Weibel-Palade–body secretion induced by infusion of activated platelets. (A) In vivo P-selectin expression in mesenteric venules. Fluorescent microspheres (1 μm) coupled to anti–P-selectin antibody were infused in mice given injections with activated or resting platelets 4 hours prior to intravital microscopy. Arrows point to the venule wall. Many microspheres were observed binding to mesenteric venules of a mouse preinfused with activated platelets (left) compared to venules of a mouse given an infusion of resting platelets (right). No microspheres decorated the arterioles (☆). Bar = 100 μm. Representative images are shown. (B) Plasma VWF levels after infusion of VWF^-/- platelets. Plasma VWF was determined using ELISA 4 hours after infusion of activated or resting VWF^-/- platelets in WT mice. VWF level of a pool of plasma of WT mice was defined as 1 U/mL. There was a significant increase in circulating VWF after infusion of activated platelets as compared to resting platelets (P < .02). Average ± SEM is shown.