Adrenaline potentiates type 2B von Willebrand factor-induced activation of human platelets by enhancing both the formation and action of thromboxanes

Investigation of cyclooxygenase and signaling pathways involved in human platelet aggregation mediated by synergistic interaction of various agonists

In the present study, the mechanism(s) of synergistic interaction of various platelet mediators such as arachidonic acid (AA) when combined with 5-hydroxytryptamine (5-HT) or adenosine diphosphate (ADP) on human platelet aggregation were examined. The results demonstrated that 5-HT had no or negligible effect on aggregation but it did potentiate the aggregation response of AA. Similarly, the combination of subeffective concentrations of ADP and AA exhibited noticeable rise in platelet aggregation. Moreover, the observed synergistic effect of AA with 5-HT on platelets was inhibited by different cyclooxygenase (COX) inhibitors, namely ibuprofen and celecoxib, with half maximal inhibitory effect (IC₅₀) values of 18.0±1.8 and 15.6±3.4 μmol/L, respectively. Interestingly, the synergistic effect observed for AA with 5-HT was, also, blocked by the 5-HT receptor blockers cyproheptadine (IC₅₀=22.0±7 μmol/L), ketanserin (IC₅₀=152±23 μmol/L), phospholipase C (PLC) inhibitor (U73122; IC₅₀=6.1±0.8 μmol/L), and mitogen activated protein kinase (MAPK) inhibitor (PD98059; IC₅₀=3.8±0.5 μmol/L). Likewise, the synergism of AA and ADP was, also, attenuated by COX inhibitors (ibuprofen; IC₅₀=20±4 μmol/L and celecoxib; IC₅₀=24±7 μmol/L), PLC inhibitor (U73122; IC₅₀=3.7±0.3 μmol/L), and MAPK inhibitor (PD98059; IC₅₀=2.8±1.1 μmol/L). Our observed data demonstrate that the combination of subthreshold concentrations of agonists amplifies platelet aggregation and that these synergistic effects largely depend on activation of COX/thromboxane A2, receptor-operated Ca²⁺ channels, Gq/PLC, and MAPK signaling pathways. Moreover, our data revealed that inhibition of COX pathways by using both selective and/or non-selective COX inhibitors blocks not only AA metabolism and thromboxane A2 formation, but also its binding to Gq receptors and activation of receptor-operated Ca²⁺ channels in platelets. Overall, our results show that PLC and MAPK inhibitors proved to inhibit the synergistic activation of platelets by several/multiple agonists.

Systemic protection through remote ischemic preconditioning is spread by platelet-dependent signaling in mice

Remote ischemic preconditioning (RIPC), the repetitive transient mechanical obstruction of vessels at a limb remote to the operative site, is a novel strategy to mitigate distant organ injury associated with surgery. In the clinic, RIPC has demonstrated efficacy in protecting various organs against ischemia reperfusion (IR), but a common mechanism underlying the systemic protection has not been identified. Here, we reasoned that protection may rely on adaptive physiological responses toward local stress, as is incurred through RIPC. Standardized mouse models of partial hepatic IR and of RIPC to the femoral vascular bundle were applied. The roles of platelets, peripheral serotonin, and circulating vascular endothelial growth factor (Vegf) were studied in thrombocytopenic mice, Tph1(-) (/) (-) mice, and through neutralizing antibodies, respectively. Models of interleukin-10 (Il10) and matrix metalloproteinase 8 (Mmp8) deficiency were used to assess downstream effectors of organ protection. The protection against hepatic IR through RIPC was dependent on platelet-derived serotonin. Downstream of serotonin, systemic protection was spread through up-regulation of circulating Vegf. Both RIPC and serotonin-Vegf induced differential gene expression in target organs, with Il10 and Mmp8 displaying consistent up-regulation across all organs investigated. Concerted inhibition of both molecules abolished the protective effects of RIPC. RIPC was able to mitigate pancreatitis, indicating that it can protect beyond ischemic insults.

CONCLUSIONS

We have identified a platelet-serotonin-Vegf-Il10/Mmp8 axis that mediates the protective effects of RIPC. The systemic action, the conservation of RIPC effects among mice and humans, and the protection beyond ischemic insults suggest that the platelet-dependent axis has evolved as a preemptive response to local stress, priming the body against impending harm.

Inhibitory effect by new monocyclic 4-alkyliden-beta-lactam compounds on human platelet activation

In the present study some new beta-lactam compounds were screened for their ability to inhibit human platelet activation. In particular four compounds differing in the group on the nitrogen atom of the azetidinone ring were investigated. A beta-lactam having an ethyl 2-carboxyethanoate N-bound group was demonstrated to inhibit, in the micromolar range, both the Ca(2+) release from endoplasmic reticulum, induced either by thrombin or by the ATPase inhibitor thapsigargin, and the Ca(2+) entry in platelets driven by emptying the endoplasmic reticulum. The compound also inhibited the platelet aggregation induced by a variety of physiological agonists including ADP, collagen, thrombin and thrombin mimetic peptide TRAP. The beta-lactam reduced the phosphorylation of pleckstrin (apparent MW 47 kDa), elicited by thrombin but not by the protein kinase C activator phorbol ester. Accordingly it did not significantly affect the aggregation evoked by phorbol ester or Ca(2+) ionophore. It was concluded that the beta-lactam likely exerts its anti-platelet-activating action by hampering the agonist induced cellular Ca(2+) movements. The beta-lactam concentration, which significantly inhibited platelet activation, only negligibly affected the cellular viability. Even if it is still premature to draw definitive conclusions, the present results suggest that this new compound might constitute a tool of potential clinical interest and the starting-point for the synthesis of new more beneficial anti-thrombotic compounds.

