Low extracellular Ca2+ enhances synaptic transmitter release from retinal photoreceptors (PRs) by increasing intracellular [Ca2+] in PRs

Purpose: To provide evidence that the synaptic transmission increase induced by nominally Ca2+• free media in the eyecup of turtle (Pseudemys scripta) or salamander (Ambystoma tigrnum) (see Piccolino er of, PNAS, l996) is the result of a paradoxical increase of [Ca2+]i in PRs consequent to increased opening of Ca2+ channels via a surface-charge mediated effect. Methods: changes in [Ca2+]o induced by application of nominally Ca2+-free media in eyecups were measured using micropipettes filled with a Ca2+ ionophore (I • Cocktail A by Fluka, # 2l048); and changes of [Ca2+]i induced in isolated PRs by bath application of Cai2+-concentrations comparable to those induced in eyecups by nominally Ca2+-free media were measured with Ca2+-imaging techniques (utilizing FLUO-3 AM). Results: In eyecups superfused from the vitreal side with nominally Ca2+•free media for periods of 30 minutes or more, [Ca2+] near the PH terminals decreased to levels around 0.7-0.3 mM. During Ca2+—free application, clear spatial gradients of [Ca2+] were evident along the sclero-vitreal axis with lower [Ca2+] near the vitreal surface. Membrane depolarisation induced in isolated PRs by soliitions containing elevated [K+] (15 mM) and normal (3 mM) [Ca2+] resulted in an increase of [Ca2+],. When the same high K+ solutions were applied in the presence of reduced [Ca2+] (0.7-0.3 mM) the increase of [Ca•2+]. signal was enhanced. Conclusions: These results justify our "surface- chargel interpretation of the transmitter release changes induced by low Ca2+ application in eyecup preparations, and invite caution in interpreting the effects of low Ca2+ media in intactor semi-intact preparations of the nervous system. Supported by HFSP Grant Non