Ionized calcium, an important second messenger, plays a great variety of roles in cellular responses, both chemical and electrical. This review focuses on one signaling pathway in which Ca2+ acts as a second messenger. The pathway begins with an increase in cytosolic Ca2+ concentration, either by opening of the plasma membrane or by receptor activation from the first messenger. With receptor activation, the receptor may activate channels via a cascade of events. These channels may contain phosphorylation sites that open in the presence of Ca2+. When cytosolic Ca2+ concentrations are elevated, Ca2+ can act on various protein kinases, one of which is a target for Ca2+. Upon binding with Ca2+, calmodulin undergoes conformational changes that allow it to activate or inhibit a large variety of enzymes and other proteins. Inhibition or activation of proteins involved in the cellular response depends on the specific target protein. Calmodulin-dependent protein kinases are one example of proteins involved in the cellular response. Inhibition and activation of proteins are part of the first messenger response.
