Calcium Signaling in Smooth Muscle

Changes in intracellular Ca2+ are central to the function of smooth muscle, which lines the walls of all hollow organs. These changes take a variety of forms, from sustained, cell-wide increases to temporally varying, localized changes. The nature of the Ca2+ signal is a reflection of the source of Ca2+ (extracellular or intracellular) and the molecular entity responsible for generating it. Depending on the specific channel involved and the detection technology employed, extracellular Ca2+ entry may be detected optically as graded elevations in intracellular Ca2+, junctional Ca2+ transients, Ca2+ flashes, or Ca2+ sparklets, whereas release of Ca2+ from intracellular stores may manifest as Ca2+ sparks, Ca2+ puffs, or Ca2+ waves. These diverse Ca2+ signals collectively regulate a variety of functions. Some functions, such as contractility, are unique to smooth muscle; others are common to other excitable cells (e.g., modulation of membrane potential) and nonexcitable cells (e.g., regulation of gene expression).