In vitro enhancement of human platelet aggregation by somatostatin

Summary Very low concentrations of somatostatin (S-14) strongly potentiate the in vitro aggregation induced by collagen, ristocetin and arachidonic acid, but not that induced by ADP or epinephrine, in both human platelet rich plasmas and gel-filtered platelet preparations. Desensitization phenomena may be induced either by repeated addition of S-14 or long lasting contact between S-14 and platelets.     

In vitro enhancement of human platelet aggregation by somatostatin

Very low concentrations of somatostatin (S-14) strongly potentiate the in vitro aggregation induced by collagen, ristocetin and arachidonic acid, but not that induced by ADP or epinephrine, in both human platelet rich plasmas and gel-filtered platelet preparations. Desensitization phenomena may be induced either by repeated addition of S-14 or long lasting contact between S-14 and platelets.     

The effect of piroxicam on platelet aggregation

Summary Piroxicam inhibited aggregation of human and dog platelets caused by collagen, but not by adenosine diphosphate (ADP). Release of platelet ADP was inhibited by piroxicam.     

The effect of piroxicam on platelet aggregation.

Piroxicam inhibited aggregation of human and dog platelets caused by collagen, but not by adenosine diphosphate (ADP). Release of platelet ADP was inhibited by piroxicam.     

Platelet antiaggregating activity in the salivary secretion of the blood sucking bugRhodnius prolixus

Summary The salivary secretion ofRhodnius prolixus inhibits both ADP and collagen induced platelet aggregation in human platelet-rich plasma. Fractionation studies show that at least 3 different inhibitors are present inRhodnius saliva.     

Dissociation between inhibition of phospholipid methylation and production of PAF-acether by rabbit platelets

Platelet-activating-factor (PAF-acether, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphorylcholine) is formed by and released from rabbit platelets stimulated with thrombin, with the ionophore A23187, with collagen and with the platelet-stimulating glycoprotein convulxin. We here show that 3-deazaadenosine (C3ado) and L-homocysteine (HCy), two inhibitors of platelet methylation, reduced the formation of PAF-acether and of its deacetylated product lyso-PAF-acether by rabbit platelets challenged with thrombin, under conditions where the accompanying aggregation was not significantly modified. In contrast, platelet aggregation induced by convulxin was completely and irreversibly blocked when C3ado and HCy were associated. Aggregation by thrombin was not affected by the methylation inhibitors even when ADP was scavenged and thromboxane formation was suppressed. Our results indicate that phospholipid methylation, thrombin-induced platelet aggregation and formation of PAF-acether can be dissociated. The formation of PAF-acether by rabbit platelets can be blocked by mechanisms other than inhibition of phospholipase A2, since the latter is not affected by C3ado and/or HCy.     

Dissociation between inhibition of phospholipid methylation and production of PAF-acether by rabbit platelets

Summary Platelet-activating-factor (PAF-acether, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphorylcholine) is formed by and released from rabbit platelets stimulated with thrombin, with the ionophore A23187, with collagen and with the platelet-stimulating glycoprotein convulxin. We here show that 3-deazaadenosine (C₃ado) and L-homocysteine (HCy), two inhibitors of platelet methylation, reduced the formation of PAF-acether and of its deacetylated product lyso-PAF-acether by rabbit platelets challenged with thrombin, under conditions where the accompanying aggregation was not significantly modified. In contrast, platelet aggregation induced by convulxin was completely and irreversibly blocked when C₃ado and HCy were associated. Aggregation by thrombin was not affected by the methylation inhibitors even when ADP was scavenged and rhromboxane formation was suppressed. Our results indicate that phospholipid methylation, thrombin-induced platelet aggregation and formation of PAF-acether can be dissociated. The formation of PAF-acether by rabbit platelets can be blocked by mechanisms other than inhibition of phospholipase A2, since the latter is not affected by C₃ado and/or HCy.     

Alpha-tocopherol: its inhibition on human platelet aggregation.

Alpha-tocopherol inhibits human platelet aggregation induced by arachidonate sodium, collagen, epinephrine, adenosine diphosphate or thrombin - arachidonate sodium being the most susceptible. The second phase of the biphasic platelet aggregation induced by epinephrine or adenosine diphosphate is preferentially inhibited.     

Alpha-tocopherol: Its inhibition on human platelet aggregation

Summary Alpha-tocopherol inhibits human platelet aggregation induced by arachidonate sodium, collagen, epinephrine, adenosine diphosphate or thrombin — arachidonate sodium being the most susceptible. The second phase of the biphasic platelet aggregation induced by epinephrine or adenosine diphosphate is preferentially inhibited.This work was supported by the Medical Research Council of Canada, Quebec Medical Council, and a Fraser Scholarship, McGill University. I thank Drs.K. N. Drummond andS. O'Regan for their comments.     

Platelet aggregation is inhibited by phycolectins

Lectins from four marine algal species were examined for interaction with human platelets. The lectin designated hypnin A, from the red algaHypnea japonica, inhibited adenosine diphosphate (ADP)-or collagen-induced human platelet aggregation in a dose-dependent manner. Complete inhibition was observed at concentrations of 100 and 5 g/ml of the lectin with, ADP (2 M) and collagen (0.2 g/ml)-induced platelet aggregation, respectively. At the inhibitory concentration of 0.5 to 100 g/ml, the lectin did not induce aggregation of resting platelets. Lectins from the other three algal species also inhibited ADP-induced human platelet aggregation. These results indicate that the algal lectins are a new group of inhibitors and may be useful to study glycoconjugates on platelet membranes and to design novel platelet aggregation inhibitors.     

