Quantitative studies on the influence of leukocytes on the vascular resistance in a skeletal muscle preparation

The effects of leukocytes on vascular resistance and capillary filtration coefficient (CFC) of the maximally vasodilated vascular bed of rat hindquarters were measured using servocontrolled isogravimetry . After determination of the pre- and postcapillary resistances and CFC with a cell-free perfusate, a leukocyte bolus was introduced (0.5 to 9 X 10(6) cells/100 g tissue). Experiments were performed at three flows: 0.7-0.9, 1.4-1.6, and 2.4-3.7 ml/(min X 100 g tissue). Changes in vascular resistance were calculated from continuous pressure-flow recordings during a 10-min period, followed by a CFC measurement. Leukocytes were counted in the effluent for calculation of the number of cells remaining in the vascular bed. Leukocyte infusion caused a transient increase of the precapillary resistance (Ra) by as much as 72% after infusion of 9 X 10(6) cells/100 g tissue at 0.7-0.9 ml/(min X 100 g tissue). After the initial peak the Ra usually fell to a lower, constant, level, but was still clearly elevated (maximum 23%) at the low flows of 0.7-0.9 and 1.4-1.6 ml/(min X 100 g tissue). This sustained increase of Ra was positively related to the number of cells remaining in the vasculature. At high flows of 2.4-3.7 ml/(min X 100 g tissue) the Ra increase was very small, irrespective of the number of cells infused or remaining in the vascular bed. These findings, together with the fact that CFC decreased proportionally to the sustained increase of Ra, indicate that the effect of leukocytes on Ra was due to a pressure-dependent occlusion of the microvasculature.