Purchase this article with an account.
Y Koutalos, C Chen, K Nakatani; Free Magnesium Concentration In Vertebrate Photoreceptor Outer Segments . Invest. Ophthalmol. Vis. Sci. 2002;43(13):1417.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
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.
This PDF is available to Subscribers Only