EFFECTS OF OSMOTIC AND LIGHT STIMULATION ON 3 H-TAURINE EFFLUX FROM ISOLATED ROD OUTER SEGMENTS AND SYNTHESIS OF TAURET IN THE FROG RETINA Andranik M. Petrosian, 1 Jasmine E. Haroutounian, 1 Kjell Fugelli, 2 and Hilde Kanli 2 1 Buniatian Inst. of Biochemistry of Natl. Acad. Sci. of Armenia, Sevag St. 5/1, Yerevan 14, Armenia: 2 Div. of Gen University Oslo, P.O. Box 1051 Blindern, 0316 Oslo 3. Norway Abstract: After injection of 3H-taurine into eyeballs of frogs and maintenance for 3 h in darkness by a gentle shaking, an almost homogenous fraction of rod outer segments (ROS) was prepared. About a 22% decrease in tonicity caused by reducing NaCl in isotonic 225 mOsm normal solution caused a rapid increase in the rate coefficient of efflux of 3 H-taurine from the ROS fraction. The peak level of increased efflux rate coefficient was 7-times higher than the basal isotonic level. This indicates that taurine could contribute essentially to the volume regulation, either via selective channels or a carrier transporter- mediated pathways. For further clarifying if taurine fluxes in the ROS are sensitive to the light, other experiments were performed. Neither light stimulation of dark-adapted ROSs fractions or dark stimulation of weakly illuminated ROSs revealed any detectable changes in the efflux rate coefficient of 3 H-taurine. These results indicate that light-induced taurine efflux, if present in the ROS, must be small, compared with hypoosmotic induced efflux. Thus the question of light-induced release of taurine from ROS still remains to be clarified. In the second part of this study, using TLC (thin layer chromatography) in combination with 3 H-taurine measurements we have tried to clarify whether frogs (Rana ridibunda) eye structures can synthesize tauret (retinylidenetaurine). In isolated retinal preparations almost no any noticeable radioactivity was detected compared with background level. The capability of the eye structures to synthesize tauret from 3 H-taurine was revealed in the second whole eye injection experiment. About 0.3 % of the total 3 H-taurine pool taken up was converted into 3 H-tauret in the dark-adapted frog retina. In the retina of frogs adapted to light compared with those which were dark adapted tauret quantities were remarkable lower - on average about half. These Taurine 4, edited by Della Corte et al. Kluwer Academic / Plenum Publishers, New York, 2000. 441