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 μM 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.