In Vitro and in Vivo Functions of Thrombin-Treated Platelets

Abstract

SummaryThrombin-induced platelet aggregation has been generally believed to be irreversible. However, thrombin-induced aggregation of washed platelets is reversible if fibrin formation is prevented or the fibrin which binds the platelets together is removed from the platelet aggregates. After treatment with high concentrations of thrombin (0.5 units/ml) single platelets can be recovered that have lost practically all of their releasable serotonin and adenine nucleotides. These platelets are able to aggregate upon addition of low concentrations of ADP in the presence of fibrinogen. They aggregate in response to the ionophore A23, 187 in the absence of added fibrinogen, whereas sodium arachidonate-induced aggregation requires added fibrinogen. Thrombin-treated platelets change their shape in response to collagen in the absence of fibrinogen, and will aggregate upon the addition of collagen providing fibrinogen is present. This response to collagen can be blocked with aspirin but not with a mixture of creatine phosphate/creatine phosphokinase. Upon a second exposure to thrombin, thrombin-pretreated platelets do not change their shape and do not undergo aggregation. Thrombin-pretreated platelets will not retract a thrombin-induced fibrin clot unless ADP, sodium arachidonate, the ionophore A23, 187 or collagen are added together with thrombin.The ability of thrombin-treated platelets to adhere to the exposed subendothelial surface of the rabbit aorta is reduced, compared with untreated control platelets. The thrombin-treated platelets shorten the bleeding time of thrombocytopenic rabbits. However, they are not as effective in shortening the bleeding time as normal control platelets. When injected into rabbits with a normal platelet count, the thrombin-treated platelets that circulate after infusion survive for the same length of time as untreated control platelets. These findings indicate that thrombin-induced platelet aggregation with extensive release of granule constituents is not irreversible and that thrombin treatment does not cause irreversible damage of all platelets that would lead to their immediate elimination from the circulation. Furthermore, these platelets can still be haemostatically effective. It is conceivable that platelets that have lost their amine storage granule contents during a release reaction in vivo, such as may occur in certain cases of intravascular coagulation and repeated episodes of thrombosis, may be found in the circulation of man.

In vivo tracking of platelets: circulating degranulated platelets rapidly lose surface P-selectin but continue to circulate and function.

“…We thereby demonstrate that circulating degranulated platelets rapidly lose surface P-selectin to the plasma pool but continue to circulate and function in vivo. Thus, our studies reconcile the results of previous studies (2,4,5,(7)(8)(9)(10) by demonstrating that platelet surface P-selectin molecules, rather than degranulated platelets, are rapidly cleared.…”

“…It has therefore been postulated that surface P-selectin-positive (degranulated) platelets may be rapidly cleared from the circulation by leukocytes (2,4,5,7,8). In apparent contradiction to this postulate, other investigators have reported that degranulated platelets are no more rapidly cleared from the circulation than control platelets (9,10). Methods have not previously been available to directly address this issue.…”

Platelet Parameters in Children with Upper Urinary Tract Infection: Is There a Specific Response?

“…In vitro platelet serotonin release has been demonstrated when platelets were incubated with endotoxin [15] and pneumococci. [16] Also, activated complement [17] and thrombin [18] can cause platelet release reaction.…”

Reversal of shortened platelet survival in rats by the antifibrinolytic agent, epsilon aminocaproic acid.

“…The first blood sample, taken just before the catheter was introduced, was assigned a value of 100%. Blood samples were taken 3,6,9, and 11 h after insertion of the catheter. Radioactivity in the samples was determined and the mean platelet life span was calculated from the radioactivity in individual samples using the gamma function described by Murphy and Bolling (9) Control animals had their femoral artery exposed and ligated in a "sham operation."…”

“…In other experiments we have shown that the reduction of platelet survival cannot be solely attributed to platelet accumulation in thrombi associated with the damaged vessel wall or catheters but seems to be related at least in part to the extent of vessel wall injury and platelet interaction with the damaged vessel wall.' Exposure of platelets to ADP, thrombin, or thromboxane A2 with which platelets are likely to come in contact during thrombus formation, does not shorten platelet survival (5,6). However, proteolytic enzymes that cleave platelet membrane glycoproteins do shorten platelet survival (7).…”