Vol. 291, Issue 1, 383-389, October 1999

Stimulatory Effect of Taurine on Calcium Ion Uptake in Rod Outer Segments of the Rat Retina Is Independent of Taurine Uptake¹

Julius D. Militante and John B. Lombardini

Department of Pharmacology (J.D.M., J.B.L.), and Department of Ophthalmology & Visual Sciences (J.B.L.), Texas Tech University Health Sciences Center, Lubbock, Texas

Taurine stimulates ATP-dependent Ca²⁺ uptake in the rat rod outer segments (ROS). This stimulation has been linked to the function of the cyclic nucleotide-gated cation channel, implying an important physiologic role for taurine in visual signal transduction. Calmodulin (CaM) has been reported to affect taurine transport in the choroid plexus and also to inhibit the cyclic nucleotide-gated channel; thus, the effects of the competitive CaM inhibitors trifluoperazine (TFP) and N-(8-aminooctyl)-5-iodonaphthalene-1-sulfonamide (J-8) were studied on Ca²⁺ and taurine uptake in the rat ROS. Pretreatment of the ROS preparation with TFP and J-8 for 5 min before measurement of Ca²⁺-uptake activity produced inhibition of the effects of taurine on ATP-dependent Ca²⁺ uptake. Both TFP and J-8 also were effective in inhibiting high-affinity taurine uptake. In both uptake systems, inhibition by TFP was noncompetitive. These data initially suggested that the stimulatory effects of taurine on ATP-dependent Ca²⁺ uptake are dependent on taurine uptake. However, competitive inhibition of taurine uptake by guanidinoethane sulfonate did not produce any effect on the stimulatory effects of taurine. Previous studies have proposed that taurine binds directly to the plasma membrane, and our study demonstrated that TFP inhibits taurine binding to the ROS. In addition, our study demonstrated that taurine uptake is unaffected by varying the concentration of Ca²⁺ and that the effects of TFP are independent of Ca²⁺, suggesting that TFP acts through a CaM-independent mechanism.