The Effect of Serotonin Inhibition on Initial Microvessel Hemostasis and Platelet Aggregation In Vivo

The effect of MCI-9042 on serotonin-induced platelet aggregation in type 2 diabetes mellitus

MCI-9042, a potent inhibitor of 5HT2 receptor, was used to examine its effects on serotonin induced-aggregation of platelets obtained from patients with type 2 diabetes mellitus (DM). The extent of platelet aggregation induced by serotonin increased in DM patients without retinopathy, but the increase was further significant in DM patients with retinopathy. MCI-9042 as well as ketanserin, inhibited significantly serotonin induced-aggregation of platelets obtained from DM patients with retinopathy. MCI-9042 dose dependently inhibited platelet aggregation induced by serotonin and collagen. These results suggest that serotonin which may be released from platelets of DM patients may activate platelets together with collagen exposed on atherosclerotic endothelium and that MCI-9042 may be inhibitory to enhanced platelet aggregability under these conditions.

The influence of ketanserin on hemostasis in vitro

Ketanserin is a new selective 5-HT2 receptor blocker. It has been used to indirectly study the influence of serotonin on hemostatic function in vitro. Coagulation and fibrinolysis in vitro were not affected. In high doses adrenalin induced platelet aggregation was inhibited but no influence was seen on collagen or ADP induced aggregation. It can be concluded that it is not very likely that serotonin plays an important role in initial hemostasis. However, the findings ought to be confirmed in vivo.

Does dihydroergotamine reduce plasma volume?

Indications on a plasma volume reduction by dihydroergotamine (DHE) has been found in earlier studies. This study was made in order to further evaluate such a change. In animal experiments (sheep, n = 10) measurements were performed of red blood cell volume and plasma volume using radioactive labelling techniques. Red blood cell volume was unchanged but a plasma volume decrease of 10% was found after two hours and it persisted for 24 hours. In postoperative patients (n = 9), who had been given an infusion of ringer-glucose (5.5%) at a rate of 100 ml/hour, DHE caused an increase in diuresis but neither ADH (n = 9) nor aldosterone (n = 3) levels were changed. The data from the present study further support the earlier findings of a reduced plasma volume after DHE administration. The main effect of dihydroergotamine is on the vascular bed and especially on capacitance vessels (increased tonus). The reduced plasma volume by DHE seems to be one additional effect of DHE.

Acute hypotension due to platelet serotonin-induced chemoreflexes after intravenous injection of dextran sulfate in the rabbit

The hypotension and bradycardia observed after intravenous injection of dextran sulfate in rabbits was prevented by prior depletion of circulating platelets, but was not prevented by depletion of the third component of complement or Hageman factor. Dextran sulfate injection caused immediate thrombocytopenia with temporary localization of platelets within lungs. Morphological analysis revealed platelet aggregates in lung capillaries. The platelets had changed shape and were in the process of degranulating. Serotonin and histamine levels in blood increased approximately 5-fold and 7-fold, respectively, after dextran sulfate injection. The cardiovascular events following dextran sulfate injection were mimicked by intravenous serotonin but not by intravenous histamine injection, although a combination of serotonin and histamine reproduced the pattern of blood pressure changes better than did either agent alone. Quantification of platelets trapped in lung revealed that the potential release of serotonin from trapped platelets could account for the rise in plasma serotonin concentration and the hemodynamic changes observed. Both the dextran sulfate-induced cardiovascular effects and serotonin-induced hypotension were markedly diminished by cutting vagus and depressor nerves, and were virtually abolished by carotid ligation in addition to nerve section. These results support the concept that platelet activation within rabbit lungs may cause hypotension via serotonin-induced chemoreflexes.