title: Free magnesium concentration in salamander photoreceptor outer segments.
abstract: 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.
mesh_terms: D000557:Ambystoma / Q000502:physiology*; D000818:Animals; D002118:Calcium / Q000502:physiology; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D006152:Cyclic GMP / Q000502:physiology; D005456:Fluorescent Dyes; D016257:Fura-2 / Q000031:analogs & derivatives*; D066298:In Vitro Techniques; D015640:Ion Channel Gating / Q000187:drug effects; D007814:Larva; D008274:Magnesium / Q000378:metabolism*; D010775:Photic Stimulation; D017949:Retinal Cone Photoreceptor Cells / Q000187:drug effects / Q000378:metabolism*; D017948:Retinal Rod Photoreceptor Cells / Q000187:drug effects / Q000378:metabolism*; D012374:Rod Cell Outer Segment / Q000378:metabolism*; D012964:Sodium / Q000502:physiology
chemical_list: D005456:Fluorescent Dyes; C058776:2-(2-(5-carboxy)oxazole)-5-hydroxy-6-aminobenzofuran-N,N,O-triacetic acid; D012964:Sodium; D006152:Cyclic GMP; D008274:Magnesium; D002118:Calcium; D016257:Fura-2