The influence of platelet activating factor on the effects of platelet agonists and antiplatelet agents in vitro

We assessed the effect of the intercellular mediator of inflammation, platelet activating factor (PAF), on platelet function. The interaction between PAF and the platelet agonists ADP, thrombin and convulxin was analyzed in vitro in whole blood with the use of flow cytometry and was further characterized with the use of receptor antagonists to PAF (ABT-491), P2Y1 (MRS-2179), and P2Y12 (cangrelor) as well as a monoclonal anti-PSGL-1 antibody (anti-CD162). Low concentrations of PAF (0.1 nM) synergistically augmented platelet activation induced by other agonists (P < 0.01). Augmentation by PAF was receptor mediated and did not require platelet-leukocyte interaction. With >99% inhibition of P2Y receptor-mediated platelet activation, greater than additive activation was still observed with the combination of ADP plus PAF. Accordingly, PAF synergistically augments platelet activation in response to ADP and thrombin, and the extent of inhibition exerted by P2Y receptor antagonists is decreased in the presence of PAF.

Severity of sepsis alters the effects of superoxide anion inhibition in a rat sepsis model

Previous analysis showed that selective inhibitors of five different host inflammatory mediators administered for sepsis, although beneficial with severe sepsis and high-control mortality rates, were ineffective or harmful with less severe sepsis. We hypothesized that severity of sepsis would also influence inhibition of superoxide anion, another inflammatory mediator. To test this, 6-h infusions of M40401, a selective SOD mimetic, or placebo were given to antibiotic-treated rats ( n = 547) starting 3 h after challenge with differing doses of intravenous Escherichia coli designed to produce low- or high-control mortality rates. There was a positive and significant ( P = 0.0008) relationship between the efficacy of M40401 on survival rate and control mortality rates. M40401 increased or decreased the log (odds ratio of survival) (means ± SE), dependent on whether control mortality rates were greater or less than the median (66%) (+0.19 ± 0.12 vs. −0.25 ± 0.10, P = 0.01). In a subset of animals examined ( n = 152) at 9 h after E. coli challenge, M40401 increased (mean effect ± SE compared with control) mean arterial blood pressure (8 ± 5 mmHg) and decreased platelets (−37 ± 22 cells × 103/ml) with high-control mortality rates but had opposing effects on each parameter (−3 ± 3 mmHg and 28 ± 19 cells × 103/ml, respectively) with low rates ( P ≤ 0.05 for the differing effects of M40401 on each parameter with high- vs. low-control mortality rates). A metaregression analysis of published preclinical sepsis studies testing SOD preparations and SOD mimetics showed that most (16 of 18) had control mortality rates >66%. However, across experiments from published studies, these agents were less beneficial as control mortality rate decreased ( P = 0.03) in a relationship not altered ( P = not significant) by other variables associated with septic challenge or regimen of treatment and which was similar, compared with experiments with M40401 ( P = not significant). Thus, in these preclinical sepsis models, possibly related to divergent effects on vascular function, inhibition of superoxide anion improved survival with more severe sepsis and high-control mortality rates but was less effective or harmful with less severe sepsis. Extrapolated clinically, inhibition of superoxide anion may be most efficacious in septic patients with severe sepsis and a high risk of death.

Role of Ca(2+) and protein kinase C in the serotonin (5-HT) transport in human platelets

Accumulation of serotonin (5-HT) into human platelets was not affected by the presence of the extra-cellular calcium chelator EGTA, while decreased by platelet incubation with the membrane permeant chelator BAPTA-AM. Serotonin uptake also diminished upon platelet exposure to EGTA/thapsigargin or EGTA/ionomycin which increased the cytosolic [Ca(2+)] to levels lower than those inducing secretion of dense granules. The latter inhibition depended in part on changes of intra-granular pH, since the accumulation of acridine orange, which is driven into the dense granules by the intra-granular acid pH gradient, was slightly decreased in the presence of EGTA/thapsigargin. These compounds also inhibited the 5-HT uptake in platelets pre-incubated with reserpine and bafilomycin that prevent 5-HT from entering into the dense granules. Inhibitors of protease, protein phosphatase, Na(+)/H(+) exchanger or ciclo-oxygenase activities did not modify the serotonin accumulation. Addition of EGTA/thapsigargin to reserpine-treated, [(14)C]5-HT-loaded, platelets caused an imipramine-insensitive release of labelled serotonin. This release was reduced by both BAPTA-AM or protein kinase C inhibitor bisindoylmaleimide (GF). The latter compound, either alone or together with EGTA/thapsigargin, inhibited the 5-HT accumulation in reserpine-treated platelets. It is concluded that both cytosolic [Ca(2+)] and protein kinase C are involved in the regulation of the plasma membrane 5-HT transport.