Xestospongin C is a marine natural product isolated from Pacific basin sponges; it antagonizes the calcium-releasing action of inositol-1,4,5-trisphosphate (IP3) at the receptor level. Xestospongin C blocks the increase in intracellular calcium in vascular smooth muscle cells completely at a concentration of 25 µM. Xe C also inhibits the sarcoplasmic reticulum Ca²⁺- ATPase pump at concentrations of 700 nM.