Increase in cytoskeletal actin induced by inositol 1,4-bisphosphate in saponin-permeated pig platelets.

Inositol 1,4-bisphosphate (IP2) which rapidly accumulates during cell activation, strongly stimulates an increase in cytoskeletal actin in saponin-permeated platelets, and the effect is insensitive to 5'-Chloro-5'-deoxyadenosine. Within 10 s, the amount of cytoskeletal actin in platelets rapidly increases by 41%, and then slowly increases further. IP2 induces the increase in cytoskeletal actin in a dose-dependent manner. The half-maximal effect requires approximately 2 microM of IP2. Inositol 1,4,5- triphosphate, the messenger for Ca2+ release, causes the increase in cytoskeletal actin, but is less effective than IP2. Inositol 1-monophosphate and inositol 2-monophosphate have no effect on cytoskeletal actin. Phorbol 12-myristate 13-acetate, which has been shown to activate IP3 5'-phosphatase through protein kinase C, stimulates the increase in cytoskeletal actin. Spermine, an inhibitor of IP3 5'-phosphatase, inhibits the thrombin stimulated increase in cytoskeletal actin. These results suggest that IP2 may be a messenger that controls the organization of actin filaments during cell activation. This study presents the first evidence for IP2 as a messenger during cell activation.