Leukocyte adherence initiation in skeletal muscle capillaries and venules.

In vivo leukocyte adherence has many physiological implications but the effect of vessel size and hemodynamic parameters on adherence initiation remains unclarified. The early phases of adherence, defined to include attachment of previously freely moving leukocytes to endothelium or leukocytes which roll along the vessel wall, are the focus of this report. Blood velocity and diameter in 121 capillaries and venules in the cremaster of 13 rats were measured. The hemodynamic and vessel size difference characterizing vessels in which either initial leukocyte wall attachment or leukocyte rolling occurred (termed "adherent vessels"), as distinguished from vessels in which neither transient stoppage nor cell rolling was observed (termed "nonadherent vessels"), was then determined. Fifty-seven percent of the vessels observed were adherent vessels. These vessels were characterized by a larger diameter, a smaller blood velocity, and a smaller calculated wall shear rate than the nonadherent vessels. The results show that leukocyte adherence can be initiated in capillaries and postcapillary vessels with diameters that are equal to and less than the leukocyte size. Within vessels less than 11 micron, adherence or nonadherence appears dependent on local hemodynamics with the possibility of a critical shear rate threshold of about 400 sec-1. In vessels with shear rates greater than this value only 7% were observed to have adherence. In vessels greater than 11 micron the absence of hemodynamic differences between adherent and nonadherent vessels suggests the presence of other adherence-initiating mechanisms. It is thought that in these larger size vessels both local erythrocyte effects and adherence initiation within upstream capillaries affect observed adherence.