We have estimated free, intracellular calcium ion concentrations ([Ca](i)) in isolated retinal ganglion cells of adult goldfish by ratio-imaging fura-2 emission intensity at two excitation wavelengths. Here we describe [Ca](i) in these cells, both at rest and during depolarization by elevated levels of extracellular potassium ions ([K](o)). [K](o) was varied between 5 and 60 mM in sodium-free, tetrodotoxin-containing salines. Ganglion cell membrane potential, measured with patch electrodes, fell with each increment of [K](o) used, from ~-70 mV in 5 mM K+ to ~-20 mV in 60 mM K+. In control saline, [Ca](i) was roughly 120 nM in cell somata and at least twofold higher in their growth cones. [Ca](i) increased in both somata and growth cones to as high as 1.5 μM in salines containing 60 mM K+. [Ca](i) exceeded 1.5 μM in some cells in high-K+ salines, although these levels could not be quantified accurately with fura-2. Increases in [Ca](i) elicited by elevated [K](o) persisted for the duration of the exposure to high-K+ saline and were blocked by replacement of most of the bath Ca2+ by Co2+. These increases in [Ca](i) were also sensitive to dihydropyridine calcium-channel ligands, viz., enhanced by BAY K 8644 (3 μM) and antagonized by nifedipine (10 μM). Partial recovery of control [Ca](i) occurred when [K](o) was reduced to 5 mM after exposure to high-K+ saline and in high-K+ saline when nifedipine was included. These results show that goldfish retinal ganglion cells can partially buffer intracellular Ca2+ in the absence of extracellular Na+ ions. These results provide measurements of the changes in [Ca](i) brought about by depolarization of goldfish retinal ganglion cells in Na+-free salines. In these salines, at least part of the increase in [Ca](i) appears to result from Ca2+ influx through a voltage-activated, noninactivating calcium conductance in the somata and growth cones of these cells. These measurements complement whole-cell patch-clamp and vibrating microprobe recordings from the somata and neurites of these cells and also immunocytochemical studies and patch-clamp measurements in amphibian, reptilian, and mammalian retinal ganglion cells.
|Original language||English (US)|
|Number of pages||12|
|Journal||Journal of Neurophysiology|
|State||Published - 1991|
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