Ca 2+ influx shutdown in neutrophils induced by Fas (CD95) cross‐linking

Summary In neutrophils, as in most other cell types, Ca 2+ signalling is important for a number of cellular activities. Although inositol(1,4,5)trisphosphate‐mediated release of Ca 2+ from intracellular stores is a necessary prelude, it is the Ca 2+ influx that is responsible for many of the neutrophil responses. We report here that although elevations of cytosolic Ca 2+ do not accompany Fas‐mediated apoptosis in neutrophils, the Ca 2+ influx component of the response to N ‐formyl‐methionyl‐leucyl‐phenylalanine (FMLP) becomes selectively inactived as the neutrophils progress towards accelerated apoptosis induced by Fas (CD95) cross‐linking. After 4 hr incubation at 37°, untreated neutrophils display an exaggerated Ca 2+ influx phase in response to FMLP. This was absent in neutrophils that had been Fas‐activated at the same time. No Ca 2+ influx component was demonstrable by the removal of extracellular Ca 2+ or by Ca 2+ channel blockade with Ni 2+ and no Mn 2+ influx was detectable. The defect could not be attributed to a decrease in receptor sensitivity, receptor coupling or receptor number because the release of stored Ca 2+ remained constant during incubation and was unaffected by Fas activation. Ca 2+ influx became uncoupled from store release before detectable gross morphological changes or phosphatidyl serine externalization and was also insensitive to caspase 3 and 8 inhibitors. These results suggest a mechanism other than caspase‐mediated proteolytic damage to components important for Ca 2+ influx.