Calcium content of frog rod outer segments and discs |
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Authors: | Ete Z Szuts Richard A Cone |
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Institution: | Thomas C. Jenkins Department of Biophysics, The Johns Hopkins University, Baltimore, Md. 21218, U.S.A. |
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Abstract: | To test the “Ca2+ hypothesis of visual excitation”, we measured the total Ca2+ content of freshly isolated bullforg rod outer segments, and have compared the total Ca2+ contents of fully dark-adapted discs with discs exposed to small amounts of light. Discs were prepared by hypotonically lysing outer segments under conditions expected to remove Ca2+ from the cytoplasm but not from the discs. Ca2+ was assayed by atomic absorption spectrophotometry. We find that both discs and outer segments contain a total of about 0.1–0.2 Ca2+ per rhodopsin molecule. Thus, each frog disc retains about . If most of this Ca2+ were free in the aqueous space inside the intact discs, the Ca2+ activity would be a few mM. Since the light-regulated Na+ channels have been reported to be highly sensitive to cytoplasmic Ca2+, this store of Ca2+ in the discs is far more than required by the Ca2+ hypothesis. However, despite several variations in experimental conditions, we did not observe any light-activated release of Ca2+ from discs in response to stimuli that photoactivated a small fraction of the rhodopsin, as required by the Ca2+ hypothesis. In the 26 experiments reported here we could have detected a release as small as 20–30% of the Ca2+ content of the disc. |
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Keywords: | To whom correspondence should be directed |
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