Platelets release their lysomal content in vivo in humans upon activation

Platelets contain, besides alpha- and delta-granules, lysosomes which store glycohydrolases able to degrade glycoproteins, glycolipids and glycosamino glycans. While several studies have shown that alpha- and delta-granule sec retion takes place "in vivo" in humans upon platelet activation, no data ar e available on the "in vivo" release of lysosomes. We have studied the rele ase of platelet lysosomal contents "in vivo" in healthy volunteers at a loc alized site of platelet activation by measuring markers of lysosomal secret ion in the blood oozing from a skin wound inflicted for the measurement of the bleeding-time. The levels of beta-N-acetylhexosaminidase (Hex) were 13. 1 +/- 0.85 mU/ml in bleeding time blood and 10.2 +/- 0.66 mU/ml in plasma ( p < 0.001). Hex in serum was 16.4 +/- 0.72 mU/ml. The levels of beta-galact osidase were also higher in bleeding-time blood than in plasma (0.85 +/- 0. 07 mU/ml vs 0.4 +/- 0.05 mU/ml, p < 0.001), In bleeding-time blood collecte d at one minute intervals, Hex rose progressively consistent with ongoing p latelet activation and flow-cytometry showed a progressive increase of the expression of LIMP and LAMP-2, two lysosomal associated proteins. In conclu sion, our data demonstrate that platelet lysosomal glycohydrolases are rele ased "in vivo" in humans upon platelet activation.