This study used imaging and electrophysiological techniques in salamander retinal slices to correlate Ca2+ and Cl- levels in rods and thus test the hypothesis of a feedback interaction between Ca2+- and Ca2+-activated Cl- channels whereby Cl- efflux through Cl- channels can inhibit Ca2+ channels. Increasing [K+]o levels produced a concentration-dependent depolarization of rods accompanied by increases in [Ca2+]i measured with Fura-2. The voltage dependence of increases in [Ca2+]i was compared with the voltage dependence of the calcium current (ICa). [Cl-] i was measured with the dye, MEQ. Depolarization with high K + to membrane potentials below -20 mV reduced [C-] i; larger depolarizations increased [Cl-]i. The Na/K/Cl cotransport inhibitor, bumetanide, shifted the apparent Cl - equilibrium potential (ECl) to more negative potentials, suggesting that this cotransporter helps establish a relatively depolarized ECl. MEQ fluorescence changes evoked by high K+ were inhibited by niflumic acid (0.1 mM), NPPB (2 μM), or replacement of Ca 2+ with Ba2+, suggesting that depolarization-evoked Cl- changes result partly from stimulation of Ca 2+-activated Cl- channels. Replacing ≥12 mM [Cl -]o with CH3SO4- produced a significant reduction in [Cl-]i. [Ca 2+]i increases evoked by 20 or 50 mM K+ were also significantly inhibited by replacing ≥12 mM [Cl-] o with CH3SO4-. Thus modest depolarization can evoke increases in [Ca2+]i that lead to reductions in [Cl-]i, and conversely, reductions in [Cl-]i inhibit depolarization-evoked [Ca 2+]i increases. These findings support the hypothesis that feedback interactions between Ca2+- and Ca 2+-activated Cl- channels may contribute to the regulation of presynaptic Ca2+ currents involved in synaptic transmission from rod photoreceptors.
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