Retinomotor movements in the frog retinal pigment epithelium: dependence of pigment migration on Na+ and Ca2+.
The ionic dependence of the screening-pigment migrations in the frog retinal epithelium (RPE) was quantitatively studied with eyecups incubated in media of different compositions. Typical migrations in response to light and darkness, equivalent to those observed in the intact animal, were fully accomplished and maintained for up to 6 hr by the isolated organ bathed in Ringer solution rich with O2. Pigment migration in either direction was completed under the appropriate illumination conditions at any time during the day, indicating that circadian influences, if present in the intact animal, can be overridden in the isolated organ by light or darkness alone. Pigment aggregation toward the dark-adapted position was inhibited by: (a) low external Ca2+, (b) high external Na+, and (c) drugs expected to increase the cytoplasmic levels of either Na+, or Ca2+, like ouabain, caffeine and the ionophore A23187. However, the inhibition caused by low Ca2+ did not occur if Na+ was also reduced in the incubation medium. On the other hand, an increase in the concentration of external Ca2+ or the addition of Co2+ to the normal Ringer facilitated pigment aggregation in the dark. Pigment dispersion to the light-adapted position was unaffected by any of the above conditions. This is the first report of full and stable pigment responses in the RPE of vertebrate eyes incubated under simple physiological conditions. The results seem to conciliate a discrepancy of previous reports on the Ca2+ dependence of RPE movements, and are compatible with current views on ionic mechanisms in analogous systems of intracellular transport.
Mondragón
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Departamento de Biología Celular, Centro de Investigación y de Estudios Avanzados del IPN, Mexico.
Frixione
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E
D000221:Adaptation, Ocular; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D010857:Pigment Epithelium of Eye / Q000502:physiology*; D011894:Rana pipiens; D012168:Retinal Pigments / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism*
D012168:Retinal Pigments; D012964:Sodium; D002118:Calcium