ULTRASTRUCTURAL LOCALIZATION AND X-RAY-ANALYSIS OF CALCIUM-INDUCED ELECTRON-DENSE DEPOSITS IN THE SKATE RETINA

The technique of Oschman & Wall (1972) was used to visualize the sites at which electron-dense deposits accumulate in neural layers of the skate retina. The method, which involves the addition of calcium to the primary fixative, resulted in the formation of electron-dense deposits along the plasma membranes of horizontal cells, within the outer segments, mitochondria and synaptic vesicles of the visual cells, and at pre- and postsynaptic membranes of the synaptic elements of the outer plexiform layer. Energy dispersive X-ray spectrometry of the electron-dense deposits yielded prominent peaks for calcium and phosphorus. In the dark-adapted retina, the electron-dense deposits were seen more frequently in association with the receptor terminal than with the processes of second-order neurons; after light adaptation, the reverse situation was observed. A light-induced translocation of electron-dense deposits was not found at any other retinal location. The addition of cobalt to the fixative in lieu of calcium produced results equivalent to those obtained with calcium; i.e. electron-dense deposits were formed at simular loci within the retina. The emergence in the X-ray spectrum of a small calcium signal in the presence of a large cobalt peak suggests that the two ions compete for similar membrane sites. On the other hand, the formation of electron-dense deposits was suppressed completely by exposing the retinas to Ca2+-free solutions, with or without the addition of Mg2+. These findings are consistent with physiological observations that suggest that Mg2+ and Co2+ act at different sites in modifying the actions of Ca2+ at synapses. © 1979.