Haemostatic Platelet Plug Formation in the Isolated Rabbit Mesenteric Preparation - An Analysis of Red Blood Cell Participation

Abstract

SummaryIn a model using an isolated rabbit mesenteric preparation microvessels were transected and the time until haemostatic plugs formed was registered. Perfusion of platelet rich plasma gave no haemostasis whereas whole blood did. Addition of chlorpromazine or adenosine to the whole blood significantly prolonged the time for haemostasis, and addition of ADP to the platelet rich plasma significantly shortened it. It is concluded that red cells are necessary for a normal haemostasis in this model, probably by a combination of a haemodynamic and ADP releasing effect.The fundamental role of platelets in haemostatic plug formation is unquestionable but there are still problems concerning the stimulus for this process to start. Three platelet aggregating substances have been discussed – thrombin, adenosine diphosphate (ADP) and collagen. Evidence speaking in favour of thrombin is, however, very minimal, and the discussion has to be focused on collagen and ADP. In an in vitro system using polyethylene tubings we have shown that "haemostasis" can be obtained without the presence of collagen but against these results can be argued that it is only another in vitro test for platelet aggregation (1).To be able to induce haemostasis in this model, however, the presence of red blood cells is necessary. To further study this problem we have developed a model where haemostatic plug formation can be studied in the isolated rabbit mesentery and we have briefly reported on this (2).Thus, it is possible to perfuse the vessels with whole blood as well as with platelet rich plasma (PRP) and different pharmacological agents of importance.

Contribution of the Haematocrit to the Bleeding Time

“…The possible mech anisms have been reviewed recently [10], and include mechanical factors (increased diffusivity of platelets as the red cell concentration increases) and biochemical factors (release of ADP from red cells). The need for red cells in haemostatic plug formation has recently been confirmed in rabbit experiments [11]: both mechanical factors and ADP release ap peared to be involved. A further possibility is that red cells bind prostacyclin [12], hence an increase in haematocrit could favour platelet adhesion and aggregation by removing pros tacyclin which inhibits platelet adhesion and aggregation.…”

Effect of ADP‐utilizing enzymes on the arterial bleeding time in rats and rabbits

“…Enzyme systems which remove ADP have been much used to provide evidence for its involvement in reactions of platelets in vitro (Haslam, 1964;Izrael et al, 1974) or to prevent the activation of platelets during their preparation (Mustard et al, 1972). Apyrase increases the bleeding time from transected vessels in isolated rabbit mesenteries (Bergqvist & Arfors, 1980) and from punctures or cuts into artificial vessels (Born et al, 1976). All this is in vitro or ex vivo evidence for the dependence of the haemostatic aggregation of platelets on free ADP in the blood.…”

Mechanisms of hemolysis‐associated platelet activation

“…Shortly thereafter, this small molecule was identified as ADP [12]. Hemolysis, permanent RBC damage, RBC deformation and shear stress all cause RBCs to release ADP [9,13,14]. Once in the bloodstream, ADP can cause platelet activation.…”