Summary
Thrombospondin, a trimeric glycoprotein contained in the platelet α-granules, has
been proposed as a marker of in vivo platelet activation. However, it is also synthesised
by a range of other cells. The extraplatelet contribution to plasma levels of thrombospondin
was therefore estimated by investigating the relationship between plasma thrombospondin
levels and platelet count in samples from profoundly thrombocytopenic patients with
marrow hypoplasia, using the platelet-specific α-granule protein β-thromboglobulin
as control. Serum concentrations of both proteins were highly correlated with platelet
count, but while plasma β-thromboglobulin levels and platelet count also correlated,
there was no relationship between the number of platelets and thrombospondin concentrations
in plasma. Serial sampling of patients recovering from bone marrow depression indicated
that the plasma thrombospondin contributed by platelets is superimposed on a background
concentration of at least 50 ng/ml probably derived from a non-platelet source, and
plasma thrombospondin levels do not simply reflect platelet release.
Keywords
Platelet - Thrombospondin - β-thromboglobulin - Marrow hypoplasia
