Electrophysiology of glutamate and sodium co-transport in a glial cell of the salamander retina.
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)
Schwartz
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EA
Department of Pharmacological and Physiological Sciences, University of Chicago, IL 60637.
Tachibana
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D000557:Ambystoma; D000818:Animals; D001224:Aspartic Acid / Q000494:pharmacology; D001665:Binding Sites; D001693:Biological Transport, Active; D004305:Dose-Response Relationship, Drug; D005971:Glutamates / Q000378:metabolism*; D015640:Ion Channel Gating / Q000187:drug effects; D007608:Kainic Acid / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects; D012160:Retina / Q000378:metabolism / Q000502:physiology*; D012964:Sodium / Q000378:metabolism*
D005971:Glutamates; D001224:Aspartic Acid; D012964:Sodium; D007608:Kainic Acid