Abstract
Ion-selective double-barreled microelectrodes inserted into a planarian ocellus were used to monitor the ocellus potential and the changes in extracellular concentrations of Ca 2+ (Ca o) and Na + (Na o) caused by a 0.5-sec light flash or sustained (120s) illumination. Ca o and Na o were slightly decreased following a flash. Sustained illumination caused a biphasic change in Ca o (a rapid decrease followed by a slow increase) and a tonic decrease in Na o. When Na + in the planarian saline was replaced by Li + or choline +, the increase in Ca o was prevented: sustained illumination induced only a decrease in Ca o. These results suggest that illumination induces influxes of both Ca 2+ and Na + into planarian photoreceptors, and that the Ca 2+ influx is rapidly followed by a Na-dependent Ca 2+ efflux due to Na-Ca exchange.
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