Effect of calcium on phenothiazine inhibition of neutrophil degranulation

The phenothiazines are known to be potent inhibitors of calmodulin and have been used as probes for examining calmodulin-dependent cellular functions. We report here that the characteristics of phenothiazine inhibition of exocytosis in neutrophils more closely resemble their interaction with the annexins in vitro. Ca(2+)-dependent aggregation of liposomes mediated by either annexin I or annexin II was inhibited by the phenothiazines. Inhibition of liposome aggregation was not caused by interference with the binding of annexins to phospholipids. Rather, the phenothiazines increased the concentration of Ca2+ required for aggregation. Likewise, in neutrophils permeabilized with streptolysin O, inhibition of degranulation by phenothiazines could be overcome by increasing [Ca2+]. These results suggest that inhibition by phenothiazines of neutrophil degranulation is secondary to the ability of these compounds to inhibit membrane-membrane contact promoted by the annexins.