December 2002
Volume 43, Issue 13
ARVO Annual Meeting Abstract  |   December 2002
Free Magnesium Concentration In Vertebrate Photoreceptor Outer Segments
Author Affiliations & Notes
  • Y Koutalos
    Physiology and Biophysics University of Colorado School of Medicine Denver CO
  • C Chen
    Physiology and Biophysics University of Colorado School of Medicine Denver CO
  • K Nakatani
    Institute of Biological Sciences University of Tsukuba Tsukuba Japan
  • Footnotes
    Commercial Relationships   Y. Koutalos, None; C. Chen, None; K. Nakatani, None. Grant Identification: Support: NIH Grant EY11351
Investigative Ophthalmology & Visual Science December 2002, Vol.43, 1417. doi:
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      Y Koutalos, C Chen, K Nakatani; Free Magnesium Concentration In Vertebrate Photoreceptor Outer Segments . Invest. Ophthalmol. Vis. Sci. 2002;43(13):1417.

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      © ARVO (1962-2015); The Authors (2016-present)

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Abstract: : Purpose: To measure the free Mg2+ concentration in the outer segments of vertebrate rod and cone photoreceptors and examine the different mechanisms involved in Mg2+ homeostasis. Methods: Living, intact rod photoreceptors were isolated from larval tiger salamander (Ambystoma tigrinum) retinas. Isolated cells were loaded with the Mg2+-sensitive probe Furaptra by incubating with the acetoxymethyl ester form of the dye. Cells were placed in a chamber on a microscope stage and changes in fluorescence were measured with a digital CCD camera. The ratio F340/F380 of dye fluorescence excited at 340 and 380 nm (emission: 540 nm) is a measure of the free Mg2+ concentration. Fluorescence ratios were converted to free Mg2+ concentrations after calibrating intracellular dye with different Mg2+ concentrations. Results: Resting free Mg2+ concentrations in bleached rod and cone cell outer segments were 0.86±0.06 mM and 0.81±0.09 mM, respectively. The outer segment free Mg2+ concentration was not significantly affected by changes in extracellular pH, Ca2+ and Na+, excluding a significant role for the respective exchangers in the regulation of Mg2+ homeostasis. The resting free Mg2+ concentration was also not significantly affected by exposure to 0 Mg2+, suggesting the lack of a significant Mg2+ influx. In cone cells, inhibition of the phosphodiesterase with 1 mM IBMX led to a significant increase in the Mg2+ concentration in the absence of Na+ and Ca2+, but not in their presence, suggesting that significant Mg2+ influx through the cGMP-gated channels may occur only in the absence of other permeant ions, but not under physiological conditions. Conclusion: The free Mg2+ concentration is the same in bleached rod and cone outer segments. Mg2+ is known to regulate enzymes involved in phototransduction, like the guanylate cyclase, but it appears that there are no influx and efflux pathways that can significantly affect the Mg2+ outer segment concentration. Therefore, it is unlikely that Mg2+ plays a significant role in the dynamic modulation of phototransduction. Support: NIH grant EY11351.

Keywords: 517 photoreceptors • 431 imaging/image analysis: non-clinical 

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