Uptake, autoradiography and release of taurine and homotaurine from retinal tissue

Uptake of 3H-taurine into bovine retina was studied. Kinetic parameters of uptake were Km = 6.3 x 10(-5) mol/l with Vmax = 43 nmol/g wet tissue x min and Km = 5 x 10(-3) mol/l with Vmax = 702 nmol/g wet tissue x min. Unlabeled homotaurine, a close structural analogue of taurine, showed only a single sodium dependent low affinity uptake with mH = 1.5 x 10(-3) mol/l and Vmax = 1322 nmol/g wet tissue x min. Both taurine and homotaurine uptake were highly sensitive to temperature and inhibited equally by addition of taurine or homotaurine. The taurine content of incubated retinae was greatly altered depending on the osmolarity of the incubation medium. Autoradiography showed 3H-taurine mainly in Müller cells and certain photoreceptor cells of rat retinae, whereas 3H-homotaurine was found exclusively in Müller cells. Electrical stimulation as well as elevated external potassium concentration resulted in a substantial release of 3H-taurine and 3H-homotaurine from preloaded retinae. However, unspecific release cannot be excluded.