Calcium and Proteases

Calcium ions (Ca2+) are critical second messengers within various eukaryotic cell-signaling pathways, functioning as regulators of diverse cellular processes. The importance of Ca2+ in muscle contraction was first observed at the end of the 19th century, and since the mid-1970s, research into the complexity and diversity of Ca2+ functions along with the development of experimental tools to investigate these functions has yielded impressive findings. Ca2+ clearly stimulates numerous cellular processes, such as muscle contraction, cellular proliferation, gene expression, hormone and neurotransmitter secretion, exocytosis, and chemotaxis, but Ca2+ is also toxic. Thus, free intracellular Ca2+ ([Ca2+]i) must be tightly regulated to properly balance Ca2+-mediated cell function and Ca2+-mediated cell injury and death. Intracellular cytoplasmic Ca2+ (∼10−8–10−7molL−1) is maintained well below the extracellular concentration 10−3molL−1, creating a gradient across the cell membrane.