Opposing interactions of ionophores (valinomycin and monensin) on calcium ion uptake in rat retinal preparations

Valinomycin is a potent inhibitor of taurine-stimulated ATP-dependent calcium ion uptake in rat retinal membrane preparations but had no effect on ATP-dependent calcium ion uptake in the absence of taurine and no effect on ATP-independent calcium ion uptake. The presence of potassium ions in the buffer systems were required for valinomycin to be inhibitory. On the contrary, monensin stimulated calcium ion uptake in the ATP-dependent system but had no effect on ATP-independent calcium ion uptake. The crude retinal homogenate was also fractionated into various subcellular components. The fraction which contained photoreceptor cell synaptosomes (P₁) had a higher, specific activity for taurinestimulated ATP-dependent calcium ion uptake than the crude homogenate or either the fractions which contained synaptosomes derived from the plexiform layer (P₂) or rod outer segments (ROS). No differences in calcium ion uptake were observed in the various subcellular fractions compared to the homogenate when assayed for ATP-dependent calcium ion uptake. Valinomucin inhibited both ATP-dependent and taurine-stimulated ATP-dependent calcium ion uptake in the P₁, P₂, and ROS fractions while monensin stimulated the ATP-dependent calcium ion uptake in the subcellular fractions.