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At least 55 records · Page 3Linked to original sources
PubMed1982

[Activable phospholipase a2 of rat blood platelets].

Rat blood platelet lysate showed high phospholipase A2 activity and contained much less than activating factor much greater than which increased phospholipase A2 activity. The phospholipase A2 and the much less than activating factor much greater than were eluted together in the same first protein fraction obtained by chromatography on a G 100 Sephadex column washed with tris-HCl buffer (pH 7.2). This mixture of proteins can be further fractionated on a Sepharose Blue CL 6B column in 2 protein fractions. The first fraction eluted by tris-HCl buffer (pH 7.2) showed 10 percent of the initial phospholipase activity; the second fraction eluted by NaCl 1 M-tris HCl buffer (pH 7.2) which showed no phospholipase activity, contains activating factor; if both fractions are mixed the initial phospholipase activity is almost totally restored. In conclusion, in rat blood platelets, phospholipase activity originates in a great part from the association of an much less than activable phospholipase much greater than and an much less than activating factor much greater than.

Animals↗
PubMed1988

Angiotensin II and Na+/H+ exchange in human blood platelets.

Human blood platelets have a high potency for Na+/H+ exchange activity; they also bear receptors for angiotensin II (Ang II). To explore the effect of Ang II on the exchange, we have used a fluorimetric analysis capable of detecting cytoplasmic changes of 0.01 pH units in platelets, together with an Ang II preparation which has been proven to be effective as a vasoconstrictor. Thrombin and adenosine diphosphate (ADP) affected Na+/H+ exchange, but Ang II did not affect Na+/H+ exchange for a wide range of concentrations and experimental conditions. It is concluded that in contrast to its function in vascular smooth muscle cells, Ang II apparently does not play a role in the regulation of Na+/H+ exchange in human blood platelets.

Angiotensin II↗
PubMed1983

Clonidine and para-aminoclonidine, partial agonists for the alpha2-adrenergic receptor on intact human blood platelets.

Human blood platelets display alpha2-adrenergic receptors, which promote platelet aggregation and inhibit the adenylate cyclase. We investigated the effects of the antihypertensive agent clonidine and its analogue para-aminoclonidine on this receptor in the intact human platelet to determine their pharmacological effects and their ability to bind to the receptor by radioligand displacement. Both agents potentiated platelet aggregation induced by a submaximal concentration of ADP. Epinephrine-induced aggregation, on the other hand, was antagonized by clonidine and para-aminoclonidine in a dose-dependent fashion. Both agents inhibited the accumulation of cyclic AMP in platelets exposed to prostaglandin E1 and a phosphodiesterase inhibitor, but to a lesser extent than the inhibition caused by epinephrine. Both antagonized this excess inhibitory action of epinephrine. Clonidine and para-aminoclonidine blocked the binding of [3H]yohimbine (a selective alpha 2-adrenergic antagonist) to intact platelets with half-maximal effects at 0.3 and 0.7 microM, respectively. No evidence for the existence of a second class of binding sites with high affinity for clonidine was seen in intact platelets, either by this technique or by direct binding of [3H]clonidine. It is concluded that these two agents are partial agonists for the alpha 2-adrenergic receptors on blood platelets and that this receptor exists predominantly in the low-affinity state in the intact cell.

Adrenergic alpha-Agonists↗
PubMed1984

Ca2+ as messenger of 5HT2-receptor stimulation in human blood platelets.

In blood platelets of man, both 5-hydroxytryptamine (5HT) and 80 nM of the Ca2+-ionophore A23187 led to rapid shape change reactions which were inhibited by prostaglandin E1 (PGE1), forskolin, 2-methyl-6-methoxy-8-nitroquinoline ( quin2 ) and chlortetracycline. The IC50-values of the inhibitors were similar in the 5HT- and the A23187-experiments. Higher amounts of A23187 abolished the inhibitory actions of PGE1 and forskolin. Furthermore, 5HT and A23187 enhanced adrenaline-induced platelet aggregation their effects showing similar time dependence. Ketanserin, an antagonist of 5HT2 -receptors, and 8-(N,N-diethyl-amino)octyl-3,4,5-trimethoxybenzoate (TMB-8), an intracellular Ca2+-antagonist, counteracted the effects of 5HT much more than those of A23187, whereas acetylsalicylate and indomethacin did not influence the actions of either 5HT or A23187. In addition, 5HT caused a concentration-dependent rise of intracellular free Ca2+ in platelets which was counteracted by ketanserin. PGE1 and forskolin reduced the resting Ca2+-levels. 5HT did not affect either the basal or the PGE1-stimulated activity of adenylate cyclase, whereas the Ca2+-ionophore A23187 slightly raised the basal activity of the enzyme. In conclusion, the functional effects of 5HT2 -receptor stimulation in human blood platelets (shape change reaction and enhancement of adrenaline aggregation) seem to be mediated by a rise of intracellular free Ca2+.

Adenylyl Cyclases↗
PubMedDate not supplied

Activation of blood platelets in chronic hepatitis and liver cirrhosis P-selectin expression on blood platelets and secretory activity of beta-thromboglobulin and platelet factor-4.

