We have investigated the role of Ca2+in the excitation of Limulus photoreceptors by intracellular injection of the Ca2+buffer, 5,5′-dibromo-BAPTA. Buffer with free Ca2+of 0.5 or 5 μM slowed the rising edge of the light response over 100-fold and greatly reduced both the transient and plateau phases of the light response, as expected if Ca2+elevation is necessary for all phases of excitation. Injection of buffers with free Ca2+of 5 or 45 μM, levels normally reached during light, evoked sustained inward current as expected if Ca2+is sufficient for excitation. The transduction cascade appears due to a single pathway that sequentially involves 1,4,5-trisphosphate inositol, Ca2+, and cyclic GMP. © 1993.
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