Free Magnesium Concentration in Salamander Photoreceptor Outer Segments

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.