Summary
Endothelial cells (ECs) synthesize and release von Willebrand factor (vWf) either
constitutively or from Weibel-Palade bodies by a regulated pathway. Although stimulated
release of vWf from ECs occurs following exposure to thrombin, histamine, interleukin,
tissue necrosis factor and fibrin in vitro, these agents are unlikely to be present
in physiologically relevant concentrations during the initial stages of primary hemostasis.
Alternatively, agents known to be released from the dense granules of activated platelets
at the sites of vascular injury may provide the initial physiological stimuli for
vWf release from ECs in vivo. We have examined the effects of the platelet secretagogues
ADP, AMP, ATP and serotonin on the release of vWf from ECs and demonstrated enhanced
release in all cases. The extent and time at which optimum vWf release was observed
depended on the agonist and its concentration. At 3 nM, optimum release occurred after
4 hours with ADP (330 %/ml) or 1 h with AMP (153%/ml) or ATP (450%/ml). At 30 nM,
optimum release was seen after 1 hour with ADP (315 %/ml) or AMP (595%/ml) and after
15 min with ATP (938%/ml). With serotonin, optimal release was seen by 30 min at 0.3
μM (1034%/ml) and after 1 h at 1 pM (745%/ml) although the response after 15 min was
nearly equivalent (667%/ml). The doses giving 50% of maximal response (ED50) after
1 h were 6.5 nM (ADP), 15.2 nM (AMP) and 2.4 nM (ATP) and 20 nM for ATP or 75 nM for
serotonin after 15 or 30 min respectively. ADP also enhanced PGI2 release from ECs
in a dose- and time-dependent manner. These observations support a mechanism whereby
activated platelets, which interact with and bind to ECs or subendothelium and release
adenine nucleotides or serotonin from platelet dense granules, could stimulate the
release of vWf from adjacent ECs thereby enhancing EC-platelet interactions and facilitating
primary hemostasis.
