Role of Ca2+ in membrane excitation and cell motility in Characean cells as a model system.
The Characeae internodal cell is excitable and generates an action potential. The depolarizing phase of the action potential primarily reflects the activation of Cl- channels, which takes place in a Ca2+-dependent manner. Namely, an increase in the Ca2+ influx takes place at the very beginning of the action potential and heightens the Ca2+ concentration in the cytoplasm ([Ca2+]c). The high [Ca2+]c activates Cl- channels at the plasmalemma, resulting in a large depolarization. The high [Ca2+]c also acts as a signal to induce a tonoplast action potential and the instantaneous cessation of cytoplasmic streaming; the tonoplast action potential also is caused by Ca2+-induced activation of Cl- channels at the tonoplast, and the cessation is a result of inhibition of the actin-myosin interaction by Ca2+. When the cytoplasm of the Characeae cell, especially in Nitella flexilis, is hydrated rapidly, [Ca2+], also increases through Ca2+ release from an intracellular store(s). The release may be triggered by the stretching of endomembranes caused by osmotic swelling of the Ca2+ store lumens. Although the origin of Ca2+ is different from that in the case of an action potential, high [Ca2+]c not only induces membrane depolarization through activation of the Cl- channel in a Ca2+-dependent manner but also inhibits cytoplasmic streaming in Characeae.
Kikuyama
M
M
Biological Laboratory, The University of the Air, Chiba, Japan.
D000200:Action Potentials / Q000502:physiology*; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling / Q000502:physiology; D002462:Cell Membrane / Q000378:metabolism* / Q000648:ultrastructure; D002465:Cell Movement / Q000502:physiology*; D018118:Chloride Channels / Q000378:metabolism; D000460:Chlorophyta / Q000378:metabolism* / Q000648:ultrastructure*; D003593:Cytoplasm / Q000378:metabolism / Q000648:ultrastructure; D003595:Cytoplasmic Streaming / Q000502:physiology; D008954:Models, Biological; D009995:Osmosis / Q000502:physiology; D014617:Vacuoles / Q000378:metabolism / Q000648:ultrastructure
D018118:Chloride Channels; D002118:Calcium