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This Article Full Text Full Text (PDF) All Versions of this Article: 553/1/125    most recent jphysiol.2003.053280v1 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 HighWire Citing Articles via Google Scholar Google Scholar Articles by Chen, C. Articles by Koutalos, Y. Search for Related Content PubMed PubMed Citation Articles by Chen, C. Articles by Koutalos, Y.

Free magnesium concentration in salamander photoreceptor outer segments

Chunhe Chen *, Kei Nakatani † and Yiannis Koutalos *

Magnesium ions (Mg²⁺) play an important role in biochemical functions. In vertebrate photoreceptor outer segments, numerous reactions utilize MgGTP and MgATP, and Mg²⁺ also regulates several of the phototransduction enzymes. Although Mg²⁺ can pass through light-sensitive channels under certain conditions, no clear extrusion mechanism has been identified and removing extracellular Mg²⁺ has no significant effect on the light sensitivity or the kinetics of the photoresponse. We have used the fluorescent Mg²⁺ dye Furaptra to directly measure and monitor the free Mg²⁺ concentration in photoreceptor outer segments and examine whether the free Mg²⁺ concentration changes under physiological conditions. Resting free Mg²⁺ concentrations in bleached salamander rod and cone photoreceptor cell outer segments were 0.86 ± 0.06 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 significant basal Mg²⁺ flux. Opening the cGMP-gated channels led to a significant increase in the Mg²⁺ concentration in the absence of Na⁺ and Ca²⁺, but not in their presence, indicating that depolarization can cause a significant Mg²⁺ influx only in the absence of other permeant ions, but not under physiological conditions. Finally, light stimulation did not change the Mg²⁺ concentration in the outer segments of dark-adapted photoreceptors. The results suggest that there are no influx and efflux pathways that can significantly affect the Mg²⁺ concentration in the outer segment under physiological conditions. Therefore, it is unlikely that Mg²⁺ plays a significant role in the dynamic modulation of phototransduction.

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