CONTRIBUTION OF SELECTINS TO LEUKOCYTE SEQUESTRATION IN PULMONARY MICROVESSELS BY INTRAVITAL MICROSCOPY IN RABBITS

1. Sequestration of leucocytes in the lung is the net result of leucoc yte rolling and sticking in pulmonary arterioles and venules and their retention in alveolar capillaries. 2. In order to investigate whether adhesion molecules of the selectin family contribute to these phenome na the effects of fucoidin (an inhibitor of L- and P-selectin) on micr ohaemodynamics and leucocyte kinetics were studied in pulmonary arteri oles, capillaries and venules by means of intravital fluorescence micr oscopy in a rabbit model. 3. Fucoidin reduced leucocyte rolling in pul monary arterioles and venules by 75 and 83%, respectively, without aff ecting leucocyte sticking. In alveolar capillaries, fucoidin reduced l eucocyte retention and accelerated leucocyte passage, thus reducing th e alveolar transit time of leucocytes by 62%. 4. It is concluded that rolling of leucocytes in pulmonary microvessels is mediated by selecti ns, whereas sticking relies on selectin-independent mechanisms. 5. Leu cocyte retention in alveolar capillaries is not due solely to mechanic al hindrance of leucocyte passage through narrow vessel segments, as p reviously hypothesized, but also depends on interaction of leucocytes with the capillary endothelium.