STORE-OPERATED CA2-60 GRANULOCYTES( INFLUX AND STIMULATION OF EXOCYTOSIS IN HL)

This study addresses the role of store-operated Ca2+ influx in the reg ulation of exocytosis in inflammatory cells. In HL-60 granulocytes, wh ich do not possess voltage-operated Ca2+ channels, the chemotactic pep tide fMet-Leu-Phe (fMLP) was able to stimulate store-operated Ca2+ inf lux and to trigger exocytosis of primary granules. An efficient trigge ring of exocytosis by fMLP required the presence of extracellular Ca2 and was inhibited by blockers of store-operated Ca2+ influx. However, receptor-independent activation of store-operated Ca2+ influx through thapsigargin did not trigger exocytosis. fMLP was unable to stimulate exocytosis in the absence of cytosolic free Ca2+ concentration [Ca2+] (c) elevations. However, a second signal generated by fMLP synergized with store-operated Ca2+ influx to trigger exocytosis and led to a lef t shift of the exocytosis/[Ca2+], relationship in ionomycin-stimulated cells. The synergistic fMLP-generated signaling cascade was long-last ing, involved a pertussis toxin-sensitive G protein and a phosphatidyl inositol 3-kinase. In summary, store-operated Ca2+ influx is crucial f or the efficient triggering of exocytosis in HL-60 granulocytes, but, as opposed to Ca2+ influx through voltage-operated Ca2+ channels in ne urons, it is not a sufficient stimulus by itself and requires synergis tic receptor-generated signals.