THE LEUKOCYTE RESPONSE TO FLUID STRESS

Leukocyte migration from a hemopoietic pool across marrow endothelium requires active pseudopod formation and adhesion, Leukocytes rarely sh ow pseudopod formation while in circulation, At question then is the m echanism that serves to minimize leukocyte pseudopod formation in the circulation, We tested the hypothesis that fluid shear stress acts to prevent pseudopod formation, When individual human leukocytes (neutrop hils, monocytes) spreading on glass surfaces in vitro were subjected t o fluid shear stress (approximate to 1 dyn/cm(2)), an instantaneous re traction of pseudopods was observed, Removal of the fluid shear stress in turn led to the return of pseudopod projection and cell spreading, When steady shear stress was prolonged over several minutes, leukocyt e swelling occurs together with an enhanced random motion of cytoplasm ic granules and a reduction of cytoplasmic stiffness, The response to shear stress could be suppressed by K+ channel blockers and chelation of external Ca2+, In rat mesentery microvessels after occlusion, circu lating leukocytes project pseudopods in free suspension or when attach ed to the endothelium, even though immediately after occlusion only fe w pseudopods were present, When flow was restored, pseudopods on adher ing leukocytes were retracted and then the cells began to roll and det ach from the endothelium. In conclusion, plasma shear stress in the ci rculation serves to reduce pseudopod projection and adhesion of circul ating leukocytes and vice versa reduction of shear stress leads to pse udopod projection and spreading of leukocytes on the endothelium.