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Journal of Neuroscience, Vol 11, 4015-4023, Copyright © 1991 by Society for Neuroscience
Modulation of calcium-activated chloride current via pH-induced changes of calcium channel properties in cone photoreceptors
S Barnes and Q Bui
Department of Medical Physiology, Faculty of Medicine, University of Calgary, Alberta, Canada.
The activity of calcium-activated chloride channels is controlled through
the complex interaction of cellular mechanisms affecting calcium entry,
buffering, and extrusion, and an unknown stoichiometric relation between
intracellular Ca concentration and Cl channel activation. Here, we show
that calcium-activated chloride current [ICl(Ca)] in cone photoreceptors is
also highly sensitive to external pH, being strongly reduced by
acidification and enhanced by alkylinization of the bathing medium. We
propose that this modulation is accounted for by the pH sensitivity of Ca
channel activation and permeation, already well characterized in other
cells, which we now extend to cone photoreceptor Ca channels. Acidification
of the external medium from a control pH of 7.4 shifts the Ca channel
activation range positively by about 10 mV at pH 6.8, reducing the
magnitude of calcium current with a consequent reduction of chloride
current. Alkylinization shifts the Ca channel activation range negatively
by about 8 mV at pH 8 and produces larger calcium currents during step
depolarizations that in turn elicit larger chloride tail currents.
Modulation of ICl(Ca) by pH suggests other consequences of the pH-induced
shift in Ca channel gating, for one, modification of Ca-dependent
transmitter release, which could be especially significant in
photoreceptors where the cell's operating voltage range overlaps only the
lower end of the Ca channel activation range.
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