CCE can be directly enhanced by an increase in intracellular cAMP, as 8-bromoadenosine 3',5'-cyclic monophosphate (8-brcAMP), forskolin, and isobutylmethylxanthine have stimulatory effects on CCE; and (4) neither of the two potent protein kinase A (PKA) inhibitors, H8 and H89, nor a specific PKA agonist, Sp-adenosine 3',5'-cyclic monophosphothioate, had a significant effect on cAMP-enhanced Ca2+ entry. The [Ca2+]i increase was not due to a release from calcium stores, hyperpolarization of the membrane potential, inhibition of calcium extrusion, or a change in pHi, suggesting that cAMP itself probably acts as a novel messenger to modulate CCE. We also conclude that activation of PKC results in an increase in CCE. cAMP and PKC seem to modulate CCE by different pathways.     

Thrombin-induced changes of intracellular [Ca2+] and pH in human platelets. Cytoplasmic alkalinization is not a prerequisite for calcium mobilization

We have studied the effects of thrombin (0.1 U/ml) on intracellular Ca2+ ([Ca2+]i) and pH (pHi) in human platelets loaded with fluorescent indicators. Thrombin produced a transient decrease of pHi which reached its maximum within 15-25 seconds (s) and was followed by a sustained alkalinization which brought pHi above the resting value. [Ca2+]i increased transiently peaking at 5-10 s. The late alkalinization induced by thrombin was antagonized by ethylisopropylamiloride, an inhibitor of Na+-H+ exchange, and by sphingosine, an inhibitor of protein kinase C, with little effect on the [Ca2+]i transient. The early acidification was not inhibited by these treatments. We conclude tha the thrombin-induced changes of [Ca2+]i and pHi are mediated by different mechanisms. The late alkalinization is due to activation of Na+/H+ exchange mediated by protein kinase C and, contrarily to previous proposals (Siffert, W. and Akkerman, J.W.N. (1987) Nature 325, 456-458), it is not necessary for calcium mobilization from intracellular stores.     

Inhibition of platelet-activating-factor-induced human platelet activation by prostaglandin D2. Differential sensitivity of platelet transduction processes and functional responses to inhibition by cyclic AMP.

It has been proposed that cyclic AMP inhibits platelet reactivity: by preventing agonist-induced phosphoinositide hydrolysis and the resultant formation of 1,2-diacylglycerol and elevation of cytosolic free Ca2+ concentration [( Ca2+]i); by promoting Ca2+ sequestration and/or extrusion; and by suppressing reactions stimulated by (1,2-diacylglycerol-dependent) protein kinase C and/or Ca2+-calmodulin-dependent protein kinase. We used the adenylate cyclase stimulant prostaglandin D2 to compare the sensitivity to cyclic AMP of the transduction processes (phosphoinositide hydrolysis and elevation of [Ca2+]i) and functional responses (shape change, aggregation and ATP secretion) that are initiated after agonist-receptor combination on human platelets. Prostaglandin D2 elicited a concentration-dependent elevation of platelet cyclic AMP content and inhibited platelet-activating-factor(PAF)-induced ATP secretion [I50 (concn. causing 50% inhibition) approximately 2 nM], aggregation (I50 approximately 3 nM), shape change (I50 approximately 30 nM), elevation of [Ca2+]i (I50 approximately 30 nM) and phosphoinositide hydrolysis (I50 approximately 10 nM). A 2-fold increase in cyclic AMP content resulted in abolition of PAF-induced aggregation and ATP secretion, whereas maximal inhibition of shape change, phosphoinositide hydrolysis and elevation of [Ca2+]i required a greater than 10-fold elevation of the cyclic AMP content. This differential sensitivity of the various responses to inhibition by cyclic AMP suggests that the mechanisms underlying PAF-induced aggregation and ATP secretion differ from those underlying shape change. Thus a major component of the cyclic AMP-dependent inhibition of PAF-induced platelet aggregation and ATP secretion is mediated by suppression of certain components of the activation process that occur distal to the formation of DAG or elevation of [Ca2+]i.     

Regulation of the plasma membrane Ca2+ pump by cyclic nucleotides in cultured vascular smooth muscle cells

We investigated the mechanisms of Ca2+ extrusion from cultured rat aortic smooth muscle cells while monitoring changes in the cytosolic Ca2+ concentration ([Ca2+]i) using fura 2 fluorescence. 45Ca2+ efflux from these cells consisted of two major mechanisms; one was dependent on the extracellular sodium concentration (Na+o) and the other was independent of Na+o. Na+o-dependent efflux increased monotonically with increasing [Ca2+]i between 0.1 and 1.0 microM, whereas Na+o-independent efflux reached a plateau at 0.6-1 microM [Ca2+]i with a half-maximum obtained at about 0.16 microM. At [Ca2+]i below 1 microM, the latter was significantly greater than the former. Unlike the Na+o-dependent mechanism, Na+o-independent 45Ca2+ efflux was inhibited almost entirely by extracellularly added La3+ or a combination of high extracellular pH (pH 8.8) and 20 mM Mg2+. It was also inhibited, although not completely, by compound 48/80, a calmodulin antagonist, and vanadate. These results strongly suggest that Na+o-dependent and Na+o-independent 45Ca2+ effluxes occur via the Na+/Ca2+ exchanger and the ATP-dependent Ca2+ pump, respectively. Sodium nitroprusside and atrial natriuretic factor, which are agents that stimulate intracellular production of cGMP, and 8-BrcGMP significantly accelerated the Na+o-independent 45Ca2+ efflux especially at low [Ca2+]i. Forskolin, dibutyryl cAMP, and 8-Br-cAMP, however, showed no stimulation. These results suggest that the plasma membrane Ca2+ pump is regulated by cGMP but not by cAMP in intact vascular smooth muscle cells.     

