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This Article Full Text Full Text (PDF) All Versions of this Article: 579/1/15    most recent jphysiol.2006.120162v1 Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Leung, Y. T. Articles by Matthews, H. R. Search for Related Content PubMed PubMed Citation Articles by Leung, Y. T. Articles by Matthews, H. R. Related Collections Cellular

¹ Physiological Laboratory, Department of Physiology, Development and Neuroscience, University of Cambridge, Cambridge CB2 3EG UK
² Departments of Physiological Science and Ophthalmology, University of California, Los Angeles, CA 90095-1606, USA

In rods and visible cone photoreceptors, multiple measurements cannot be made of intracellular Ca²⁺ concentration from the same cell using fluorescent dyes, because a single exposure of the measuring light bleaches too large a fraction of the rod or cone photopigment. We have therefore identified and characterized UV-sensitive cones of the zebrafish, whose wavelength of maximum sensitivity is at 360 nm which is far enough from the wavelength of our measuring light (514.5 nm) so that it has been possible to make multiple determinations of photocurrent and Ca²⁺ concentration from the same cells. We show that for a limited number of measurements, for which the bleaching of the cone photopigment is too small to affect flash kinetics, the outer segment Ca²⁺ concentration closely follows the wave form of the flash response convolved with the dominant time constant for Ca²⁺ removal by Na⁺–Ca²⁺–K⁺ exchange. For a larger number of measurements, significant acceleration of the response kinetics by pigment bleaching inevitably occurs, but the Ca²⁺ concentration nevertheless rises and falls in approximate agreement with the flash wave form. During exposure to steady background light, the Ca²⁺ concentration falls in proportion to the steady-state current for dim backgrounds at all times and for bright backgrounds at steady state. At early times following the onset of bright backgrounds, however, the Ca²⁺ concentration is markedly higher than expected from the current of the cone. We show this to be the result of light-dependent Ca²⁺ release by bright background light, which can be abolished by pre-exposure of the cone to the membrane-permeant acetoxymethyl ester of the Ca²⁺ chelator BAPTA. Our results therefore demonstrate that the cone outer segment Ca²⁺ concentration is predominantly a function of the rate of influx and efflux of Ca²⁺ across the plasma membrane, but that a release of Ca²⁺ in bright light most probably from buffer sites within the cell can transiently elevate the Ca²⁺ concentration above the level expected from the open probability of the light-dependent channels.

(Received 4 September 2006; accepted after revision 21 November 2006; first published online 23 November 2006)
Corresponding author H. R. Matthews: Physiological Laboratory, Department of Physiology, Development and Neuroscience, Downing Street, Cambridge CB2 3EG, UK. Email: hrm1{at}cam.ac.uk