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 Ca(2+)-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 Ca(2+)- 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 Ca(2+)- 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.