2. G protein-coupled membrane receptor, or metabotropic receptor constitute a large family with more than 500 members o has seven transmembrane ahelices o Prolonged exposure of a cell to a particular agonist often leads to the cells becoming less responsive to that agonist via down-regulation and desensitization of the receptors2. G protein-coupled membrane receptor, or metabotropic receptor • Prolonged exposure of a cell to a particular agonist often leads to the cell’s becoming less responsive to that agonist via down-regulation and desensitization of the receptors. • has seven transmembrane αhelices • constitute a large family with more than 500 members