Xestospongins are marine natural products first isolated from pacific basin sponges, and noted to have vasodilatory properties. Inositol phosphates (IPs) are important signal transduction messengers acting via IP3 receptors to promote the mobilization of Ca2+ from intracellular stores. XestosponginA antagonizes the calcium-releasing action of inositol-1,4,5-trisphosphate at the receptor level in cerebral microsomes, with an IC50 of 2.5 µM. It is thus about ten times less potent than XestosponginC as an antagonist of the IP3 receptor.