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G.L. Fain, H.R. Matthews; Light and Calcium in Salamander Rods . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2001.
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Purpose: In darkness, Ca2+ enters through cGMP-gated channels in the rod outer segment and exits via Na+/Ca2+-K+ exchange. Light closes the channels and decreases Ca2+i , but the relationship between light, current, and Ca2+i has remained unclear. We have previously shown that intense light evokes a release of Ca2+ within the outer segment, most clearly seen when a rod is superfused with a 0Ca2+/0Na+ solution designed to oppose surface membrane Ca2+ fluxes. It seemed possible that this release would contribute to changes in Ca2+i. Methods: Rods isolated from salamanders killed humanely by stunning, decapitation and pithing were loaded with the fluorescent indicator dye fluo 5F and illuminated with a laser to measure free Ca2+i. Electrical responses were measured with suction electrode recording, and solutions were changed as previously described by moving the microscope stage with a stepping motor. Results: After exposure to saturating light, the circulating current, Ca2+i and the light-releasable pool of Ca2+ all recover with a nearly identical exponential time course. There is no observable lag in recovery of Ca2+i. Steady background light produces a maintained decrease in current, which is accompanied by a nearly proportional decrease both in Ca2+i and in the light-releasable Ca2+ pool. The amount of Ca2+ in the light-releasable pool appears to be determined principally by Ca2+i except during bright light exposure. This is most clearly seen if the rod is illuminated in 0Ca2+/0Na+ solution, since relatively small bleaches in this solution deplete the releasable pool even in the absence of a light-induced decline in Ca2+i, indicating some special effect of pigment bleaching on Ca2+ buffering and/or sequestration. This effect is spatially localised, since exposure to a narrow slit of intense light in 0Ca2+/0Na+ solution eliminates the releasable pool in that location but not in neighbouring unilluminated regions of the outer segment. Conclusions: These experiments show that the major determinant of Ca2+i in the outer segment following just-saturating illumination or in the presence of background light is the rate of influx through the channels, and that the light-releasable pool makes only a minor contribution. Bleaching produces specific depletion of the light-releasable pool, but the functional consequences of this light-induced calcium release for the rods is unknown.
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