fMLP-stimulated neutrophils increase endothelial [Ca²⁺] _i and microvessel permeability in the absence of adhesion: Role of reactive oxygen species

Our previous study demonstrated that firm attachment of leukocytes to microvessel walls does not necessarily increase microvessel permeability (Am J Physiol Heart Circ Physiol 283: H2420-H2430, 2002). To further understand the mechanisms of the permeability increase associated with leukocyte accumulation during acute inflammation, we investigated the direct relation of reactive oxygen species (ROS) release during neutrophil respiratory burst to changes in microvessel permeability and endothelial intracellular Ca²⁺ concentration ([Ca²⁺]_i) in intact microvessels. ROS release from activated neutrophils was quantified by measuring changes in chemiluminescence. When isolated rat neutrophils (2 × 10⁶/ml) were exposed to formyl-Met-Leu-Phe-OH (fMLP, 10 μM), chemiluminescen transiently increased from 1.2 ± 0.2 × 10⁴ to a peak value of 6.7 ± 1.0 × 10⁴ cpm/min (n = 12). Correlatively, perfusing individual microvessels with fMLP-stimulated neutrophils in suspension (2 × 10⁷/ml) increased hydraulic conductivity (L_p) to 3.7 ± 0.4 times the control value (n = 5) and increased endothelial [Ca²⁺]_i from 84 ± 7 nM to a mean peak value of 170 ± 7 nM. In contrast, perfusing vessels with fMLP alone did not affect basal L_p. Application of antioxidant agents, Superoxide dismutase, vitamin C, or an iron chelator, deferoxamine mesylate, attenuated ROS release in fMLP-stimulated neutrophils and abolished increases in L_p. These results indicate that release of ROS from fMLP-stimulated neutrophils increases microvessel permeability and endothelial [Ca²⁺]_i independently from leukocyte adhesion and the migration process.