Accumulation of calcium by retinal outer segments.

Two preparations of bovine photoreceptors, including intact and lysed outer segments, are used to study accumulation of Ca++ in order to differentiate between binding and transport. The two preparations accumulate initially roughly equal amounts of Ca++ in the presence or absence of ATP. In the absence of ATP no further accumulation occurs. With ATP accumulation continues most rapidly in intact outer segments. The additioesults in kinetics of uptake similar to those in the absence of ATP. Thus the initial phase of Ca++ accumulation is predominantly due to binding, while that induced by ATP is due to transport. Accumulation of Ca++ by outer segments can be stimulated by preloading them with ATP suggesting that ATP enters the structures and acts intracellularly. Illumination decreases binding of Ca++ to outer segments but does not consistently affect transport. After illumination more Ca++ is retained by intact than lysed outer segments indicating that plasma membrane acts as a barrier to the escape of Ca++. Phospholipids and protein appear to be responsible for Ca++ binding of outer segments as suggested by a reduced binding ability after removal of these constituents. Phosphatidyl ethanolamine is the main phospholipid class of outer segments.