摘要:

Calpain, a Ca2+activated intracellular protease and its endogeneous protein inhibitor, calpastatin are abundant in platelets. The structure and enzymological properties of calpain, including its isozymes, and of calpastatin in platelets have been fully characterized. Also, platelet calpain has been shown to cleave various endogeneous polypeptides. However, the mode of activation and the physiological function of platelet calpains have not been clarified. Our recent investigations on platelet calpains with cell permeable calpain antagonists and specific antibodies reveal that calpains are not involved in the early phases of platelet activation such as shape change and aggregation, but in the later phases of platelet activation such as cytoskeletal reorganization and Ca2+uptake.

ISSN:0953-7104    

DOI:10.3109/09537109509078452

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor

摘要:

Human blood platelets carry a high affinity, but low capacity, saturable system for the uptake of noradrenalhe. The uptake is partially Na+dependent but cannot be categorised as uptake. It is distinct from the uptake system responsible for 5-hydroxytryptamine transport into the platelet since the selective inhibitors of the platelet uptake system for 5-hydroxytryptamine (citalopram, paroxetine) Wer from those for the uptake system for noradrenaline (normetanephrine, methylisoprenaline). 5-hydroxytryptamine inhibits noradrenaline uptake but with properties inconsistent with competition for the same uptake system while noradrenaline does not inhibit 5-hydroxytryptamine uptake. Neither noradrenaline nor 5-hydroxytryptamine uptake by human platelets is inhibited by dopamine.

ISSN:0953-7104    

DOI:10.3109/09537109509078453

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor

摘要:

We studied the effects in Vitro of the calcium channel blocker verapamil (0.1, 0.2 or 0.3 mM) on platelet aggregation, on cytoplasmic Ca+ +levels and on TxB2production after activation of platelets with adenosine diphosphate (ADP) (100µM), collagen (20µg/ml) or thrombin (1 U/ml). A Platelet Ionized Calcium Aggregometer was used and washed, aequorin loaded platelets were employed. The drug was able to inhibit similarly and always significantly aggregation, Ca+ +fluxes and TxB2production when collagen was the agonist. Furthermore, inhibition of aggregation and TxB2production was significant at all the concentrations tested when platelets were activated by ADP or thrombin, but in this case inhibition of Ca+ +fluxes was observed only with the higher concentrations of the drug (0.2 or 0.3 mM). Hence, with these two last agonists inhibition of Ca+ +movements was less pronounced than inhibition of aggregation or TxB2production. These data suggest that platelet activation by collagen depends directly and almost exclusively on Ca+ +fluxes through biological membranes, while activation by ADP or thrombin is less strictly related to Ca+ +movements. Indeed, with these last two agonists verapamil may inhibit platelet activation also by calcium-independent mechanism(s).

ISSN:0953-7104    

DOI:10.3109/09537109509078454

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor

摘要:

Gamma-glutamyltransferase (GGT) activity in human platelet sonicates was 13.6 u/g of protein (range: 7.9–25.0) in 13 healthy, non-smoking, female volunteers; corresponding values in 16 males were: 20.3 (10.1–26.0). These values Mered significantly (p = 0.034). Platelet and serum GGT activity correlated significantly (p>0.04). Platelets seem to contain only the isoenzyme GGT 4. Part of this enzyme activity is in the form of aggregates or linked with membranes/proteins. This activity is released by Triton X-100 and trypsin and migrates as GGT 4. Serum GGT activity, a measurement in routine use, could be influenced by GGT released by platelets. It is therefore of interest that serum GGT activity can be increased in clinical conditions (e.g. myocardial infarction, diabetes, peripheral vascular disease) associated with platelet hyperactivity. Platelet GGT may influence intracellular S-nitrosoglutathione (a putative nitric oxide donor) levels. Potential associations between serum GGT activity and platelet function indices deserve investigation.

ISSN:0953-7104    

DOI:10.3109/09537109509078455

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor

摘要:

Human melanoma cell Line, HMV-I, has been shown to induce platelet aggregation without thrombin generation. The platelet-aggregating activity of HMV-I cells increased with increasing culture periods of the cells. However, data indicate that the platelet-aggregating activity of HMV-I cells is dependent upon the cell density of the culture and does not result from the culture periods- nor cell cycle-dependent regulation. It is suggested that this effect of cell density is referred to as density-induced upregulation of platelet-aggregating activity.

ISSN:0953-7104    

DOI:10.3109/09537109509078456

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor

摘要:

We have examined the action of a range of transition metal nitrosyl compounds in the inhibition of ADP-induced platelet aggregation. Inhibition results from the formation of the activated nitric oxide (NO) complex of guanylate cyclase, hence increasing platelet [cGMP]. Nitrosylation of guanylate cyclase may occur by release of NO from a nitrosyl complex, or, indirectly, by nitrosation of a thiol group followed by decomposition of the S-nitrosyl thiol to give NO. The latter process might be expected to be more efficient for compounds with a greater NO+character, and hence nitrosating ability, of the nitrosyl complex, but the results did not show a consistent relationship between NO character and the inhibitory potency on platelets. Inhibition of aggregation by Rousin's black salt, Na[Fe4S3(NO)7], was abolished by haemoglobin, and enhanced in the presence of M&B22948. These findings indicate that activation of guanylate cyclase is mediated by extracellular release of NO. For sodium nitroprusside, inhibition of platelet aggregation became progressively less sensitive to addition of haemoglobin, indicating that another process, such as release of cyanide, became significant as the incubation time was increased.

