In a large number of tissues, calcium is released as a consequence of the hydrolysis of phosphatidyl inositol. The inositol 1,4,5 triphosphate (InsP3) formed is able to release calcium from internal stores. Ref: Skeletal Muscle in Health and Disease: A Textbook of Muscle Physiology, David A. Jones, Joan M. Round - 1990, Page 49 ; Elements of Molecular Neurobiology By Christopher Upham Murray Smith 2002, Page 231