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 (ICa). [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 (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 microM), or replacement of Ca2+ with Ba2+, suggesting that depolarization-evoked Cl- changes result partly from stimulation of Ca2+-activated Cl- channels. Replacing >/=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 >/=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.
Thoreson
Wallace B
WB
Department of Ophthalmology, University of Nebraska Medical Center, Omaha, Nebraska 68198, USA. wbthores@unmc.edu
Bryson
Eric J
EJ
Rabl
Katalin
K
D000557:Ambystoma; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism; D018118:Chloride Channels / Q000378:metabolism; D002712:Chlorides / Q000378:metabolism* / Q000494:pharmacology; D007424:Intracellular Fluid / Q000187:drug effects / Q000378:metabolism; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D017948:Retinal Rod Photoreceptor Cells / Q000378:metabolism*
D015220:Calcium Channels; D018118:Chloride Channels; D002712:Chlorides; D002118:Calcium