PECAM-1^−/− platelets demonstrate increased adhesion to immobilized fibrillar collagen in the absence of Mg²⁺.

Calcine am-loaded platelets derived from wild-type (open symbols) and PECAM-1^−/− (closed symbols) mice were allowed to adhere, in the absence of Mg²⁺, to plates coated with increasing concentrations of fibrillar collagen (circles) or to plates coated with a constant concentration of polyL-lysine (squares) for 15, 30, or 60 minutes at 37°C. Wild-type and PECAM-1^−/− platelets were labeled to a similar extent with calcine am (not shown). After removal of nonadherent platelets, fluorescence associated with adherent platelets was measured as described in “Materials and methods.” Each assay was performed in triplicate, and each data point represents the mean of triplicate measurements ± SD. Similar results were obtained from 3 independent experiments. Note that wild-type and PECAM-1^−/− platelets bound to a similar extent to polyL-lysine–coated plates, whereas at later time points, higher levels of adhesion to fibrillar collagen, in the absence of Mg²⁺, was observed in PECAM-1^−/− platelets relative to the wild-type platelets.