Calcium Pumping

 

There are two parts to every signaling process. The part that usually receives most attention is the ON part. Most research has been directed towards the mechanisms that elevate cytoplasmic Ca2+ concentration. However, the OFF part of the process is just as important. An uncontrolled rise in cytoplasmic Ca2+ concentration will very rapidly (within seconds) kill a cell. The responsibility for keeping cytoplasmic Ca2+ concentration low when the cell is at 'rest', and for recovering a low cytoplasmic Ca2+ concentration following a Ca2+ signal, usually rests with the Ca2+ pump. The Ca2+-pump (or Ca2+-ATPase) is one of two primary active transporters universally distributed throughout cells. Just as the Na+ pump is the powerhouse for maintenance of the membrane potential and for secondary active transport so the Ca2+ pump is responsible for controlling and containing Ca2+ signals.

In addition to the Ca2+ pump, there is another transport system used by cells to remove Ca2+ from the cytoplasm namely Na+/Ca2+ exchange. Sodium/Calcium exchange is an example of an antiport cotransport system. The Na+ gradient (directed into the cell) is used to drive Ca2+ out of the cell. Sodium/Calcium exchange has a higher capacity for Ca2+ transport than the Ca2+ pump and it will also operate at a much faster rate than the Ca2+ pump. In other words it can move more Ca2+ more quickly than the Ca2+ pump. What Na+/Ca2+ exchange cannot do is to drive the Ca2+ concentration to the very, very low levels required by most cells at ‘rest’ (Intracellular Ca2+ is about 10-7 M/l and extracellular Ca2+ is about 10-3 M/l, i.e the intracellular Ca2+ concentration is 1/10,000 the extracellular Ca2+ concentration). However, both processes work together very efficiently, Na+/Ca2+ exchange will rapidly reduce a high cytoplasmic Ca2+ concentration and then the Ca2+ pump can take over and finish the job.

 

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© Pete Smith 1998