ADP-induced changes in [32P]phosphate labeling of phosphatidylinositol-4,5-bisphosphate in washed rabbit platelets made refractory by prior ADP stimulation

Functional expression of a P2T ADP receptor in Xenopus oocytes injected with megakaryocyte (CMK 11-5) RNA

Since the P2T purinergic (ADP) receptor is unique to the megakaryocytic/platelet lineage, cells of this lineage were screened for the relative effects of ADP and ATP in intracellular Ca2+ levels. Like platelets, CMK 11-5 cells responded with an increase in intracellular Ca2+ mobilization in response to ADP but not to ATP or adenosine. In contrast, both nucleotides increased intracellular Ca2+ mobilization in the megakaryoblastic cell lines MO7E and Meg-01, indicating that they contain P2Y receptors or a mixed complement of purinergic receptors. Pharmacological responsiveness of CMK 11-5 cells to nucleotides paralleled those of platelets, in which ADP and ADP-alpha-S are active as agonists and ATP and ATP-alpha-S are inactive as agonists but act as antagonists. [3H]ADP and 35S-ATP-alpha-S bound to CMK 11-5 cells at a high-affinity site (Kd1 and Ki1, 262 and 125 nmol/L, respectively) and a low-affinity site (Kd2 and Ki2, 10,100 and 5400 nmol/L, respectively) with 2 x 10(6) to 6 x 10(6) sites per cell. ADP bound at both sites was competed with ADP, ATP, and ATP-alpha-S with affinities in a rank order similar to that found for platelets (ATP-alpha-S approximately ATP approximately ADP > or = ADP-beta-S approximately adenosine), suggesting the presence of a P2T receptor on CMK 11-5 cells. Photoaffinity labeling of intact CMK 11-5 cells with 35S-ATP-alpha-S resulted in the labeling of the alpha-subunit of GP IIb as found with platelets, although this was confirmed to be independent of ADP receptors. After RNA from CMK 11-5 cells was microinjected into Xenopus oocytes, only ADP and ADP-alpha-S stimulated 45Ca2+ efflux, which was not observed with ATP, 2-methylthio-ATP, alpha, beta-methylene-ATP, ATP-gamma-S, ATP-alpha-S, or adenosine. In addition, incubation of RNA-injected oocytes with ATP or ATP-alpha-S but not adenosine blocked the 45Ca2+ response to ADP. These experiments demonstrate that a nascent receptor that responded specifically to ADP but not to other P1, P2Y, P2X, and P2U agonists was expressed in functional form on Xenopus oocytes.

ADP and, indirectly, ATP are potent inhibitors of cAMP production in intact isoproterenol-stimulated C6 glioma cells

When added to intact C6 glioma cells in the micromolar range of concentrations, ADP and ATP induce an inhibition of the isoproterenol-elicited cAMP responses. ATP is rapidly hydrolyzed by the ectonucleotidases present on these cells, with an apparent Km of 50 microM and a Vmax of 1.1 nmol/min/10(6) cells. cAMP responses are also inhibited by millimolar concentrations of either ATP in the presence of an ATP-regenerating system to prevent ADP accumulation or AMP-PCP. These observations show that, in C6 glioma cells, ADP is a more potent inhibitor of cAMP production than ATP, the latter acting indirectly, via its rapid hydrolysis to ADP. The additive inhibition of isoproterenol-elicited cAMP responses induced, on one hand, by the treatment of the cells with a phorbol ester and by addition of ADP to the cells, and, on the other hand, by the progressive disappearance of the effects of ADP and ATP when cells are treated with increasing concentrations of Pertussis toxin, demonstrate that ADP and ATP exert their action in C6 glioma cells via a P2 purinoceptor probably negatively coupled to adenylate cyclase and a G regulatory protein.

Structure, function and subcellular localization of a human platelet Ca2+-ATPase

Human platelets contain a Ca2+-ATPase in internal membranes that is essential for Ca2+ homeostasis. This Ca2+ pump has enzymatic properties quite similar to the sarcoplasmic reticulum (SR) Ca2+ pumps. Antibodies against the SR Ca2+ pump crossreact with the human platelet protein. However, the platelet Ca2+-ATPase is approximately 10 kD larger than the SR pumps and exhibits a larger mRNA coding for the protein in a megakaryocyte tumor cell line. In addition, the platelet Ca2+-pump may be localized in specialized internal membrane structures that function in Ca2+ uptake and release. These results suggest that the platelet Ca2+-ATPase may represent a new class of internal membrane Ca2+-pumps.

Effect of PCR 4099 on ADP-induced calcium movements and phosphatidic acid production in rat platelets

Antiplatelet activity of PCR 4099, an analogue of ticlopidine, resides in its specific effect against exogenous as well as released ADP. This study investigated in rat platelets the effects of the drug on ADP-induced shape change, elevation of cytosolic free Ca2+ concentration ([Ca2+]i) and hydrolysis of inositol phospholipids, monitored as [32P]phosphatidic acid formation. Shape change and influx of Ca2+ ions across the plasma membrane were not modified after PCR 4099 administration using aspirin-treated platelets. On the other hand, phosphatidic acid formation and calcium mobilization from internal stores were strongly inhibited. These results suggest that PCR 4099 leaves intact the machinery involved in ADP-induced platelet shape change and influx of calcium ions, but inhibits an early step in the ADP-response coupling leading to inositol phospholipid hydrolysis and aggregation.

