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Iris pigmentation and atropine mydriasis.

M Salazar, K Shimada, P N Patil

    The Journal of Pharmacology and Experimental Therapeutics
    |April 1, 1976
    PubMed
    Summary

    Pigment in the iris significantly affects how atropine works, causing it to accumulate in pigmented tissues and prolonging its effects. This explains differences in drug efficacy and duration between pigmented and non-pigmented eyes.

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    Area of Science:

    • Pharmacology
    • Ophthalmology
    • Cell Biology

    Background:

    • Atropine is known to cause mydriasis (pupil dilation), but its effect is influenced by ocular pigmentation.
    • The interaction between atropine and ocular tissues, particularly pigmented iris, requires further elucidation.
    • Understanding drug accumulation in pigmented tissues is crucial for predicting pharmacological responses.

    Purpose of the Study:

    • To investigate the pigment-dependent mydriatic effect of atropine in the eye.
    • To quantify the in vitro accumulation and retention of atropine in pigmented versus non-pigmented ocular tissues.
    • To correlate in vitro findings with in vivo observations of atropine's duration of action.

    Main Methods:

    • In vitro studies using 3H-atropine to measure drug accumulation in pigmented and non-pigmented rabbit iris and stomach fundus strips.
    • Drug washout experiments to determine the retention half-life (T 1/2) in different tissues.
    • In vivo experiments measuring the duration of atropine-induced mydriasis in rabbits with varying pigmentation and atropinesterase levels.

    Main Results:

    • Pigmented iris accumulated significantly higher amounts of 3H-atropine compared to non-pigmented iris and stomach fundus strips.
    • Atropine was retained much longer in pigmented iris tissues than in non-pigmented tissues after repeated washings.
    • In vivo, atropine's duration of mydriasis was markedly longer in non-albino (heavily pigmented) rabbits compared to albino rabbits, especially those negative for atropinesterase.

    Conclusions:

    • Ocular pigment acts as a reservoir for atropine, leading to its accumulation in pigmented iris tissues.
    • This accumulation reduces the free concentration of atropine available to muscarinic receptors, potentially explaining lower efficacy at high concentrations in pigmented irises.
    • The slow release of accumulated atropine from pigment contributes to the prolonged mydriatic effect observed in heavily pigmented eyes.

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