Influence of calcium on retinal ATPases

The activity of Mg2+-ATPase and Na+-K+-ATPase was measured in media of calcium concentrations ranging from 10(-9)M to 10(-3)M, with retinal homogenates from rat and rabbit. In both species calcium stimulated the Mg2+-ATPase and inhibited Na+-K+-ATPase. In the rat, activity of Na+-K+-ATPase fell by 75% as calcium was increased from 10(-8)M to 10(-7)M and reached 90% inhibition only at 10(-3)M. By contrast, the activity in the rabbit fell gradually to a 90% inhibited state, over the range 10(-8)M to 10(-3)M. Calcium activated the Mg2+-ATPase by a maximum of 50% in each species, at a concentration of 10(-8)M in the rat and over a broader concentration range between 10(-5)M and 10(-4)M in the rabbit. It is postulated that maintenance of intracellular calcium at low levels by the Ca2+-activated Mg2+-ATPase or other cellular mechanisms is essential for the activity of the membrane-bound Na+-K+-ATPase of the retina.