Surfactant Releases Internal Calcium Stores in Neutrophils by G Protein-Mediated Pathway

Pulmonary surfactant with surfactant-associated proteins (PS+SAP) decreases pulmonary inflammation by suppression of neutrophil activation. We have observed that PS+SAP inserts channels into artificial membranes, depolarizes neutrophils, decreases calcium influx following stimulation, and depresses neutrophil functions in vitro. We hypothesize that PS+SAP suppresses neutrophil activation by insertion of cation channels into plasma membrane, depolarization of neutrophils, and 0 protein-dependent release of Oa++ stores, and that gramicidin - a monovalent, cation channel protein - mimics these effects. Human neutrophils were monitored for Oa++1 responses after exposure to gramicidin alone, gramicidin reconstituted with phospholipid (PLG), one of two different PS+SAP preparations, or a PS-SAP preparation. tOa++1 responses were reexamined following preexposure to 0 protein or internal Ca++ release inhibitors. We observed that: (1)1% PS+SAP but not PS-SAP - causes transient increases of neutrophil Oa++% within seconds of exposure; (2)1% PLG - but not gramicidin alone - closely mimics the effect of PS+SAP upon Ca++ response; (3) PS+SAP, gramicidin alone and PLG equally depolarizes neutrophils despite differences among neutrophil Oa++ responses; (4) direct inhibition of internal Ca++ store release or G protein activation suppresses Ca++ responses to PS+SAP and PLO; and 5) preexposure to either PS+SAP or PLG inhibits Ca++ influx following tMLP stimulation. We conclude that PS+SAP independently depolarizes neutrophils, releases Oa++ from internal stores by a G protein-mediated pathway, and alters subsequent neutrophil response to physiologic stimulants. The mimicking of these results by PLG supports the hypothesis that PS+SAP initiates depolarization via channel insertion into neutrophil plasma membrane.