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Free Magnesium Concentration in Salamander Photoreceptor Outer Segments
Author(s) -
Chen Chunhe,
Nakatani Kei,
Koutalos Yiannis
Publication year - 2003
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.2003.053280
Subject(s) - biophysics , magnesium , extracellular , visual phototransduction , chemistry , depolarization , ion transporter , kinetics , ion , calcium , homeostasis , biochemistry , biology , membrane , microbiology and biotechnology , retinal , physics , organic chemistry , quantum mechanics
Magnesium ions (Mg 2+ ) play an important role in biochemical functions. In vertebrate photoreceptor outer segments, numerous reactions utilize MgGTP and MgATP, and Mg 2+ also regulates several of the phototransduction enzymes. Although Mg 2+ can pass through light‐sensitive channels under certain conditions, no clear extrusion mechanism has been identified and removing extracellular Mg 2+ has no significant effect on the light sensitivity or the kinetics of the photoresponse. We have used the fluorescent Mg 2+ dye Furaptra to directly measure and monitor the free Mg 2+ concentration in photoreceptor outer segments and examine whether the free Mg 2+ concentration changes under physiological conditions. Resting free Mg 2+ concentrations in bleached salamander rod and cone photoreceptor cell outer segments were 0.86 ± 0.06 and 0.81 ± 0.09 m m , respectively. The outer segment free Mg 2+ concentration was not significantly affected by changes in extracellular pH, Ca 2+ and Na + , excluding a significant role for the respective exchangers in the regulation of Mg 2+ homeostasis. The resting free Mg 2+ concentration was also not significantly affected by exposure to 0 Mg 2+ , suggesting the lack of significant basal Mg 2+ flux. Opening the cGMP‐gated channels led to a significant increase in the Mg 2+ concentration in the absence of Na + and Ca 2+ , but not in their presence, indicating that depolarization can cause a significant Mg 2+ influx only in the absence of other permeant ions, but not under physiological conditions. Finally, light stimulation did not change the Mg 2+ 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 Mg 2+ concentration in the outer segment under physiological conditions. Therefore, it is unlikely that Mg 2+ plays a significant role in the dynamic modulation of phototransduction.