The F-actin cytoskeleton modulates slow secretory components rather than readily releasable vesicle pools in bovine chromaffin cells.

Adrenal chromaffin cells were used to test the role of the peripheral cytoskeleton of F-actin in controlling different vesicle pools. Phorbol 12-myristate 13-acetate and calyculin A, two substances affecting phosphorylation-dephosphorylation cycles, produced different degrees of F-actin reorganization, inducing the partial and the almost total disassembly of this structure, respectively, as visualized using rhodamine-phalloidin staining. Consequently, electron microscopy studies revealed the higher efficiency of calyculin-A over phorbol 12-myristate 13-acetate in promoting vesicle access to the plasmalemma boundary. Surprisingly, only the phorbol ester enhanced fast kinetics and the population of rapidly releasable vesicle pools as studied by single-cell amperometry, whereas both agents, as well as the F-actin severing compound, Latrunculin A, promoted an increase in the population of vesicles recruited in response to prolonged or repetitive stimulations. Taken together, our data support the notion that the F-actin peripheral barrier controls primary granule recruitment from reserve vesicle pools, whereas the phorbol ester effect on the rapidly releasable pools might be related to the alteration of late secretory stage through protein kinase C-dependent phosphorylation of an unidentified target.