Illumination of barnacle (Balanus amphitrite) photoreceptors is known to increase the membrane permeability to sodium and Ca2+ ions resulting in a depolarizing receptor potential. In this report, we show that lanthanum (La3+), a known inhibitor of Ca-binding proteins, reversibly eliminates the receptor potential of barnacle photoreceptors when applied to the extracellular space. Similar reversible elimination of the light response was obtained by removing extracellular Ca2+ by application of the calcium chelating agent EGTA. Iontophoretic injection of Ca2+, but not K+ into the cells protected both the transient and the steady-state phases of the receptor potential from elimination by EGTA while only the transient phase was protected in the presence of La3+. The EGTA experiments suggest that internal Ca2+ is necessary for light excitation of barnacle photoreceptors while the La3+ experiments suggest that La3+-sensitive inward current is necessary to maintain excitation during prolonged light.
Calcium is necessary for light excitation in barnacle photoreceptors
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