Xestospongins and araguspongins are marine natural products first isolated from Pacific basin sponges, and noted to have vasodilatory properties. Inositol phosphates (IP) are important signal transduction messengers acting via IP3 receptors to promote the mobilization of Ca2+ from intracellular stores. Araguspongin B antagonizes the calcium-releasing action of inositol 1,4,5-trisphosphate at the receptor level in cerebral microsomes, with an IC50 of 0.6 µM. It is nearly as potent as xestospongin C as an antagonist of the IP3 receptor.