Reciprocal interactions between calcium and chloride in rod photoreceptors

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 (I-Ca). [Cl-](i) was measured with the dye, MEQ. Depolarization with high K+ to membrane potentials below - 20 mV reduced [Cl-](i); larger depolarizations increased [Cl-](i). The Na/K/Cl cotransport inhibitor, bumetanide, shifted the apparent Cl- equilibrium potential (E-Cl) to more negative potentials, suggesting that this cotransporter helps establish a relatively depolarized E-Cl. MEQ fluorescence changes evoked by high K+ were inhibited by niflumic acid (0.1 mM), NPPB ( 2 muM), or replacement of Ca2+ with Ba2+, suggesting that depolarization-evoked Cl- changes result partly from stimulation of Ca2+-activated Cl- channels. Replacing greater than or equal to 12 mM [Cl-](o) with CH3SO4- produced a significant reduction in [Cl-](i). [Ca2+](i) increases evoked by 20 or 50 mM K+ were also significantly inhibited by replacing greater than or equal to 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 [Ca2+](i) increases. These findings support the hypothesis that feedback interactions between Ca2+- and Ca2+- activated Cl- channels may contribute to the regulation of presynaptic Ca2+ currents involved in synaptic transmission from rod photoreceptors.