Summary
von Willebrand factor (vWf) is released from endothelial cell storage granules after
stimulation with thrombin, histamine and several other agents that induce an increase
in cytosolic free calcium ([Ca2+]i). In vivo, epinephrine and the vasopressin analog DDAVP increase vWf plasma levels,
although they are thought not to induce vWf release from endothelial cells in vitro.
Since these agents act via a cAMP-dependent pathway in responsive cells, we examined
the role of cAMP in vWf secretion from cultured human umbilical vein endothelial cells.
vWf release increased by 50% in response to forskolin, which activates adenylate cyclase.
The response to forskolin was much stronger when cAMP degradation was blocked with
IBMX, an inhibitor of phosphodiesterases (+200%), whereas IBMX alone had no effect.
vWf release could also be induced by the cAMP analogs dibutyryl-cAMP (+40%) and 8-bromo-cAMP
(+25%); although their effect was weak, they clearly potentiated the response to thrombin.
Epinephrine (together with IBMX) caused a small, dose-dependent increase in vWf release,
maximal at 10-6 M (+50%), and also potentiated the response to thrombin. This effect is mediated
by adenylate cyclase-coupled β-adrenergic receptors, since it is inhibited by propranolol
and mimicked by isoproterenol. In contrast to thrombin, neither forskolin nor epinephrine
caused an increase in [Ca2+]j as measured by fura-2 fluorescence. In addition, the effects of forskolin and thrombin
were additive, suggesting that they act through distinct signaling pathways. We found
a close correlation between cellular cAMP content and vWf release after stimulation
with epinephrine and forskolin. These results demonstrate that cAMP-dependent signaling
events are involved in the control of exocytosis from endothelial cells (an effect
not mediated by an increase in [Ca2+]i) and provide an explanation for epinephrine-induced vWf release.
