Calcium signaling in human platelets

Thrombin causes neurite retraction in neuronal cells through activation of cell surface receptors

The mechanism by which thrombin induces neurite retraction was studied in NB2a mouse neuroblastoma cells. The rapid effect of thrombin (completed within minutes) appears to involve an interaction between its anion-binding exosite and the thrombin receptor. Structural alterations of this site increase the EC50 for thrombin-mediated retraction, and a hirudin C-terminal peptide that blocks this site inhibits the response. The thrombin effect was mimicked by a 14 amino acid peptide starting with Ser-42, at the proposed cleavage site of the human thrombin receptor. The protein kinase inhibitors staurosporine and H-7 blocked thrombin-induced retraction. It is therefore proposed that thrombin-mediated neurite retraction is caused by cleavage-induced activation of the thrombin receptor and involves stimulation of a protein kinase(s).

Role of cGMP and cGMP-dependent protein kinase in nitrovasodilator inhibition of agonist-evoked calcium elevation in human platelets

Most platelet agonists activate and elevate the cytosolic free calcium concentration in human platelets through receptor-dependent mechanisms that are antagonized by cAMP- and cGMP-elevating agents. Nitrovasodilators such as nitroprusside and endothelium-derived relaxing factor are potent cGMP-elevating platelet inhibitors. In the present study, the role of cGMP and cGMP-dependent protein kinase in nitrovasodilator inhibition of ADP- and thrombin-evoked calcium elevation and activation of human platelets was investigated. Preincubation of platelets with 8-(4-chlorophenylthio)guanosine 3',5'-cyclic monophosphate (8-pCPT-cGMP; a membrane-permeant selective activator of the cGMP-dependent protein kinase that does not significantly affect cGMP-regulated phosphodiesterases) inhibited the thrombin-induced phosphorylation mediated by myosin light chain kinase and protein kinase C. Nitrovasodilator-induced protein phosphorylation in human platelets was distinct from that induced by cAMP-elevating prostaglandins and could be mimicked by 8-pCPT-cGMP. Preincubation of human platelets with nitrovasodilators or 8-pCPT-cGMP inhibited the ADP- and thrombin-evoked calcium elevation in the presence and absence of external calcium. Nitrovasodilators and 8-pCPT-cGMP also inhibited the agonist-induced Mn2+ influx, but stopped-flow experiments indicated that the ADP receptor-operated cation channel was not significantly inhibited. These results suggest that in human platelets nitrovasodilators inhibit the agonist-induced calcium mobilization from intracellular stores and the secondary store-related calcium influx but not the ADP receptor-operated cation channel. The results also suggest that these nitrovasodilator effects are mediated by cGMP and the cGMP-dependent protein kinase.

Cyclic nucleotide-dependent regulation of agonist-induced calcium increases in mouse megakaryocytes

1. The regulatory effects of cyclic AMP and cyclic GMP on ADP- and thrombin-induced increases in [Ca2+]i were studied in mouse bone marrow megakaryocytes. Changes in [Ca2+]i were continuously monitored in single Fura-2-loaded cells using microspectrofluorometry, and cyclic nucleotides were directly introduced into the single cells using the whole-cell patch-clamp technique. 2. ADP increased [Ca2+]i in a concentration-dependent fashion, and its threshold concentration was in the order of 0.01 microM. A low dose of ADP (below 0.1 microM) induced a transient response of [Ca2+]i which recovered to original levels during the stimulation. A high dose of ADP (0.3-10 microM) induced a biphasic response of [Ca2+]i with an initial peak and a plateau lasting until the end of the stimulation. Repeated stimulation with the same dose of ADP induced a reduced response, probably as a result of desensitization. 3. Thrombin increased [Ca2+]i in a concentration-dependent manner. The time courses of the responses were different from those caused by ADP. Thrombin-induced responses lacked the initial sharp peak observed in ADP-induced responses, and caused a sustained response. 4. The ADP-induced increase in [Ca2+]i was antagonized by the presence of prostaglandin E1 (PGE1, 100-1000 nM), in the medium, and by direct injection of cyclic AMP (100-500 microM) or cyclic GMP (500 microM) into the megakaryocyte. When 500 microM-cyclic AMP was injected into the cells, the rise of [Ca2+]i induced by ADP was reduced by 85%. Effects of these antagonists were inhibited by treatment with a protein kinase inhibitor, H-8. Thrombin-induced increases in [Ca2+]i were reduced by direct injection of cyclic AMP or cyclic GMP. 5. ADP could induce an increase in [Ca2+]i in the absence of external Ca2+. The time course of the response was essentially similar to that observed in the normal condition (1 mM-CaCl2), but the size of the response was reduced by 33%. Thus, 67% of the rise in [Ca2+]i induced by ADP could be accounted for by calcium mobilization from internal storage pools. The presence of NiCl2 (5 mM) duplicated the effects of external Ca2+ removal, suggesting the involvement of a Ca2+ influx pathway, which could be inhibited by Ni2+ in ADP stimulation. 6. Injection of cyclic AMP or cyclic GMP reduced ADP-induced increases in [Ca2+]i under conditions of inhibited Ca2+ influx by NiCl2 (5 mM).(ABSTRACT TRUNCATED AT 400 WORDS)

