ABSTRACT The homeostasis of calcium ion and its role in phototransduction of rod cells is reviewed here. Calcium is highly concentrated (mM) in the rod outer segment, for the most part stored into the disk membranes. In the dark, the level of Ca2+ in the cytoplasm is about 0.5 µM, as a result of calcium buffering systems and calcium influx through the cGMP-gated channels and efflux through the Na+/ Ca2+,K+ exchanger. After illumination, the level of the cytoplasmic calcium of the rod outer segment decreases and this fact has consequences on the cGMP metabolism and in turn on the phototransduction events. The role of the disks is discussed in view of the finding of a Ca2+-ATPase isolated on the disk membranes.
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