The effect of inhibition of prostaglandin synthesis on microvascular haemostasis and macromolecular leakage

Microvascular thrombosis: experimental and clinical implications

A significant amount of clinical and research interest in thrombosis is focused on large vessels (eg, stroke, myocardial infarction, deep venous thrombosis, etc.); however, thrombosis is often present in the microcirculation in a variety of significant human diseases, such as disseminated intravascular coagulation, thrombotic microangiopathy, sickle cell disease, and others. Further, microvascular thrombosis has recently been demonstrated in patients with COVID-19, and has been proposed to mediate the pathogenesis of organ injury in this disease. In many of these conditions, microvascular thrombosis is accompanied by inflammation, an association referred to as thromboinflammation. In this review, we discuss endogenous regulatory mechanisms that prevent thrombosis in the microcirculation, experimental approaches to induce microvascular thrombi, and clinical conditions associated with microvascular thrombosis. A greater understanding of the links between inflammation and thrombosis in the microcirculation is anticipated to provide optimal therapeutic targets for patients with diseases accompanied by microvascular thrombosis.

Influence of platelet-vessel wall interactions on leukocyte rolling in vivo

The influence of platelet-vessel wall interactions on leukocyte rolling was investigated in rabbit mesenteric venules (diameter, 21-40 microns) using intravital videomicroscopy. Puncture of the wall with glass micropipettes (tip, 6-8 microns) evoked the formation of a thrombus in all venules. In most vessels, emboli were produced as well. The rolling of leukocytes (i.e., their movement along the vessel wall at a velocity clearly lower than that of the other blood cells) was quantitated simultaneously in vessel segments upstream and downstream from a thrombus up to 10 minutes after puncture. During embolization the number of rolling leukocytes decreased significantly from the upstream to the downstream vessel segment (median decrease, 45%; p less than or equal to 0.001). It was still decreased by approximately 50% after embolization had stopped, indicating that the decrease in leukocyte rolling was not caused by inclusion of leukocytes in the emboli. In venules without embolization, leukocyte rolling did not change systematically, indicating that fluid dynamic changes induced by the thrombus do not influence leukocyte rolling. Inhibition of prostaglandin formation with aspirin (100 mg/kg) almost completely abolished the influence of the thromboembolic reaction on leukocyte rolling, but blockade of thromboxane A2 receptors with sulotroban (30 mg/kg) had no effect. In conclusion, this is the first report on a functional interaction in vivo, at a site of vessel wall injury, between platelets, vascular cells, and leukocytes. The findings suggest that substances produced by activated platelets and/or damaged vascular cells diminish leukocyte rolling. The identity of these substances is not yet clear, but the present study indicates that prostaglandins other than thromboxane A2 are involved.

The effect of normovolemic hemodilution on microvascular hemostasis in the rabbit

Hemostatic plug formation has been studied in the rabbit mesenteric microcirculation during normovolemic hemodilution. A hemodilution method has been developed which maintains a constant platelet count. Using this method the primary hemostatic plug formation time (PHT) and the total hemostatic plug formation time (THT) has been assessed for different degrees of hemodilutions. Both PHT and THT are significantly prolonged for arterioles perfused with low hematocrits (10.6 and 14.2). At a hematocrit of 19.0 the hemostasis is most effective with the shortest hemostatic plug formation times.

Bradykinin and prostaglandin E1, E2 and F2alpha-induced macromolecular leakage in the hamster cheek pouch

The hamster cheek pouch prepared for intravital observations on macromolecular permeability with fluorescein labelled dextran was used in four series of 5 hamsters each, all pretreated with indomethacin. Bradykinin, PGE1, PGE2 and PGF2alpha increased macromolecular leakage at postcapillary venules, and this leakage was reversible on removal of agent. A linear relation was found between the logarithmic value of dose of bradykinin and the mean number of leakage sites. No tachyphylaxis to bradykinin was seen. The effect of either PGE1, PGE2 or PGF2alpha applied simultaneously with bradykinin was to significantly (p less than 0.05) potentiate the bradykinin response. Bradykinin and these prostaglandins appeared to have the same site of action for their effect of increasing permeability, e.g. the postcapillary venule.

The effect of O-(beta-hydroxyethyl)-rutoside (HR) on macromolecular leakage, thrombosis and haemostasis in experimental animals

O-(beta-hydroxyethyl)-rutoside (HR) (Venoruton, Zyma AS, Nyon, Switzerland) has been investigated experimentally to evaluate the effect on microvascular permeability and thromboembolism. Permeability to macromolecules is diminished in a hamster cheek-pouch model. Haemostatic plug formation is impaired whereas laser-induced intravascular platelet aggregation is uninfluenced. There is a small but insignificant protection against sodium morrhuate (Eli Lilly and Co., Indianapolis, Indiana) induced femoral vein thrombosis.

Platelet aggregation in the cerebral microcirculation: effect of aspirin and other agents

After a certain period of time filtered ultraviolet light produces platelet aggregation in microvessels on the cerebral surface of the mouse, but only when sodium fluorescein is first injected intravascularly to provide a light-absorbing, heat-generating target. The platelet aggregates fluoresce. They occur only in the illuminated field and adhere to arteriolar and venular walls. Vasoconstriction is not detected prior to or up to 30 seconds after aggregation. Electron microscopy reveals damaged endothelium and undamaged red cells, as well as aggregates consisting almost exclusively of platelets in varying stages of aggregation, pseudopod formation, and degranulation. The time between onset of the noxious stimulus and recognition of the first aggregate can be measured as the vessels are observed microscopically. This "time of aggregation" is prolonged by pentobarbital as opposed to urethane anesthesia, and also is related to time elapsed after craniotomy. We also found that aspirin and indomethacin significantly prolong time to first aggregate, but only on the arteriolar side of the circulation. This is so even though the composition of the aggregates is the same on both the arteriolar and venular sides. Heparin has no effect.