Two Possibly Distinct Prostaglandin E1 Receptors in N1E-115 Clone: One Mediating Inositol Trisphosphate Formation, Cyclic GMP Formation, and Intracellular Calcium Mobilization and the Other Mediating Cyclic AMP Formation

Prostaglandin E1 (PGE1)-mediated transmembrane signal control systems were investigated in intact murine neuroblastoma cells (clone N1E-115). PGE1 increased intracellular levels of total inositol phosphates (IP), cyclic GMP, cyclic AMP, and calcium ([Ca2+]i). PGE1 transiently increased inositol 1,4,5-trisphosphate formation, peaking at 20 s. There was more than a 10-fold difference between the ED50 for PGE1 at cyclic AMP formation (70 nM) and its ED50 values at IP accumulation (1 microM), cyclic GMP formation (2 microM), and [Ca2+]i increase (5 microM). PGE1-mediated IP accumulation, cyclic GMP formation, and [Ca2+]i increase depended on both the concentration of PGE1 and extracellular calcium ions. PGE1 had more potent intrinsic activity in cyclic AMP formation, IP accumulation, and cyclic GMP formation than did PGE2, PGF2 alpha, or PGD2. A protein kinase C activator, 4 beta-phorbol 12 beta-myristate 13 alpha-acetate, had opposite effects on PGE1-mediated IP release and cyclic GMP formation (inhibitory) and cyclic AMP formation (stimulatory). These data suggest that there may be subtypes of the PGE1 receptor in this clone: a high-affinity receptor mediating cyclic AMP formation, and a low-affinity receptor mediating IP accumulation, cyclic GMP formation, and intracellular calcium mobilization.     

Inhibition of prostaglandin E1-induced elevation of cytoplasmic free calcium ion by protein kinase C-activating phorbol esters and diacylglycerol in Swiss 3T3 fibroblasts

In quiescent cultures of Swiss 3T3 cells, prostaglandin E1 (PGE1) known to elevate cAMP increased rapidly cytoplasmic free Ca2+ concentration ([Ca2+]i) as measured with the fluorescent Ca2+ indicator quin2. The primary source of the PGE1-induced elevation of [Ca2+]i was extracellular. Pretreatment of the cells with various doses of 12-O-tetradecanoylphorbol-13-acetate (TPA), a potent protein kinase C-activating phorbol ester, inhibited the PGE1-induced elevation of [Ca2+]i in a dose-dependent manner. Inversely, TPA enhanced slightly the PGE1-induced increase of cAMP. TPA alone did not affect the basal level of [Ca2+]i or cAMP in the absence of PGE1. The inhibitory action of TPA on the PGE1-induced elevation of [Ca2+]i was mimicked by other protein kinase C-activating agents such as phorbol 12,13-dibutyrate and 1-oleoyl-2-acetylglycerol. 4 alpha-Phorbol 12,13-didecanoate known to be inactive for protein kinase C was ineffective in this capacity. Prolonged treatment of the cells with phorbol 12,13-dibutyrate resulted in the down-regulation and disappearance of protein kinase C. In these protein kinase C-deficient cells, PGE1 still elevated [Ca2+]i to the same extent as that in the control cells, but TPA did not inhibit the PGE1-induced elevation of [Ca2+]i. These results strongly suggest that protein kinase C serves as an inhibitor for PGE1-induced Ca2+ influx in Swiss 3T3 cells.     

Cyclic GMP modulates store-operated calcium entry inducing phosphatidylserine translocation at the surface of megakaryocytic cells

When subjected to stimulation, cells from the vascular compartment show a spontaneous collapse of the plasma membrane phospholipid asymmetry and phosphatidylserine is exposed at the external leaflet. Thus, phosphatidylserine externalization is essential for normal hemostasis and phagocytosis. The mechanism governing the migration of phosphatidylserine to the exoplasmic leaflet is not yet fully understood. We have proposed that store-operated calcium entry (SOCE) constitutes a key step of this process. Here, interaction of [Ca(2+)](i), cAMP and cGMP pathways and phosphatidylserine exposure was examined in human megakaryocytic cells. The membrane permeable cAMP and cGMP analogues, pCPT-cAMP and pCPT-cGMP, enhanced the Ca(2+) signal induced by ionophore and SOCE. Responses to pCPT-cAMP and pCPT-cGMP were independent of protein kinase A, protein kinase G (PKG) or ERK pathways. Inhibition of small G-proteins reduced or abolished the increase of [Ca(2+)](i) induced by pCPT-cAMP or pCPT-cGMP, respectively. pCPT-cGMP but not pCPT-cAMP enhanced the ability of cells to expose phosphatidylserine. This effect was not prevented by the inhibition of PKG or small G-proteins. These results show the differential role of cyclic nucleotides in the Ca(2+)-dependent membrane remodeling. Hence, pCPT-cGMP is another regulatory element for the completion of SOCE-induced phosphatidylserine transmembrane redistribution in HEL cells through a mechanism implicating small G-proteins.