Stimulation of macromolecular synthesis by endotoxin-treated 3T6 fibroblasts

The interaction of endotoxin with cultured fibroblasts resulted in a depression of cellular proliferation and an increased synthesis of macromolecules, namely collagenous and non-collagenous proteins. The collagen salt-soluble fraction was increased at the expense of the insoluble fraction, and both the salt-soluble fraction and collagen secreted into the medium was underhydroxylated.     

Stimulation of macromolecular synthesis by endotoxin-treated 3T6 fibroblasts

Summary The interaction of endotoxin with cultured fibroblasts resulted in a depression of cellular proliferation and an increased synthesis of macromolecules, namely collagenous and non-collagenous proteins. The collagen salt-soluble fraction was increased at the expense of the insoluble fraction, and both the salt-soluble fraction and collagen secreted into the medium was underhydroxylated.     

Effect of catecholamines on ADP aggregation of rat platelets in vivo

Summary The influence of catecholamines on the platelet count was studied in an in vivo experimental model obtained with cannulation of the carotid and of the femoral vein. The i.v. infusion of epinephrine and l-norepinephrine induces a low drop in the platelet count and also potentiate aggregation by ADP.     

Colony stimulating activity in acute and chronic endotoxinemia in man

Summary Blood granulocyte-macrophage colony stimulating activity (GM CSF) was measured in 6 normal individuals challenged with low-dose endotoxin and in 63 unselected patients with nonhaematological disorders. 5/63 patients were febrile and 5 other patients showed detectable endotoxin levels, as measured by the Limulus assay. CSA levels showed a rapid increase in normal individuals following endotoxin administration, but were in the normal range in patients with chronic endotoxinemia or in those with febrile disorders. Thus, unlike acute endotoxinemia, chronic endotoxinemia is not associated with elevated activity that promotes growth of myeloid commited stem cells. In addition, fever per se did not coincide with elevated blood CSA levels.     

Colony stimulating activity in acute and chronic endotoxinemia in man.

Blood granulocyte-macrophage colony stimulating activity (GM CSF) was measured in 6 normal individuals challenged with low-dose endotoxin and in 63 unselected patients with nonhaematological disorders. 5/63 patients were febrile and 5 other patients whoed detectable endotoxin levels, as measured by the Limulus assay. CSA levels showed a rapid increase in normal individuals following endotoxin administration, but were in the normal range in patients with chronic endotoxinemia or in those with febrile disorders. Thus, unlike acute endotoxinemia, chronic endotoxinemia is not associated with elevated activity that promotes growth of myeloid commited stem cells. In addition, fever per se did not coincide with elevated blood CSA levels.     

Endotoxemia and adrenaline-hyperreactive death in mice.

Mice given i.v. a sublethal dose of endotoxin in advance died with shock-like symptoms on administration of sublethal adrenaline dose. The lethal adrenaline-hyperreaction induced by endotoxin appeared gradually within a few h, showed maximum response after several h and almost disappeared 24 h after endotoxin administration.     

Endotoxemia and adrenaline-hyperreactive death in mice

Summary Mice given i.v. a sublethal dose of endotoxin in advance died with shock-like symptoms on administration of sublethal adrenaline dose. The lethal adrenaline-hyperreaction induced by endotoxin appeared gradually within a few h, showed maximum response after several h and almost disappeared 24 h after endotoxin administration.Thanks are due to Dr Masami Kurokawa, Director of the Department, for his kind suggestions.     

Cell surface adenylate kinase activity regulates the F1-ATPase/P2Y13-mediated HDL endocytosis pathway on human hepatocytes

We have previously demonstrated on human hepatocytes that apolipoprotein A-I binding to an ecto-F₁-ATPase stimulates the production of extracellular ADP that activates a P2Y₁₃-mediated high-density lipoprotein (HDL) endocytosis pathway. Therefore, we investigated the mechanisms controlling the extracellular ATP/ADP level in hepatic cell lines and primary cultures to determine their impact on HDL endocytosis. Here we show that addition of ADP to the cell culture medium induced extracellular ATP production that was due to adenylate kinase and nucleoside diphosphokinase activities, but not to ATP synthase activity. We further observed that in vitro modulation of both ecto-NDPK and AK activities could regulate the ADP-dependent HDL endocytosis. But interestingly, only AK appeared to naturally participate in the pathway by consuming the ADP generated by the ecto-F₁-ATPase. Thus controlling the extracellular ADP level is a potential target for reverse cholesterol transport regulation. Received 13 July 2006; received after revision 29 August 2006; accepted 19 September 2006     

Lack of platelet factor-3 activation after incubation of platelet-rich plasma with kaolin in the rat

Summary Stypven times, measured in rat platelet-rich plasma (P. R. P.) after incubation with kaolin, did not shorten as incubation proceeded, thus reflecting the lack of development of platelet factor-3 (PF₃) availability in this test system. Repeated freezing and thawing of P. R. P. or aggregation with collagen did result in PF-3 availability. Aggregation and PF-3 availability were inhibited by the compound VK774. These findings add another aspect to the list of species differences in platelet function.This study was partly supported by a grant from I. W. O. N. L.     

Collagen treated with (+)-catechin becomes resistant to the action of mammalian collagenase

Treatment of radioactively labeled guinea-pig skin soluble collagen or calf skin collagen with the flavonoid (+)-catechin makes the collagen resistant to the action of mammalian collagenase but not to the action of bacterial collagenase. Complete resistance to the action of the mammalian enzyme may be achieved by incubating 0.6 mg of collagen (dry weight) with 0.1 mM (+)-catechin, followed by dialysis to remove the unbound flavonoid. Since incubation of the mammalian enzyme with (+)-catechin does not inhibit its activity, it is postulated that (+)-catechin binds tightly to collagen and modifies its structure sufficiently to make it resistant to enzyme degradation.