Surfactant modulates calcium response of neutrophils to physiologic stimulation via cell membrane depolarization

Pulmonary surfactant (PS) reduces inflammation in the lung by poorly unders tood mechanisms. We have observed that surfactant-associated proteins (SAP) insert monovalent cation channels in artificial membranes. Neutrophils are primary mediators of acute pulmonary inflammation, and their functions are activated by increases in cytosolic ionized calcium concentration ([Ca2+]) and by changes in membrane potential. We hypothesize that PS inserts SAP-d ependent cation channels in neutrophils, causing membrane depolarization, a ltered [Ca2+] response, and depressed activation. Human neutrophils were is olated, exposed to PS+SAP (1% Survanta), PS-SAP (1% Exosurf), or buffer, an d washed before activating with selected stimulants. PS+SAP reduced phorbol ester- and formyl peptide-stimulated adherence and aggregation by 38% (p < 0.05) and 54% (p < 0.02), respectively. PS+SAP also inhibited the formyl p eptide-induced [Ca2+] response of neutrophils (p < 0.01), but only in the p resence of external Ca2+. Further characterization of this inhibition demon strated that PS+SAP blocked formyl peptide-induced influx of both Ca2+ and Mn2+, and that this inhibition was present during activation by other neutr ophil stimulants (IL-8, immune complexes). Prior depolarization of neutroph ils with gramicidin-D similarly inhibited the [Ca2+] response of neutrophil s to formyl peptide, and analysis of neutrophil membrane potential by 3,3'- dipentyloxaearbocyanine iodide (diOC(5)(3)) fluorescence revealed that PS+S AP induced rapid neutrophil depolarization. In contrast, PS-SAP exhibited l ittle effect on neutrophil function, [Ca2+], or membrane potential. We conc lude that PS+SAP decreases neutrophil adherence and aggregation responses, blocks Ca2+ influx after physiologic stimulation, and decreases membrane po tential. We speculate that these effects are caused by membrane depolarizat ion via SAP-dependent cation channel insertion, and that all of these effec ts contribute to the antiinflammatory properties of PS+SAP.