Is the bradykinin-induced Ca2+ influx and the formation of endothelium-derived relaxing factor mediated by a G protein?

In cultured porcine aortic endothelial cells bradykinin produced a long-lasting Ca2+ influx. In contrast to the G protein-independent Ca2+ entry evoked by ionomycin or digitonin, bradykinin-induced Ca2+ influx was antagonized by Ni2+ with an IC50 value of about 50 microM. Since identical IC50 values for Ni2+ were found when Ca2+ entry was induced by sodium fluoride or GTP gamma S, we suggest that stimulation of G protein(s) results in the activation of the same Ca2+ channels as stimulation by bradykinin. This conclusion is supported by our findings that inhibition of GTPase by mepacrine amplified bradykinin-stimulated Ca2+ influx, but did not interfere with the effect of the Ca2+ ionophore A23187. Similar to its effect on Ca2+ influx, mepacrine also potentiated endothelium-derived relaxing factor (EDRF) formation by bradykinin and sodium fluoride, but did not affect A23187-induced EDRF biosynthesis. We therefore suggest that in endothelial cells the bradykinin-induced Ca2+ influx and the resulting formation of EDRF are regulated by a G protein.

Agonist-induced Ca2+ influx into human platelets is secondary to the emptying of intracellular Ca2+ stores

We have studied the relation between the filling state of the intracellular Ca2+ stores and the plasma-membrane permeability to Mn2+, used here as a Ca2+ surrogate for Ca2+ channels. Emptying of the intracellular Ca2+ stores either by incubation in Ca(2+)-free medium or by treatment with low concentrations of the Ca2+ ionophore ionomycin accelerated the influx of Mn2+. Refilling of the Ca2+ stores by incubation in Ca(2+)-containing medium restores low Mn2+ permeability. This Ca(2+)-store-regulated permeability was inhibited by Ni2+ and by cytochrome P-450 inhibitors. Stimulation of platelets with thrombin produced Ca2+ release from the intracellular stores, which was followed, after a temperature-dependent lag (2 s at 37 degrees C; 5 s at 18 degrees C), by an acceleration of Mn2+ influx. Cytochrome P-450 inhibitors prevented the thrombin-induced Mn2+ influx, with little effect on the Ca2+ mobilization from the intracellular stores. Ki values were similar to those estimated for inhibition of the store-regulated permeability in non-stimulated platelets. Similar results were found in platelets stimulated by platelet-activating factor or by ADP. We propose that agonist-induced Ca2+ (Mn2+) influx in platelets is secondary to the emptying of the intracellular Ca2+ stores. The activation of the plasma-membrane Ca2+ (Mn2+) pathway may take place by a mechanism involving microsomal cytochrome P-450, similar to that described previously in thymocytes [Alvarez, Montero & García-Sancho (1991) Biochem. J. 274, 193-197] and neutrophils [Montero, Alvarez & García-Sancho (1991) Biochem. J. 277, 73-79].

