1. Müller cells were isolated from salamander retinas and their membrane voltage was controlled with a whole‐cell voltage clamp. External D‐aspartate, L‐aspartate and L‐glutamate each induced a membrane current. D‐Glutamate, kainate, quisqualate and N‐methyl‐D‐aspartate were more than 100x less effective than L‐aspartate. Kynurenic acid had no effect on the current produced by L‐glutamate, L‐aspartate or D‐aspartate. 2. The current induced by an acidic amino acid (AAA) was completely dependent on the presence of external Na+. Neither Li+, Cs+, choline nor TEA+ were able to substitute for Na+. The relationship between external Na+ concentration and current amplitude can be explained if the binding of three Na+ ions enabled transport. The apparent affinity constant for Na+ binding was 41 mM. Altering K+, H+ and Cl‐ concentrations demonstrated that these ions are not required for transport. 3. The shape of the current‐voltage relation did not depend on the external amino acid concentration. The relationship between D‐aspartate concentration and current amplitude can be described by the binding of D‐aspartate to a single site with an apparent affinity constant of 20 microM. 4. Influx and efflux of AAA were not symmetric. Although influx was electrogenic, efflux did not produce a current. Moreover, influx stimulated efflux; but efflux inhibited influx. 5. Removing external Na+ demonstrated that Na+ carried a current in the absence of an AAA. Li+ was a very poor substitute for Na+. This current may be due to the uncoupled movement of Na+ through the transporter. The relationship between the external Na+ concentration and the amplitude of the uncoupled current can be explained if the binding of two or three Na+ ions enabled the translocation of Na+ in the absence of an AAA. The apparent affinity constant for Na+ binding was approximately 90 mM. 6. The temperature dependence of the AAA‐induced current had a Q10 between 8 and 18 degrees C of 1.95. The Q10 is consistent with a rate constant for influx of 10(4) s‐1 (at ‐70 mV and 20 degrees C). The maximum rate of influx was measured following a concentration jump produced by the photolysis of ‘caged’ L‐glutamate. The onset of the observed current was limited by the 1.3 ms resolution of the recording system. Hence, the rate constant for influx must be faster than 10(3) s‐1.(ABSTRACT TRUNCATED AT 400 WORDS)
The Journal of Physiology – Wiley
Published: Jul 1, 1990
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