DECREASED HALF-LIFE TIME OF PLASMA VON-WILLEBRAND-FACTOR COLLAGEN-BINDING ACTIVITY IN ESSENTIAL THROMBOCYTHEMIA - NORMALIZATION AFTER CYTOREDUCTION OF THE INCREASED PLATELET COUNT

Patients with essential thrombocythaemia (ET) exhibit a decrease of la rge von Willebrand fatter (VWF) multimers in plasma, which is inversel y related to the platelet count. In the present study we investigated whether the decrease of large VWF multimers in plasma with increasing platelet counts is the consequence of increased turnover of large VWF multimers in vivo. To that end we measured the half-life times of endo genously released VWF:Ag and VWF:CBA (collagen binding activity) after intravenous administration of desmopressin (DDAVP) to nine ET patient s and nine control subjects (N). In addition, the half-life times of V WF:Ag and VWF:CBA were also measured in four ET patients after cytored uction of the increased platelet count to normal or nearly normal valu es. Estimated half-life times of VWF:Ag did not differ between ET pati ents and normals (11.0 +/- 4.0h v 12.4 +/- 2.5h, P>0.05). Estimated ha lf-life times of VWF:CBA were significantly lower in ET patients as co mpared with normal individuals (6.1 +/- 2.0h v 8.4 +/- 1.5 h, P <0.05) . After cytoreduction of the increased platelet count to (nearly) norm al values in all four ET patients the half-life time of VWF:CBA signif icantly (P = 0.014) increased from 5.2 +/- 1.2, h to 8.7 +/- 2.0 h. Ou r data suggest that platelets may play a role in the homeostasis of ci rculating von Willebrand factor, which may compromise normal haemostas is at fairly increased platelet counts.