Basal and induced prothrombinase expression in platelets from patients with essential thrombocythaemia (ET).

Both the basal and the collagen plus thrombin-stimulated prothrombinase activities have been measured in platelets from eight patients with essential thrombocythaemia (ET). On a mean basis these activities were significantly lower than the values recorded for a group of age and sex matched control subjects. When the Ca2+ ionophore A23187 was substituted for the agonist mixture the induced expression of prothrombinase activity was essentially the same for the patient and control groups. It is suggested that the defect in prothrombinase expression seen with ET platelets may reside in either membrane signal transduction processes concerned with Ca2+ mobilization or in Ca2+ sensitive cytoskeletal control of membrane phospholipid topography.