Resting and ADP-evoked changes in cytosolic free sodium concentration in human platelets loaded with the indicator SBFI

1. Cytosolic free Na+ concentration, [Na+]i, was investigated in human platelets loaded with the fluorescent indicator SBFI (sodium-binding benzofuran isophthalate). 2. SBFI fluorescence from platelet suspensions was measured at excitation wavelengths of 340 and 385 nm and the 340/385 nm fluorescence ratio was calibrated in terms of [Na+]i in situ. [Na+]i was set to known values by resuspending cells in media with various [Na+], in the presence of the Na(+)-K+ ionophore, gramicidin. 3. Basal free [Na+]i was 5.5 +/- 0.3 mM (n = 50). This is considerably lower than estimates of total platelet Na+, suggesting that much intracellular Na+ is sequestered or bound. 4. ADP (40 microM) evoked a rise in [Na+]i from 6.4 +/- 0.7 to 18.3 +/- 1.1 mM (n = 8). The ADP-evoked rise in [Na+]i was abolished when external Na+ was replaced with N-methyl-D-glucamine. This indicates that the rise in [Na+]i was due to Na+ entry. 5. In platelets loaded with the fluorescent pH indicator, BCECF, 40 microM-ADP was shown to evoke a fall in cytosolic pH (pHi) from 7.21 +/- 0.03 to 7.12 +/- 0.03 (n = 10). Three minutes after ADP addition pHi had only recovered to 7.15 +/- 0.03. The recovery was dependent on external Na+, suggesting it was mediated by Na(+)-H+ exchange. However, this would only account for an increase in [Na+]i of approximately 0.5 mM, indicating most of the ADP-evoked Na+ entry occurred by other mechanisms. 6. Stopped-flow fluorimetry showed that the ADP-evoked rise in [Na+]i commenced without measurable delay and peaked within 1 s. The initial kinetics were thus similar to those reported for ADP-evoked rises in [Ca2+]i. 7. Cell-attached patch-clamp recordings showed that ADP evoked single-channel inward currents when included in the pipette-filling solution. The currents were similar whether Ca2+ was present or absent from the pipette. The slope conductance was 11 pS in the presence of external Ca2+ and 10 pS in its absence. Current-voltage relationships were similar and the reversal potentials were close to 0 mV under both conditions. 8. SK & F 96,365 (20 microM), a blocker of receptor-mediated Ca2+ entry in several non-excitable cells, blocked the ADP-evoked rise in [Na+]i. This compound has been shown to only partly block the biphasic ADP-evoked rise in [Ca2+]i, being selective for the fast, receptor-operated phase of entry. 9. These data suggest that ADP rapidly activates a channel in that platelet plasma membrane which is permeable to Na+ and divalent cations.

Rat platelets are deficient in internal Ca2+ release and require influx of extracellular Ca2+ for activation

Calcium fluxes were studied in fura-2-labeled rat platelets. Thrombin, ADP and ionomycin induced rapid mobilization of internally stored Ca2+, which resulted in only a moderate increase of cytosolic [Ca2+]i. Thrombin and ADP stimulated influx of extracellular Ca2+, which was monitored as uptake of 45Ca2+ and of Mn2+. With either agonist, the influx of Ca2+ magnified the initial increase of [Ca2+]i. Since responses of rat platelets were dependent on external [Ca2+], we conclude that Ca2+ influx complements the mobilization of internal stores to reach sufficiently high [Ca2+]i for full activation. A regulatory effect of protein kinase C modulators was observed on both agonist-induced elevation of [Ca2+]i and receptor-mediated Ca2+ entry.

Cytosolic ionized calcium in human platelets: the influence of collagen and a novel antiplatelet agent

Two phases of calcium mobilization were observed when aequorin-loaded human platelets, suspended in a nominally calcium-free medium containing 0.1 mM EGTA, were stimulated with collagen. The first phase coincided with platelet shape change, and the second phase corresponded to aggregation. On the other hand, only one [Ca2+]i peak was found in systems containing 1.0 mM Ca, or 1.0 or 2.0 mM EGTA. A novel antiplatelet compound alpha,alpha'-bis [3-(N,N-diethylcarbamoyl)piperidino]-p-xylene dihydrobromide, inhibited both [Ca2+]i peaks. It is suggested that inhibition of the mobilization of intraplatelet calcium stores as well as the blocking of transmembrane calcium flux may be responsible for the platelet aggregation-inhibitory action of this compound.

