Ca²⁺ can influence various cellular outcomes by either entering the cell from the extracellular milieu or by being released from intracellular stores, such as the endoplasmic reticulum (ER). Ca²⁺ release from the ER is influenced by a number of factors like the activation of G-protein coupled receptors and receptor tyrosine kinases which activate downstream pathways leading to Ca²⁺ release. There are also membrane proteins located on the ER membrane which are known to influence Ca²⁺ release. Such proteins include IP₃R, ryanodine receptors (RyRs) and various pumps. Many compounds can either promote or inhibit Ca²⁺ release from the ER.