Light-evoked and kainic-acid-induced disc shedding by rod photoreceptors: Differential sensitivity to extracellular calcium

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Abstract
In order to study the light and Ca2+ dependence of disc shedding by rod photoreceptors, we have used eyecups prepared from adult Rana pipiens frogs that had been kept in constant light for 4 days. Disc shedding was initiated by a treatment involving 1 hour of darkness followed by exposure to light or by treatment with kainic acid. Maximal L-evoked disc shedding occurred quickly (within 30–60 minutes) after light onset and could be triggered by brief (15 minutes) exposure to light. L-evoked disc shedding was completely blocked by omission of Ca2+ from culture medium or by treatment with 3mM Co2+ or 12 mM Mg2+ in the presence of Ca2+ (2 mM). The response was also blocked by the organic Ca2+ antagonist nifedipine. Experiments designed to distinguish between Ca2+ dependence of the dark or light-dependent processes necessary for shedding suggest that volage-sensitive channels mediate a Ca2+-dependent process involved in light-triggering. Kainic acid caused a dose-dependent stimulation of disc shedding under lighting conditions (continuous culture in light or darkness) that did not normally result in a significant response in the absence of the drug. Disc shedding induced by kainic acid was similar in time course and magnitude to that induced by light. However, kainic-acid-induced disc shedding was not inhibited by medium Ca2+ reduction or by the presence of Co2+. The latter observation suggests that kainic acid activates disc shedding directly, bypassing the Ca2+-dependent process involved in the L-evoked response. The Ca2+-dependent process may involve release of an effector of disc shedding that is mimicked by kainic acid.