The procoagulant activity of platelets induced by
collagen,
thrombin, and
collagen plus
thrombin, measured as their capacity to promote the conversion of
prothrombin to
thrombin in the presence of factors Va and Xa, was studied in patients with alpha, alpha delta, and delta
storage pool deficiency (SPD),
thrombasthenia, and in two new patients with isolated defects in platelet
coagulant activity, and compared with that in
Scott syndrome. The most significant abnormality in the new patients, whose defect may differ from that in
Scott syndrome, is an impairment in
collagen plus
thrombin-induced
prothrombinase activity in the absence of added
factor Va. In one of these patients this may be caused by an abnormality in platelet alpha-granule
factor V distinct from that described for
factor V Quebec, alpha delta-SPD, or alpha-SPD (
gray platelet syndrome).
Prothrombinase activity in response to all agonists was impaired in delta-SPD and was associated with an inability of these platelets to maintain elevated intracellular
calcium levels. Both the rapid decline in agonist-induced [Ca2+]i levels and the impaired
prothrombinase activation in delta-SPD platelets were corrected by the addition of
adenosine diphosphate (
ADP) after stimulation. These findings suggest that secreted
ADP may play an important role in the generation of
prothrombinase activity by contributing to the maintenance of a critical [Ca2+]i level necessary to maintain aminophospholipids on the outer surface of the platelet membrane, and provide evidence that dense granules may be a major source of
ADP which can contribute to
calcium influx in stimulated platelets. Parallel alterations, including both increases and decreases, in the [Ca2+]i and
prothrombinase responses were also observed in
thrombasthenia, depending on the agonist and stirring conditions. Both responses were increased in
collagen-stimulated, unstirred platelets, whereas an inability to maintain increased [Ca2+]i levels, associated with decreased
prothrombinase activity in all but one atypical patient, was seen in stirred
collagen plus
thrombin-activated platelets. Although the parallel alterations in these responses in
thrombasthenia, as in SPD, further show the close association between the generation of
prothrombinase activity and the maintenance of increased intracellular Ca2+ levels, the specific role that GPIIb-IIIa may play in both these events remains unresolved. Our findings of both enhancement and inhibition of these activation-related events in thrombasthenic platelets may be related to previous conflicting reports on the promotion or inhibition of
fibrin formation by GPIIb-IIIa, and could be relevant to the use of specific inhibitors of GPIIb-IIIa as
antithrombotic agents. In addition, the study provides further support for the concept that the development of agents that could induce a
Scott syndrome defect in normal platelets may provide a new approach to antithrombotic
therapy.