A novel signaling mechanism between gas and blood compartments of the lung

Propagation of inflammatory signals from the airspace to the vascular space is pivotal in lung inflammation, but mechanisms of intercompartmental sign aling are not understood. To define signaling mechanisms, we microinfused s ingle alveoli of blood-perfused rat lung with TNF-alpha, and determined in situ cytosolic Ca2+ concentration ([Ca2+](i)) by the fura-2 ratio method, c ytosolic phospholipase A(2) (cPLA(2)) activation and P-selectin expression by indirect immunofluorescence. Alveolar TNF-alpha increased [Ca2+](i) and activated cPLA(2) in alveolar epithelial cells, and increased both endothel ial [Ca2+](i) and P-selectin expression in adjoining perialveolar capillari es. All responses were blocked by pretreating alveoli with a mAb against TN F receptor 1 (TNFR1). Crosslinking alveolar TNFR1 also increased endothelia l [Ca2+](i). However, the endothelial responses to alveolar TNF-alpha were blocked by alveolar preinjection of the intracellular Ca2+ chelator BAPTA-A M, or the cPLA(2) blockers AACOCF(3) and MAFP. The gap-junction uncoupler h eptanol had no effect. We conclude that TNF-alpha induces signaling between the alveolar and vascular compartments of the lung. The signaling is attri butable to ligation of alveolar TNFR1 followed by receptor-mediated [Ca2+]( i) increases and cPLA(2) activation in alveolar epithelium. These novel mec hanisms may be relevant in the alveolar recruitment of leukocytes.