Reciprocal Interactions Between Calcium and Chloride in Rod Photoreceptors

1 Department of Ophthalmology, University of Nebraska Medical Center, Omaha, Nebraska 68198 2 Department of Pharmacology, University of Nebraska Medical Center, Omaha, Nebraska 68198 3 Department of General Zoology and Neurobiology, University of Pécs, H-7601 Pécs, Hungary Submitted 17 October 2002; accepted in final form 22 April 2003 This study used imaging and electrophysiological techniques in salamander retinal slices to correlate Ca 2 + and Cl – levels in rods and thus test the hypothesis of a feedback interaction between Ca 2 + - and Ca 2 + -activated Cl – channels whereby Cl – efflux through Cl – channels can inhibit Ca 2 + channels. Increasing [K + ] o levels produced a concentration-dependent depolarization of rods accompanied by increases in [Ca 2 + ] i measured with Fura-2. The voltage dependence of increases in [Ca 2 + ] 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 µM), or replacement of Ca 2 + with Ba 2 + , suggesting that depolarization-evoked Cl – changes result partly from stimulation of Ca 2 + -activated Cl – channels. Replacing 12 mM [Cl – ] o with CH 3 SO 4 – 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 CH 3 SO 4 – . Thus modest depolarization can evoke increases in [Ca 2 + ] 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 Ca 2 + - and Ca 2 + -activated Cl – channels may contribute to the regulation of presynaptic Ca 2 + currents involved in synaptic transmission from rod photoreceptors. Address for reprint requests: W. B. Thoreson, Department of Ophthalmology, University of Nebraska Medical Center, 985540 Nebraska Medical Center, Omaha, NE 68198-5540 (E-mail: wbthores{at}unmc.edu ).