To test if the light-evoked hyperpolarization of the rods or retinal pigment epithelium (RPE) is important for the light-evoked component of rod disc shedding, eyecups of the anuran,Xenopus, were incubated in media that have been reported to have different effects on the membrane potential of these cells.
Hyperpolarization was induced by transferral to medium with Na+ entirely replaced by choline+, or mostly replaced by Li+. Shedding in darkness was increased 4-fold in both cases (Figs. 1, 2 and 3).
To prevent light-evoked hyperpolarization, eyecups were transferred to medium containing 0.5 mmol/l ouabain. However, light-evoked shedding was not inhibited; instead, it was activated further (Fig. 4). Moreover, ouabain increased shedding in darkness by about 9-fold (Fig. 5).
Ouabain likewise activated shedding without light in eyecups of the frog,Rana (Fig. 6). InRana, shedding is entirely light-evoked, so that, unlike the case represented byXenopus eyecups, there was no question that inhibition of the light-evoked component could have been masked by over-whelming stimulation of an endogenous component.
Strophanthidin, which, in contrast to ouabain, binds reversibly to Na+-K+-ATPase, also activated shedding inXenopus eyecups kept in darkness, even when it was washed out 20 min after light onset (Fig. 7).
These results suggest that a particular change in the membrane potential of the rod or RPE cells is not directly responsible for the occurrence of light-evoked rod disc shedding.
In addition, the results identify pharmacological treatments that activate shedding in darkness. These treatments should provide a useful tool for future studies of rod disc shedding. It is noted that common effects of these treatments possibly include: (1) inhibition of Na+, K+-ATPase activity, with a resulting increase in ATP; (2) an increase in intracellular Ca2+; (3) alteration of melatonin levels.
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- ROS :
rod outer segment
- RPE :
retinal pigment epithelium
- Na +,K + -ATPase :
Sodium, potassium-activated adenosine triphosphatase
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Williams, D.S., Wilson, C., Linberg, K. et al. Effects of low sodium, ouabain, and strophanthidin on the shedding of rod outer segment discs. J. Comp. Physiol. 155, 763–770 (1984). https://doi.org/10.1007/BF00611593
- Membrane Potential
- ATPase Activity
- Retinal Pigment Epithelium