Effects of taurolithocholate, a Ca2(+)-mobilizing agent, on cell Ca2(+) in rat hepatocytes, human platelets and neuroblastoma NG108-15 cell line

The monohydroxy bile acid taurolithocholate permeabilizes the endoplasmic reticulum to Ca2+ in rat liver cells. To assess whether this action on the endoplasmic reticulum was restricted to this tissue, the effects of bile acid were investigated in two cell types quite unrelated to rat hepatocyte, namely human platelets and neuronal NG108-15 cell line. The results showed that taurolithocholate (3-100 microM) had no effect on free cytosolic [Ca2+] in human platelets and NG108-15 cells. whereas it increased it from 180 to 520 nM in rat hepatocytes. In contrast, in cells permeabilized by saponin, taurolithocholate initiated a profound release of the stored Ca2+ from the internal Ca2+ pools in the three cell types. The bile acid released 90% of the Ca2+ pools, with rate constants of about 5 min-1 and half-maximal effects at 15-30 microM. The results also showed that, in contrast with liver cells, which displayed an influx of [14C]taurolithocholate of 2 nmol/min per mg, human platelets and the neuronal cell line appeared to be resistant to [14C]taurolithocholate uptake. The influx measured in these latter cells was about 100-fold lower than in rat liver cells. Taken together, these data suggest that human platelets and NG108-15 cells do not possess the transport system for concentrating monohydroxy bile acids into cells. However, they show that human platelets and neuronal NG108-15 possess, in common with liver cells, the intracellular system responsible for taurolithocholate-mediated Ca2+ release from internal stores.

Octimibate, a potent non-prostanoid inhibitor of platelet aggregation, acts via the prostacyclin receptor

1. Octimibate, 8-[(1,4,5-triphenyl-1H-imidazol-2-yl)oxy]octanoic acid, is reported to have antithrombotic properties. This is in addition to its antihyperlipidaemic effects which are due to inhibition of acylCoA:cholesterol acyltransferase (ACAT). The aim of this study was to investigate the mechanism of the antithrombotic effect of octimibate, and to determine whether the effects of octimibate are mediated through prostacyclin receptors. 2. In suspensions of washed (plasma-free) human platelets, octimibate is a potent inhibitor of aggregation; its IC50 is approx. 10 nM for inhibition of aggregation stimulated by several different agonists, including U46619 and ADP. The inhibitory effects of octimibate on aggregation are not competitive with the stimulatory agonist; the maximal response is suppressed but there is no obvious shift in potency of the agonist. In platelet-rich plasma, octimibate inhibits agonist-stimulated aggregation with an IC50 of approx. 200 nM. 3. Octimibate also inhibits agonist-stimulated rises in the cytosolic free calcium concentration, [Ca2+]i, in platelets. Both Ca2+ influx and release from intracellular stores are inhibited. The effects of octimibate on aggregation and [Ca2+]i are typical of agents that act via elevation of adenosine 3':5'-cyclic monophosphate (cyclic AMP). Similar effects are seen with forskolin, prostacyclin (PGl2) and iloprost (a stable PGl2 mimetic). 4. Octimibate increases cyclic AMP concentrations in platelets and increases the cyclic AMP-dependent protein kinase activity ratio. Octimibate stimulates adenylyl cyclase activity in human platelet membranes, with an EC50 of 200 nM. The maximal achievable activation of adenylyl cyclase by octimibate is 60% of that obtainable with iloprost. Octimibate has no effect on the cyclic GMP-inhibited phosphodiesterase (phosphodiesterase-ITI), which is the major cyclic AMP-degrading enzyme in human platelets.5. Octimibate inhibits, apparently competitively, the binding of [3H]-iloprost (a stable PGl2 mimetic) to platelet membranes; the estimated Ki is 150 nm. 6. The platelets of different species show considerable differences in the apparent potency of their inhibition of aggregation by octimibate; platelets from cynomolgus monkeys are 3 fold more sensitive than those from humans, while rat, cat and cow platelets are 50, 100, and 250 fold less sensitive than human platelets. The sensitivity of these different species to iloprost, however, varies over a range of only 10 fold with no obvious difference between primates and non-primates. 7. Octimibate appears to be a potent agonist (aggregation), or partial agonist (adenylyl cyclase), at prostacyclin receptors and is the first non-prostanoid agent of this type to be identified. The species differences in relative potency of octimibate and iloprost may reflect the existence of receptor subtypes.