Composition of platelet phosphatidylinositol and phosphatidylcholine after ethanol withdrawal

Platelet phosphatidylinositol and phosphatidylcholine composition, ADP-induced platelet aggregation and associated thromboxane B2 formation were studied in alcoholics after a period of heavy drinking and in healthy non-alcoholic volunteers. The composition of these phospholipids in alcoholics was different from that seen in the control subjects. The most prominent change was a decrease in the relative amount of stearoyl-arachidonoyl species in the phosphatidylinositol fraction. Particularly this species of PI might be involved in the transmission of transmembrane signals. During detoxification changes were also observed in the extent of ADP-induced platelet aggregation and the amount of thromboxane B2 produced. Changes in platelet phospholipid composition might influence platelet reactivity in alcoholics.

Involvement of phosphoinositide metabolism in potentiation by adrenaline of ADP-induced aggregation of rabbit platelets

Changes in phosphoinositide metabolism were examined in washed rabbit platelets stimulated with 0.5 microM-ADP, 50 microM-adrenaline, or ADP and adrenaline in combination. Adrenaline does not stimulate platelet aggregation when used alone, but does potentiate aggregation stimulated by ADP. In platelets prelabelled with [32P]Pi and [3H]glycerol, adrenaline was found to potentiate the ADP-induced changes in platelet phospholipids, causing larger increases in the amount and labelling of phosphatidylinositol 4-phosphate (PIP) and phosphatidic acid than was observed with ADP alone. The combination of ADP and adrenaline did not produce a greater decrease in phosphatidylinositol 4,5-bisphosphate (PIP2) than was produced by ADP alone. In platelets prelabelled with [3H]inositol, adrenaline potentiated the increases in labelling of inositol phosphate and inositol bisphosphate stimulated by ADP; no increase in inositol trisphosphate labelling was detected with ADP alone or with the combination of ADP and adrenaline. Phentolamine, an alpha-adrenergic-receptor antagonist, blocked potentiation by adrenaline of ADP-induced changes in phosphoinositide metabolism. Propranolol and sotalol, beta-adrenergic-receptor antagonists, augmented the potentiation; this is consistent with the concept that the effect of adrenaline is mediated by beta-adrenergic receptors. The effect of adrenaline on phosphoinositide metabolism appears to be to potentiate the mechanisms by which ADP causes turnover of PIP and possibly degradation of PI, rather than the mechanism by which PIP2 is decreased.

The phosphoinositides exist in multiple metabolic pools in rabbit platelets

The labelling of the phosphoinositides and phosphatidic acid in washed rabbit platelets incubated with [32P]phosphate or [3H]glycerol was studied in the presence of isotope and after unincorporated isotope had been removed. With both isotopes the increase in the specific radioactivity of phosphatidylinositol 4,5-bisphosphate (PIP2) lagged behind that of phosphatidylinositol 4-phosphate (PIP) but the specific radioactivity remained higher after unincorporated isotope had been removed. This result was consistent with the presence of a second pool of PIP2, which interconverted slowly with the pool of PIP2 which was in direct equilibrium with PIP, proposed to explain the increase in specific radioactivity of PIP2 which accompanies the decrease in amount of PIP2 at 10 s in ADP-stimulated platelets. In platelets labelled with [3H]glycerol, the specific radioactivity of PIP2 became higher than that of PIP and the specific radioactivity of PIP became higher than that of phosphatidylinositol (PI). These results were interpreted to indicate that there were two pools of PIP; of these the pool with the higher specific radioactivity was the precursor of PIP2. Similarly, two pools of PI were proposed. The presence of pools of the phosphoinositides with different specific radioactivities necessitates the measurement of chemical amount of these compounds when studying the effect of stimulation of the platelets, since changes in labelling may not accurately reflect changes in the amount of the phosphoinositides.

The decrease in phosphatidylinositol 4,5-bisphosphate in ADP-stimulated washed rabbit platelets is not primarily due to phospholipase C activation

