Mechanisms that mediate the calcium influx in mammalian horizontal
cells were studied. Horizontal cells (HCs) enzymatically
dissociated from the rabbit retina were recorded by the whole-cell
configuration of the patch-clamp technique and by calcium image
ratioespectrophotometry of Fura-2 loaded cells. AMPA-preferring
glutamate receptors were shown to permeate Ca2+ in
mammalian HCs by ionic substitution experiments. Furthermore,
after blocking the L-type calcium current with nifedipine (100
μM), calcium current through the AMPA-preferring glutamate
receptors was measured. Calcium image ratioespectrophotometry
was performed on the dissociated HCs in order to determine the
changes in the intracellular calcium
([Ca2+]i). Fura-2
microspectrophotometry showed that in HCs, K+-induced
cell depolarization promoted an increase in
[Ca2+]i, mediated by the L-type
calcium channels, since it was abolished in the presence of
nifedipine. The increase in [Ca2+]i
upon cell depolarization was observed throughout each cell;
however, it was maximal at the cell soma. Activation of glutamate
receptors in dissociated HCs by glutamate, AMPA or kainate promoted
an increase in [Ca2+]i. This
increase in [Ca2+]i was abolished
in nominally Ca2+-free solution (0 mM Ca2+);
in contrast, nifedipine decreased the glutamate-induced influx
of calcium in ca. 50%. The present study demonstrates
that calcium may permeate through glutamate receptors expressed
in HCs of the rabbit retina.
Noise analysis of cytosolic calcium image data