BACKGROUND/AIMS: Blood platelets are cells that quite often undergo damage in chronic liver diseases. Endotoxemia and hyperkinetic circulation influence platelets in an active manner. The role of platelets in the development of hepatitis and liver fibrosis is speculative. The aim of the study was to determine the influence of chronic liver diseases on platelets morphologic parameters, their secretory activity and P-selectin expression. METHODOLOGY: The examination was completed in the group of 29 patients with chronic hepatitis and 27 with liver cirrhosis of postinflammatory etiology (HBV, HCV). Liver biopsies were carried out in all patients. Thirty-two healthy individuals were the control group. Platelets morphological parameters (number, volume, platelet crit, micro- and macrothrombocyte fraction) were estimated. beta-thromboglobulin concentration and platelet factor 4 in blood serum as well as P-selectin expression on resting platelets and after thrombin activation were also examined. RESULTS: Number, volume, and platelet crit decreased with the advancement of a liver disease. Megathrombocyte fraction increased inversely with the severity of liver damage. The concentration of beta-thromboglobulin and platelet factor 4 alpha-granule contents in blood serum was higher 2- and 7-times, respectively than in healthy controls. P-selectin expression on resting platelets was considerably higher. After stimulation with thrombin, P-selectin expression was equal (chronic hepatitis) or higher (liver cirrhosis) than in the control. CONCLUSIONS: There are changes of platelet morphological parameters, with accompanying megathrombocyte fraction increase that occur in chronic liver diseases. Thrombocytes in chronic liver diseases and liver cirrhosis are more activated. Platelet sensitivity to stimuli in these ailments is higher (liver cirrhosis) than in the healthy controls.

Adolescent↗
PubMed1997

Evidence for tryptophan hydroxylase and hydroxy-indol-O-methyl-transferase mRNAs in human blood platelets.

Human blood platelets were tested for the presence of mRNAs coding for tryptophan hydroxylase (TPOH) and hydroxy-indol-o-methyl-transferase (HIOMT). Total RNA was extracted from platelets (12.9 +/- 3.3 mg RNA/100 ml blood, mean +/- SEM of 6 preparations) and cDNA synthesized by reverse transcription using random hexamers, oligo-dT or TPOH- or HIOMT-specific primers, designed to amplify a 254 bp fragment for TPOH and a 301 bp fragment for HIOMT. Positive controls were performed using RNA extracted from human normal or tumoral pineal glands. The PCR products were analyzed by gel electrophoresis, transferred to a nylon membrane and hybridized with a 32P-labeled internal probe. When random hexamers, oligo-dT or specific primers were used for reverse transcription, amplification products of the predicted sizes were detectable following electrophoresis in the case of pineal glands and following transfer and hybridization in the case of platelets. These results show TPOH and HIOMT mRNAs to be present in human blood and support the hypothesis that serotonin and melatonin may be synthesized in blood and, more particularly, in platelets.

Acetylserotonin O-Methyltransferase↗
PubMedDate not supplied

[An optimal method of determining the level of cathepsin A in blood platelet homogenates].

Blood platelet cathepsin A hydrolyzes Cbz-Glu-Tyr most intensively at pH 5.0, it does so too, with Cbz-Glu-Phe, but to a smaller extent. Using the ninhydrin method for estimation of the released tyrosine++ or phenylalanine previous elimination of amino acids++ and low-molecular peptides is necessary as it is too including alpha-amine nitrogen released by platelet proteases from peptides and proteins.

Animals↗
PubMed1980

Epinephrine induces Ca2+ uptake in human blood platelets.

Human blood platelets isolated by albumin density gradient centrifugation take up Ca2+ during 10(-6)M epinephrine-induced primary aggregation but not during 10(-6)M ADP-induced primary aggregation. Platelet uptake of Ca2+ is dose-dependent over a range of 10(-7) to 10(-5) M epinephrine. Antagonism of the platelet alpha-receptor by phentolamine (10(-6)M) results in inhibition of both epinephrine-stimulated Ca2+ uptake and aggregation. The Ca2+ antagonist verapamil (50 micro M) blocks Ca2+ uptake and epinephrine-induced aggregation, but not ADP-induced aggregation. The verapamil inhibition of aggregation is reduced on Ca2+ addition. These results suggest that epinephrine acts to stimulate primary platelet aggregation through a specific receptor interaction that results in a selective increase in platelet membrane permeability to Ca2+.

Adenosine Diphosphate↗
PubMed1987

Surface membrane clearing of receptor-ligand complexes in human blood platelets.

Human blood platelets were challenged sequentially in vitro with polyclonal anti-platelet antibodies and cationized ferritin. Both ligands bound to the surface membrane and were sequestered by internalization into a surface-connected membrane system (SCS) with a cleared surface membrane as the eventual result. Patching and capping of bound antibody preceded internalization, and platelets cooperated in the clearing process by adhering to each other at capped areas and by mutual covering up, a process dubbed platelet hugging. The internalization process was repeated upon challenge with the second ligand, the two ligands being sequestered as separate deposits in the SCS, mirroring the two cycles of internalization. Platelets were activated during internalization process and tended to aggregate. In aggregates, surfaces exposed to the medium were cleared of ligand, which accumulated in the intercellular spaces and within the SCS of the aggregated platelets. Aggregation but not internalization and hugging was prevented by adenosine and adenosine monophosphate.

Adenosine↗
PubMed1979

On the degradation of heparin by human blood platelets.

Human blood platelets are able to degrade heparin from different tissues and species. The main degradation product is an oligosaccharide. Low molecular weight components such as inorganic sulfate or monosaccharides, i.e. products released by exoenzymes are not detected. The in vitro degradation of heparin by the crude enzyme is observed at pHs below 6.5 with an optimum temperature around 37 degrees C. The presence of sulfate in the substrate structure is shown to be essential for the enzyme activity. Since the oligosaccharides formed have only 10 per cent of the anticoagulant activity of the heparins tested, it is conceivable that the platelet enzyme may play an important role in the inactivation of some of the biological properties of heparin.

Blood Platelets↗