Addition of 10 micron-ADP to washed rabbit platelets caused platelet shape change and aggregation without release of the contents of the amine-storage granules, and caused a transient decrease (8.8% at 10 s) in the amount of phosphatidylinositol 4,5-bisphosphate (PIP2). By 20 s the decrease in PIP2 was no longer apparent, but by 60 s the amount of PIP2 was again decreased. Addition of thrombin (1 unit/ml), which causes platelet shape change, aggregation and the release of the contents of the amine-storage granules, caused a decrease in the amount of PIP2 (8.0% at 10 s); at 60 s the amount of PIP2 was not significantly different from that in controls. In platelets prelabelled with [3H]glycerol, the specific radioactivity of PIP2 was increased at 10 s in ADP-stimulated platelets, and unchanged in thrombin-stimulated platelets. In platelets prelabelled with [3H]inositol and incubated with 20 mM-Li+ to inhibit the degradation of the inositol phosphates to inositol, there was no increase in the labelling of inositol trisphosphate (IP3) upon stimulation with ADP. In contrast, stimulation with thrombin caused a significant increase in the labelling of IP3 at 10 s. These differences in the changes in polyphosphoinositide metabolism in ADP- and thrombin-stimulated platelets are consistent with the hypothesis that the decrease in PIP2 in ADP-stimulated platelets may be due not to degradation of PIP2 by phospholipase C, but rather to a shift in the equilibrium between PIP2 and phosphatidylinositol 4-phosphate (PIP). Increases in the labelling of phosphatidic acid at 10 s and of inositol bisphosphate and inositol phosphate after 20 s are consistent with phospholipase C being stimulated through some other mechanism that leads to the degradation of PIP and phosphatidylinositol; one possibility is that ADP causes an increase in cytoplasmic Ca2+.

Mobilization of different pools of glucose-incorporated calcium in pancreatic beta-cells after muscarinic receptor activation

Muscarinic receptor activation resulted in a biphasic mobilization of Ca2+ from isolated pancreatic islets. Glucose was essential for preparing the beta-cells to respond with the initial stimulatory phase. This effect seems to depend on the ability of the sugar to promote active sequestration of Ca2+ in the endoplasmic reticulum.

Hydrolysis of polyphosphoinositides in human platelets

Phospholipase C was purified 110 fold from human platelets. The activity of the enzyme was totally dependent upon Ca2+. The activity of the enzyme was markedly enhanced in the presence of arachidonic acid and was strongly inhibited by aminoglycoside antibiotics. The enzyme hydrolyzed endogenous polyphosphoinositides in addition to PI in Ca2+ dependent manner, suggesting the involvement of this enzyme in stimulus-linked rapid hydrolysis of polyphosphoinositides in platelets. The stimulation by thrombin of 32P-labelled human platelets induced about 30% decrease in 32P-TPI and about 220% increase in 32P-PA at the first 10 sec. The degree of hydrolysis of TPI was dependent upon the amount of agonist and it was not affected by the extracellular concentration of Ca2+. The changes in 32P-phospholipids in thrombin-stimulated platelets in the absence of Ca2+ were inhibited in a dose dependent manner by preincubation with relatively higher amount of quin 2 AM. The inhibition was completely overcome by an addition of CaCl2 to the suspending buffer. By such treatment in the absence of extracellular Ca2+, the intracellular Ca2+ concentration was significantly lowered below the basal level (less than 100 nM). Those observations suggest that TPI breakdown in thrombin-stimulated platelets is primary mediated by the agonist receptor coupling and requires at least the basal level of intracellular Ca2+.

Inositol triphosphate-induced Ca2+ release from human platelet membranes

Inositol (1,4,5) triphosphate (IP3) was observed to induce release of sequestered Ca2+ from crude human platelet membranes. This activity was also shown to be present in purified membranes enriched in Ca2+-ATPase activity. Maximal Ca2+ release occurred at 8 microM IP3 and half maximal activity was at 0.4 microM. Release was quite rapid and was complete by 40 s. Released Ca2+ was pumped back into the membrane vesicles and the rate of this reuptake was increased by the presence of phosphate. These results demonstrate that internal platelet membranes that possess an active Ca2+-pump will release sequestered Ca2+ in the presence of the second messenger IP3. IP3 did not induce release of Ca2+ from skeletal muscle sarcoplasmic reticulum when ATP was present.

Inositol trisphosphate, a novel second messenger in cellular signal transduction

There has recently been rapid progress in understanding receptors that generate intracellular signals from inositol lipids. One of these lipids, phosphatidylinositol 4,5-bisphosphate, is hydrolysed to diacylglycerol and inositol trisphosphate as part of a signal transduction mechanism for controlling a variety of cellular processes including secretion, metabolism, phototransduction and cell proliferation. Diacylglycerol operates within the plane of the membrane to activate protein kinase C, whereas inositol trisphosphate is released into the cytoplasm to function as a second messenger for mobilizing intracellular calcium.

The role of phosphatidylinositol 4,5 bisphosphate breakdown in cell-surface receptor activation

The activation of Ca2+-mobilising receptors on hepatocytes and many other cells leads to a prompt reduction in the cellular content of inositol phospholipids. The primary event which underlies these changes is, most probably, a phospholipase C-catalysed attack upon phosphatidylinositol 4,5 bisphosphate. The receptor-mediated breakdown of this lipid in stimulated cells is: (i) not mediated by an increase in cytosol [Ca2+] and (ii) closely coupled to receptor occupation. Phosphatidylinositol 4,5 bisphosphate degradation may be studied by measuring the appearance of the water-soluble product, inositol trisphosphate (and its metabolites: inositol bisphosphate and inositol monophosphate), in stimulated cells. Recent evidence indicates that inositol trisphosphate and the lipid soluble product of phosphatidylinositol 4,5 bisphosphate breakdown, 1,2 diacylglycerol, may act as 'second messengers' which mediate the effects of many extracellular signals in stimulated cells.