Xestospongin C is a marine natural product which was first isolated from Pacific basin sponges, and noted to have vasodilatory properties.[1] Xestospongin C antagonizes the calcium-releasing action of inositol-1,4,5-trisphosphate (IP3) at the receptor level. Inositol phosphates are important signal transduction messengers acting via IP3 receptors to promote the mobilization of Ca2+ from intracellular stores. Xestospongin C blocks the increase in intracellular calcium in vascular smooth muscle cells completely at a concentration of 25 μM.[2] Xestospongin C also inhibits the sarcoplasmic reticulum Ca2+ ATPase pump at concentrations of 700 nM.[3]

Reference:
[1]. Nakagawa, M., and Endo, M. Structures of xestospongin A, B, C and D, novel vasodilative compounds from marine sponge, Xestospongia exigua. Tetrahedron Letters 25(30), 3227-3230 (1984).
[2]. Gafni, J., Munsch, J.A., Lam, T.H., et al. Xestospongins: Potent membrane permeable blockers of the inositol 1,4,5-triphosphate receptor. Neuron 19, 723-733 (1997).
[3]. Castonguay, A., and Robitaille, R. Xestospongin C is a potent inhibitor of SERCA at a vertabrate synapse. Cell Calcium 32(1), 39-47 (2002).