ISSN:0953-7104    

DOI:10.3109/09537109509078457

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor

摘要:

An unusual mechanism has been demonstrated for the in vitro proaggregating interaction between human platelets and human epidermoid carcinoma A431 cells. A431 cells induce platelet aggregation in a dodependent manner, depending on the rate of ADP release from tumour cells, which occurs in the presence not only of platelet rich plasma (PRP) but also of platelet poor plasma (PPP) or serum. This ADP release appears to be correlated to C3cleavage and binding of C3cto the A431 cell membrane. The interaction between A431 cells and PRP is characterized by typical morphological changes of A431 cells, leading to formation of mixed aggregates showing long projections of tumour cells deeply penetrating into the aggregate. These features, lacking in the presence of gel-filtered platelets (GFP), and reduced in the presence of thrombin degranulated platelets (TDP), are inhibited by cytochalasin and RGDS. The same activation of A431 cell cytoskeleton is induced by PDGF, but not by ADP or thromboxane receptor agonist U46619 or TGFP. These findings suggest a cooperative mechanism of tumour cell platelet interaction, in which a complementdependent ADP release from A431 cells induces platelet degranulation, PDGF release and aggregation. PDGF may induce in A431 cells Ca2+influx, cytoskeleton activation and changes in exposition of surface adhesion molecules, while fibrinogen binding causes mixed tumour cell-platelet aggregates to form.

ISSN:0953-7104    

DOI:10.3109/09537109509078458

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor

摘要:

Homeostatic regulation of cytoplasmic pH (pHeyt) against acid and alkaline challenges was studied in the human platelet using the intracellular indicator 2′,7′-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Activation of a Na+/H+exchanger in the plasma membrane is a known mechanism by which the platelet resists cytoplasmic acidification. The present study demonstrates an additional Na+-independent H extrusion mechanism which persists at low external Na+concentration in the presence of 100µM ouabain. This mechanism is inhibited by rotenone, oligomycin and an inhibitor of glycolysis, and is tentatively identified as a plasmalemmal H+-ATPase. The Na+-independent H extrusion mechanism partially restores cytoplasmic pH (pHeyt) after the cytoplasm is acidified by addition of 1µM nigericin. The restoration process is energy-dependent and has a t1/2of 7-21 min. The Na+-independent H+extrusion mechanism is also shown capable of maintaining pHeyt≊6.0 against an acidic pH, of 5.3 in an energy-dependent manner. The present study also revealed a Na+-independent, DIDS-inhibitable Cl−/HCO3−exchange activity. It removes alkaline equivalents from the cytoplasm with a half-time of 2.0±0.4 min after alkaline loading with 25 mM NH4Cl. Its activity was also revealed in chloride to gluconate and gluconate to chloride perturbations of the external medium which raised or lowered pHeytby 0.17±0.05 or 0.14±0.04 units, respectively. The activity of the anion exchanger allows the platelet to maintain pHext= 7.00±0.11 at the alkaline pHextof 8.25. The combined activities of the Na+-independent H+extrusion and Cl−/HCO3−exchange mechanisms make the platelet cytoplasm very resistant to changes in external pH. For variation of pHextbetween 5.0 and 8.5, pHeytvaries between 6.0 and 7.0, or roughly one-third as much.

ISSN:0953-7104    

DOI:10.3109/09537109509078459

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor

摘要:

The organization and reorganization of mobile receptors, GPIIb/IIIa and GPIb/IX, on surface- and suspension-activated platelets have been studied in detail, but their distribution on resting, discoid platelets is uncertain. The present study has treated platelets in suspension with cytochalasin E before mounting on formvar grids or glass slide fragments in order to preserve their discoid appearance, then probed the organization of GPIIb/IIIa with fibrinogen coupled to gold particles (Fgn/Au) and GPIb/IX with bovine or ristocetin-activated human plasma detected by combined anti-vWF antibody and protein A coupled to gold particles. Multimers of vWF had the same tortuous, linear distribution from edge to edge observed previously on surface-activated platelets. However, the gold particles marking the complex of vWF-anti-vWF bound to GPIb/M were closer together on the discoid cells. Fgn/Au particles bound to GPIIb/IIIa receptors were uniformly distributed from edge to edge on many discoid platelets. On others they tended to clump or cluster in strips or patches. The latter organization of Fgn/Au-GPIIb/IIIa receptors may be due to the rugose nature of the discoid platelet surface or an influence of cytochalasin E. Definition of mobile receptor organization on discoid cells provides a useful baseline for determining their fate following surface or suspension activation.

ISSN:0953-7104    

DOI:10.3109/09537109509078460

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor

摘要:

Platelets contribute vitally to the complex processes involved in haemostasis and thrombosis. Even more complex is their contribution to atherogenesis. Many studies have been devoted to analysing the processes involved in platelet activation since, clearly, prevention of activation may have clinical value. There are now at least two systems of platelet activation under intensive study: (a) agonist (e.g. ADP and thrombin) induced platelet activation when fibrinogen is the ligand; this process occurs at low shear forces and is aspirin sensitive; (b) secondly, in marked contrast, at high shear forces, shear itself activates the platelets and von Willebrand's factor (vWf) is the ligand, and this process is aspirin insensitive.1

ISSN:0953-7104    

DOI:10.3109/09537109509078461

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor