Swelling of the Müller fibers in the chicken retina.
A high potassium concentration (33 meq) in the solution superfusing the isolated chicken retina causes an increase in the tissue transparency. An L-glutamate (1 mM) or L-proline (10 mM) solution has the same effect. Swelling of the Müller fibers, which have a radial position in the retina, could explain the transparency increase. This possibility was investigated in electron micrographs of retinas subjected to these treatments and fixed by freeze-substitution to preserve the water distribution in the tissue. The Müller fibers in the controls had a mean diameter of 0.22 micrometer. The fibers in retinas bathed for 2 min in the high [K+] solution were more than three times as thick (0.74 micrometer); the fibers in glutamate-treated retinas were more than twice as thick (0.49 micrometer). The fibers in the proline-treated retinas had a diameter of 0.39 micrometer. The glutamate- and proline-induced swelling may be due to a K+ release from neuronal elements, acting on the Müller fibers. The fiber swelling was postulated to be the expression of different Donnan equilibriums of fibers bathed in solutions of different K+ concentrations. The observed swelling caused by the high [K+] solution was compared with the theoretical swelling of the fiber as an ideal Donnan system, postulating permeabilities for different ions of the fiber membrane. This suggested that the high [K+] solution causes an increase in Na+ permeability in addition to the permeability of the membrane for K+, Cl-, and HCO3-. Chemical fixation with glutaraldehyde and formaldehyde in an Na-phosphate buffer yielded micrographs in which the Müller fibers of retinas treated with a high [K+] or a glutamate solution had diameters similar to those of the control preparations.
D000818:Animals; D002645:Chickens; D004305:Dose-Response Relationship, Drug; D005971:Glutamates / Q000494:pharmacology*; D008854:Microscopy, Electron; D009412:Nerve Fibers / Q000187:drug effects* / Q000648:ultrastructure; D009900:Optic Nerve / Q000033:anatomy & histology / Q000187:drug effects; D011188:Potassium / Q000494:pharmacology*; D011392:Proline / Q000494:pharmacology; D012160:Retina / Q000033:anatomy & histology / Q000187:drug effects*; D009435:Synaptic Transmission / Q000187:drug effects*
D005971:Glutamates; D011392:Proline; D011188:Potassium