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

Abstract

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.