Stabilization of calcium uptake in rat rod outer segments by taurine and ATP

Calcium ion (Ca2+) uptake was measured in rod outer segments (ROS) isolated from rat retina in the presence of varying concentrations of CaCl2 in the incubation buffer (1.0-2.5 mM). It is known that taurine increases Ca2+ uptake in rat ROS in the presence of ATP and at low concentrations of CaCl2 (Lombardini, 1985a); taurine produces no significant effects when CaCl2 concentrations are increased to 1.0 and 2.5mM. With the removal of both taurine and ATP, Ca2+ uptake in rat ROS increased significantly in the presence of 2.5mM CaCl2. Taurine treatment in the absence of ATP was effective in decreasing Ca2+ uptake at the higher levels of CaCl2 (2.0 and 2.5 mM). Similar effects were observed with ATP treatment. The data suggest that taurine and ATP, alone or in combination, limit the capacity of the rat ROS to take up Ca2+ to the extent that a stable uptake level is achieved under conditions of increasing extracellular Ca2+, indicating a protective role for both agents against calcium toxicity.