Transient pulmonary platelet sequestration during endotoxemia in dogs

We evaluated the time-course of regional platelet sequestration, following a bolus dose of endotoxin in anesthetized dogs. Autologous indium 111 labeled platelets, representing less than 1% of the circulating platelets, were injected 35-60 min prior to administering endotoxin intravenously to dogs. A gamma camera was used to monitor the distribution of these platelets within the thorax and abdomen. Alterations in the circulating blood platelet count paralleled the changes in blood radioactivity, enabling us to use external imaging to evaluate platelet kinetics. Marked hypotension and thrombocytopenia occurred within 6 min after administering endotoxin. The platelet pool in the lungs peaked at 9 min and was temporally related to the decrease in circulating platelet count, hypotension and increase in liver size. Translocation of platelets from the lungs to the circulating platelet pool occurred during the subsequent hour with sequestration occurring in the liver and possibly other organs. During this phase there was a recovery in platelet count to 35% of baseline levels but without significant recovery in mean arterial pressure. Based on these results we propose that endotoxin-induced thrombocytopenia results from pulmonary and hepatic sequestration of platelets, but that sequestration of platelets in the lungs is only transient. The mechanism and significance of subsequent translocation of platelets from the lungs to other sites, particularly the liver and the circulating platelet pool, remain to be investigated.