The uptake and release of [3H]glycine in the goldfish retina.

1. In the goldfish retina, uptake of exogenous [3H]glycine follows Michaelis‐‐Menten kinetics with increasing concentrations of glycine. This uptake can be explained kinetically by the presence of two independent affinity systems: a ‘high‐affinity’ mechanism with an apparent Km(H) of 8.1 microM and a Vmax(H) of 9.12 p‐moles/min. mg protein, and a ‘low‐affinity’ mechanism with an apparent Km(L) of 0.63 mM and a Vmax(L) of 430 p‐mole/min . mg protein. 2. The high‐affinity mechanism, and probably also the low‐affinity mechanism, is temperature‐ and Na+‐dependent. 3. The low‐affinity mechanism for glycine uptake is not affected by 5 mM‐isoleucine, methionine and valine in the medium. However, it is inhibited more than 90% by 5 mM‐alanine, proline and serine in the medium. This result indicates that the low‐affinity transport for glycine may go through system A of the neutral amino acid transport system which is present in most tissues to transport glycine and certain neutral amino acids for metabolic purposes. 4. The high‐affinity mechanism for glycine uptake is, however, not affected by the presence of up to 100‐fold excess of all amino acids examined. 5. Autoradiographic studies show that at least one type of amacrine cell and one type of probable interplexiform cell take up [3H]glycine both in the presence and absence of 5 mM‐alanaine, proline and serine, indicating that these neurones possess the high‐affinity mechanism for glycine uptake. 6. [3H]Glycine accumulated in the retina can be released by increasing the external K+ concentration. This release is probably Ca2+‐dependent since it is blocked by 10 mM‐Co2+ in the medium. Additionally, autoradiographic studies show that [3H]glycine taken up by the glycine‐accumulating neurones can also be released by Ca2+‐dependent, K+‐depolarization of the retina.