Blockade of Visual Excitation and Adaptation in Limulus Photoreceptor by GDP-β-S

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Abstract
Light causes both depolarization and adaptation to light in Limulus ventral photoreceptors. Both visual excitation and adaptation were blocked by guanosine 5′- O -(2-thiodiphosphate) (GDP-β-S), a metabolically stable analog of guanosine 5′-diphosphate (GDP). However, GDP-β-S did not block the excitation caused by injection of inositol 1,4,5-trisphosphate into the cell. These results suggest a molecular cascade of visual excitation and adaptation: Light isomerizes the visual pigment rhodopsin, which in turn activates a guanyl nucleotide-binding protein. The binding protein then stimulates production of inositol 1,4,5-trisphosphate, which causes release of calcium from the endoplasmic reticulum.