How is calcium used as a second messenger?

As a second messenger, calcium is used ubiquitously. When released into the cytosol, calcium will bind to and activate calmodulin, an intracellular messenger protein. Once activated, calmodulin acts as an effector molecule and initiates its effects by binding to various downstream targets, such as protein kinases, ion channels and other proteins, and regulating their biological activities. Cellular functions regulated by calcium/calmodulin include cell cycle, intracellular signaling, muscle contraction, neurotransmitter release, fertilization, metabolism, inflammation and the immune response.