Effects of latrunculin reveal requirements for the actin cytoskeleton during secretion from mast cells

To investigate the role of the actin cytoskeleton in exocytosis, we have tested the effects of latrunculin B, a microfilament-disrupting drug, on secretion from intact and permeabilised rat peritoneal mast cells. The toxin strongly inhibited secretion from intact cells (attached or in suspension) responding to a polybasic agonist, compound 48/80. However, this effect was revealed only after a profound depletion of actin filaments. This was achieved by a long (1 h) exposure of cells to the drug before activation, together with its presence during activation. Maximal inhibition of secretion by such treatment was 85% at 40 microgram/ml latrunculin B. These results indicate that minimal actin structures are essential for the exocytotic response. In contrast, stimulus-induced cell spreading was prevented by latrunculin (5 microgram/ml) applied either before or after activation. The effects of the toxin on intact cells were fully reversible. The responses of permeabilised cells were affected differentially: secretion induced by calcium was more sensitive to latrunculin than that induced by GTP-gamma-S. The calcium response, therefore, is more dependent upon the integrity of the actin cytoskeleton than the response induced by GTP-gamma-S. Again, maximal inhibitory effects (approximately 65 and 25% at 40 microgram/ml) were observed only when cells were exposed to the toxin both before and after permeabilisation. Since the permeabilised cells system focuses on the final steps of exocytosis, the incomplete inhibition suggests that actin plays a modulatory rather than a central role at this stage.