Characterization of a specific L-[3H]glutamic acid binding site on cilia isolated fromParamecium tetraurelia

Summary Cilia isolated fromParamecium tetraurelia possess a specific, high affinity L-[³H]glutamic acid binding site, defined by an ED₅₀ of 3.0×10^–8M. The structural specificity of this site was probed by testing the competition between L-glutamate and various analogues for binding to cilia. The binding site is stereo-specific for L-glutamic acid, and requires the presence of all three ionizable groups on the glutamate molecule for optimal ligand: receptor interaction.Specific binding of L-[³H]glutamic acid to cilia is rapid in onset but transient, reaching peak values within 6 min, and then declining thereafter. This transience may represent a form of sensory adaptation during prolonged exposure to the ligand.