Upon injury in a blood vessel, platelets become activated and adbere to prevent blood loss. Activated platelets express adhesion molecules on their surface that also make leukocytes adhere to the injury. One such molecule, which also is expressed on endothelium, is the glycoprotein P-selectin. The present studies were focused on how soluble factors and surfaces can affect P-selectin surface expression on human platelets.
This thesis presents an enzyme-linked immunosorbent assay (ELISA) application to study platelet expression and release of P-selectin. The well-known platelet activators collagen, epinephrine, ADP, ristocetin, PMA, and thrombin induced surface expression of P-selectin with different potency and time-dependency in vitro. One of the most powerful platelet activators, thrombin, rapidly mediated both surface expression and release. The surface expression decreased after a short peak upon maximal thrombin-activation. The protein kinase C activator PMA induced surface expression equivalently to thrombin but the release of Pselectin was less as compared to thrombin. Nitric oxide (NO) donors, which mimic a vessel wall mechanism to inhibit platelet activation, decreased expression and release of P-selectin upon activation. Adenosine, another agent that is produced in vivo, acted in concert with NO and totally inhibited P-selectin expression induced by thrombin. NO and adenosine act by increasing the second messengers cGMP and cAMP, respectively.
The effects of an infusion of nicotine on platelet P-selectin expression were studied in nicotine users with normal and impaired renal function. After a peak directly after infusion, the plasma concentration of nicotine declined towards the basal level and the level of NO-products was lower as compared to baseline 2 h after infusion. At the same time, P-selectin expression induced by weak activators, but not by thrombin, increased in both groups. A transient and weak inhibition of collagen-induced platelet aggregation was observed directly after infusion. Studies in vitro showed that nicotine per se inhibits P-selectin expression. Thus, nicotine appears to initially inhibit and then increase platelet activation in vivo.
The adhesion of platelets to surfaces that can be exposed upon vessel wall injury and the subsequent expression of P-selectin were found to be dependent on divalent cations. Mg2+ dose-dependently increased platelet adhesion to both collagen and fibrinogen in the physiological range, but supraphysiological concentrations decreased the adhesion to fibrinogen. Ca2+ did only increase platelet adhesion to fibrinogen. Platelet adhesion to collagen caused more expression of P-selectin than adhesion to fibrinogen. Thus, the surface and the access of divalent cations regulate platelet adhesion and P-selectin surface expression.
In summary, multiple factors rep1late and affect platelet adhesion and P-selectin surface expression. The methods presented in this thesis are suitable for studies to further understand and make pharmacological modulation possible of these complex mechanisms and consequences.
Linköping: Linköpings universitet , 2000. , 86 p.
2000-10-06, Berzeliussalen, Universitetssjukhuset, Linköping, 13:00 (Swedish)