Platelet-activating factor promotes shear rate-dependent leukocyte adhesion in postcapillary venules.

Reductions in shear rate are generally associated with a recruitment of rolling and adherent leukocytes within the microcirculation. In this study, we determined if platelet-activating factor (PAF) affects shear rate-dependent leukocytes adhesion in cat mesenteric venules and whether the adhesion glycoproteins CD11/CD18 and ICAM-1 contribute to this process. Shear rate was varied in postcapillary venules (25-35 microns) by graded occlusion of the superior mesenteric artery. Vessel diameter, red blood cell velocity, leukocyte rolling velocity, and the number of rolling and adherent leukocytes were measured at each shear rate either in the presence or absence of PAF superfusion. PAF superfusion resulted in an increased number of adherent, but not rolling, leukocytes at basal and reduced venular shear rates. A monoclonal antibody (MAb) directed against CD11/CD18 prevented the PAF-induced recruitment of adherent leukocytes, while an ICAM-1 specific MAb had no effect. These results indicate that PAF increases shear rate-dependent leukocyte adhesion, presumably through activation and/or increased surface expression of CD11/CD18 on granulocytes.