Growing evidence supports substantial pathophysiological impact of platelets and their interactions on the development of septic lung failure. We developed a rat model of endotoxemia for direct in situ visualization of pulmonary microcirculation by in vivo fluorescence videomicroscopy. Male Sprague-Dawley rats were assigned to control, endotoxemia (Escherichia coli LPS, 15 mg/kg, i.v.), and fluid management for treatment of LPS-induced hypovolemia (Ringer lactate, hydroxyethyl starch [HES] 6%) groups (n = 7 each). Leukocytes were labeled in vivo by rhodamine, and 5 x 10(6) Calcein-AM-labeled nonactivated platelets were injected. Microcirculatory parameters (vessel diameter, ventilation-perfusion ratio) and adhesive characteristics of platelets and leukocytes (velocity, rolling, sticking) within the pulmonary microcirculation were quantified after endotoxin application under various regimens of fluid substitution for 60 min. A reduction of cell velocity and enhanced cell adhesion was seen in leukocytes and platelets (P < 0.05) after LPS injection. Fluid treatment with HES 6% resulted in a significant increase of platelet's velocity compared with the LPS group (442.86 +/- 20.60 vs. 343.93 +/- 11.17; P < 0.05), whereas Ringer lactate showed no beneficial effects. Similarly, HES 6% normalized LPS-induced platelet rolling and sticking as well as alterations in ventilation-perfusion ratio. Using direct visualization of the pulmonary microcirculation, we observed that platelet and leukocyte interactions are enhanced in the lung during LPS endotoxemia. Fluid therapy with HES 6% seems to have restorative effects on these cellular functions within the pulmonary microcirculation.
