Resistance of retinal extracellular space to Ca2+ level decrease: implications for the synaptic effects of divalent cations.

Ion-sensitive microelectrodes were used to measure the variations of [Ca2+]o induced by application of low Ca2+ media in the superfused eyecup preparation of the Pseudemys turtle. The aim of the experiments was to evaluate the possibility, suggested by previous studies, that in the deep, sclerad, layers of the retina [Ca2+]o may remain high enough to sustain chemical synaptic transmission even after prolonged application of low-Ca2+ saline. It was found that, at depths of 100-200 micron from the vitreal surface, [Ca2+ ]o did not fall below 1 mM even after application for periods of 30-60 min of nominally Ca2+-free media, and it was >0.3 mM after 30-min application of media containing EGTA and with a Ca2+ concentration of 1 nM. Previous studies in isolated salamander photoreceptors have shown that a reduction of [Ca2+ ]o to 0.3-1.0 mM may result in a paradoxical increase of Ca2+ influx into synaptic terminals due to the reduced screening of negative charge on the external face of the plasma membrane. On the basis of these results, the persistence or enhancement of synaptic transmission from photoreceptors to horizontal cells observed in various retinas treated with low-Ca2+ media may be accounted for within the classical Ca2+-dependent theory of synaptic transmission without invoking a Ca2+-independent mechanism.