Complex regulation of human neutrophil activation by actin filaments: dihydrocytochalasin B and botulinum C2 toxin uncover the existence of multiple cation entry pathways

In human neutrophils, the chemotactic peptide, N‐formyl‐L‐methionyl‐L‐leucyl‐L‐phenalalanine (fMLP), the Ca2+‐ATPase inhibitor, thapsigargin, and the lectins, concanavalin A (Con A) and mistletoe lectin I (ML I), stimulate the entry of Ca2+ and Na+ with subsequent activation of exocytosis and superoxide anion (O2 –) formation. We studied the role of actin in neutrophil activation. The actin filament‐disrupting substances, dihydrocytochalasin B (dhCB) and botulinum C2 toxin (C2 toxin) potentiated fMLP‐ and lectin‐stimulated Ca2+‐ and Na+ entry. Lectin‐induced Mn2+ entry was enhanced by actin disruption, whereas fMLP‐triggered Mn2+ entry was unaffected. dhCB and C2 toxin inhibited fMLP‐ and lectin‐stimulated Ba2+ influx. The actin disrupters also inhibited fMLP‐ and ML I‐induced Sr2+ influx, whereas Con A‐stimulated Sr2+ entry was not influenced by dhCB and C2 toxin. Thapsigargin‐stimulated cation entry was not altered by actin disruption. DhCB and botulinum C2 toxin potentiated lysozyme release induced by all four stimuli. Con A and ML I per se activated O2 – formation only in the presence and not in the absence of dhCB. Con A potentiated the stimulatory effects of ML I on O2 – formation in the presence of dhCB and primed neutrophils to respond to ML I in the absence of dhCB. Our data indicate the following: (1) dhCB and C2 toxin uncover the existence of multiple cation entry pathways in neutrophils; (2) actin disruption facilitates exocytosis and O2 – formation by enhancement of Ca2+‐ and Na+ entry and by altering the function of proteins involved in activation of secretion and O2 – formation; and (3) Con A and ML I, which possess different sugar specificities, activate different signaling pathways in neutrophils. J. Leukoc. Biol. 61: 703–711; 1997.