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A method has been developed for the preparation of large numbers of glial (Muller) cells from the turtle retina. After proteolytic dissociation of the retina, Muller cells were separated from retinal neurons by velocity sedimentation at unit gravity. Fractions containing >90 percent morphologically identifiable Muller cells were prepared by this procedure. Fractions containing only Muller cells were obtained by drawing selected cells individually into a micropipette under visual observation. Biochemical analyses of isolated Muller cells showed that (a) these cells did not synthesize and accumulate acetylcholine, 
-aminobutyric acid, or catecholamines when incubated with appropriate radioactive precursors; (b) the specific activities of choline acetyltransferase (EC 2.3.1.6), glutamate decarboxylase (EC 4.1.1.15), and tyrosine hydroxylase (EC 1.14.16.2) in these cells were less than 2 percent of those found in the retina; (c) Muller cells, however, contained high activities of transmitter degrading enzymes-acetylcholinesterase (EC 3.1.1.7) and -aminobutyrate- transamine (EC 2.6.1.19); and (d) the cells also possessed high levels of two presumably glial-specific-enzymes-glutamine synthetase (EC 6.3.1.2) and carbonic anhydrase (EC 4.2.1.1). These results, together with other findings, suggest that Muller cells are not capable of neurotransmitter syntheses but possess the enzymes necessary for two important roles in the retina: (a) the inactivation of certain transmitters after synaptic transmission by uptake and degradation, and (b) the maintenance of acid-base balance and the provision of a stable microenvironment in the retina by the removal of metabolic products such as carbon dioxide and ammonia.
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