Effects of extracellular Ca++, K+, and Na+ on cone and retinal pigment epithelium retinomotor movements in isolated teleost retinas.

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Abstract
The effects of changes in extracellular ionic composition on cone and retinal pigment epithelium (RPE) retinomotor movements was studied in cultured isolated teleost retinas. In vivo, the myoid portion of teleost cones contracts in the light and elongates in the dark: RPE pigment disperses in the light and aggregates in the dark. In vitro, cones of dark-adapted (DA) retinas cultured in constant darkness contracted spontaneously to their light-adapted (LA) positions if the culture medium contained .gtoreq. 10-3 M Cao2+. DA cones retained their long DA positions in a medium containing .ltoreq. 10-6 M Cao2+. Low [Ca2+]o (10-5-10-7 M) also permitted darkness to induce cone elongation and RPE pigment aggregation. Light produced cone contraction even in the absence of Cao++, but the extent of contraction was reduced if [Ca2+]o was < 10-3 M. Thus, full contraction appeared to require the presence of external Ca2+. High [K+]o (.gtoreq. 27 mM) inhibited both light-induced and light-independent Ca2+-induced cone contraction. Low [Na+]o (3.5 mM) in the presence of .ltoreq. 10-6 M Cao2+ did not mimic light onset by inducing cone contraction in the dark. High [K+]o also promoted dark-adaptive cone and RPE movements in LA retinas cultured in the light. All results obtained in high [K+]o were similar to those observed when DA or LA retinas were exposed to treatments that elevate cytoplasmic cAMP content.

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