Brenda Shafer

Platelet von Willebrand factor: Comparison with plasma von Willebrand factor

Platelet von Willebrand factor (vWf) was compared to its plasma counterpart. The platelet vWf was different from plasma vWf in that the multimeric organization was different, larger multimers were present, and the ratio of vWf activity to antigen was higher. Platelet and plasma vWf were similar in their antigenic reactivity in the electroimmunoassay and by liquid phase radioimmunoassay. The amount of vWf activity in large platelets was significantly higher than in normal platelets while the antigen content, although somewhat higher, was not significant. These studies show differences between normal platelet and plasma vWf, and also suggest that platelet size must be considered when platelet vWf is measured in disease states.

Monoamine oxidase activity in different density gradient fractions of human platelets

Monoamine oxidase (MAO) activity measured in human platelets is reportedly altered by such drugs as epinephrine, lithium carbonate, and imipramine, and also reduced in a number of clinical disorders. To evaluate whether MAO activity might differ in platelet supopulations, density gradient centrifugation with arabino-galactan was used to prepare four platelet fractions that differed in weight and volume. MAO activity in the lightest and smallest platelet subpopulation was approximately one-half that in the heaviest and largest subpopulation. Because platelet weight and size are thought to be related to platelet age, it is possible that some drug effects on platelet MAO activity might represent changes in platelet turnover. Factors other than platelet turnover rates may contribute to individual differences in platelet MAO activity, however, since one group of individuals with markedly reduced platelet MAO activity exhibited no shift in the proportion of lighter versus heavier platelets nor in the relative amount of MAO activity in each density gradient subfraction.

Maximal packet size for serotonin in storage vesicles of intact human platelets.

Abstract A procedure has been developed to measure maximal packet size for serotonin in vesicles of intact, washed human platelets. Vesicles were trace-labelled with H 3 -5HT and subsequently permitted to accumulate larger amounts of C 14 -5HT from the extracellular medium. Parallel platelet aliquots were incubated with unlabelled 5HT, and total and thrombin-releasable 5HT were measured by an enzymatic assay. Prior to incubation, 5HT packet size ranged from 0.05 to 0.53 × 10 −17 moles/platelet. No net addition of 5HT to vesicles occurred when this compartment contained more than 2–3 × 10 −17 moles/platelet of 5HT, suggesting that maximal packet size was 2.8 × 10 −17 moles/platelet. Under these conditions, a typical vesicle would contain 4 × 10 −18 molesof 5 HT, at an estimated concentration of 0.6 M. Nevertheless, when vesicles were full, they continued to exchange C 14 -5HT from the extracellular medium with H 3 -5HT sequestered in vesicles.

Platelet activation and alpha granule secretion in type IIB von Willebrand's disease

Type IIB von Willebrand disease is characterized by enhanced ristocetin‐induced platelet aggregation, spontaneous platelet aggregation, thrombocytopenia and the absence of the largest plasma von Willebrand factor (vWf) multimers. The absence of the largest plasma vWf multimers is related to their enhanced binding to platelets. The abnormal affinity of the IIB von Willebrand factor to platelets results in thrombocytopenia, but the mechanism is not known. We have studied the platelets from three patients with type IIB von Willebrand disease and have found evidence of platelet activation and alpha granule secretion as defined by increased amounts of von Willebrand factor, fibrinogen and the alpha granule protein PADGEM/GMP‐140 on the surface of these platelets. The degree of thrombocytopenia appears to be directly related to the number of platelets with fibrinogen bound to the surface. PADGEM/ GMP‐140, an alpha granule membrane protein, fuses with the platelet plasma membrane after activation and is a site on platelets which binds to neutrophils or monocytes. This alpha granule protein may play an additional role in platelet clearance and thrombocytopenia in type IIB von Willebrand disease. This may, in part, explain the absence of thromboembolic phenomena despite the presence of activated platelets in patients with type IIB von Willebrand disease.