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

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Abstract
In the goldfish retina uptake of exogenous [3H]glycine followed Michaelis-Menten kinetics with increasing concentrations of glycine. This uptake explained kinetically by 2 independent affinity systems, a high-affinity mechanism with an apparent Km (H) of 8.1 .mu.M and a Vmax (H) of 9.12 pmol/min per mg protein, and a low-affinity mechanism with an apparent Km (L) of 0.63 mM and a Vmax (L) of 430 pmol/min per mg protein. The high-affinity mechanism and probably the low-affinity mechanism was temperature- and Na+-dependent. The low-affinity mechanism for glycine uptake was not affected by 5 mM-isoleucine, methionine and valine in the medium. It was inhibited more than 90% by 5 mM alanine, proline and serine. 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. The high-affinity mechanism for glycine uptake was not affected by presence of up to 100-fold excess of all amino acids examined. At least 1 type of amacrine cell and 1 type of probable interplexiform cell incorporated [3H]glycine in the presence and absence of 5 mM-alanine, proline and serine. These neurons apparently possessed the high-affinity mechanism for glycine uptake. [3H]glycine accumulated in the retina was released by increasing the external K+ concentration. This release was probably Ca2+-dependent since it was blocked by 10 mM Co2+ in the medium. Autoradiography revealed that [3H]glycine taken up by the glycine-accumulating neurons was released by Ca2+-dependent, K+-depolarization of the retina.