Potassium‐Stimulated Release of Radiolabelled Taurine and Glycine from the Isolated Rat Retina

Abstract

The release of preloaded [3H]glycine and [3H]taurine in response to a depolarizing stimulus (12.5-50 mM KCl) was studied in the superfused rat retina. High external K+ concentration immediately increased the spontaneous efflux of [3H]glycine, the effect of 50 mM K+ apparently being abolished by omitting Ca from the superfusing medium. In contrast, although high K+ concentrations increased the spontaneous efflux of [3H]taurine from the superfused rat retina, this release was not evident until the depolarizing stimulus was removed from the superfusing medium. The magnitude of this late release of [3H]taurine was dependent on external K+ concentrations, and appeared immediately after cessation of the stimulus, irrespective of whether it was applied for 4, 8 or 12 min. K+(50 mM)-induced release of taurine appeared partially Ca-dependent, being significantly reduced (P < 0.01) but not abolished by replacing Ca with 1 mM EDTA in the superfusate. High-affinity uptake systems for both [3H]glycine and [3H]taurine were demonstrated in the rat retina in vitro (Km values, 1.67 .mu.M and 2.97 .mu.M; Vmax values, 19.3 and 23.1 nmol/g wet wt tissue per h, respectively). The results are discussed with respect to the possible neurotransmitter roles of both amino acids in the rat retina.