Light induced sodium dependent accumulation of calcium and potassium in the extracellular space of bee retina

Intense illumination of long duration induced a large transient increase in extracellular calcium (Δ[Ca²⁺]_o) and potassium (Δ[K⁺]_o) during and after light in bee retina when measured with ion-selective microelectrodes. Whenever a large Δ[Ca²⁺]_o appeared, it was accompanied by a transient afterdepolarization (TA). Both the increase in [Ca²⁺]_o, [K⁺]_o and the TA were reduced or abolished when sodium was replaced by arginine, choline or lithium (Li⁺) ions. At 0-Na conditions a Na independent decrease in [Ca²⁺]_o was observed during illumination only. A pronounced transient depolarization of the photoreceptor in the dark due to transient anoxia did not result in a significant change in [Ca²⁺]_o. In some retinae the elevated level of [K⁺]_o after light was absent, however a small Na-dependent TA was still observed. The above findings suggest that intense long illumination induces a large Ca²⁺ influx into the photoreceptors which is followed by Na-dependent Ca²⁺ efflux due to Na-Ca exchange. The light-induced afterdepolarization arises mainly from K⁺ accumulation in the extracellular space but partially from the electrogenicity of Na-Ca exchange.