After prolonged exposure to isoproterenol, which binds to b-adrenergic receptors and activates adenylate cyclase, cells become refractory and cease responding. To investigate this phenomenon, you treat cells for various times with isoproterenol, wash it out, and then assay for b-adrenergic receptors by measuring the binding of dihydroalprenolol, which is hydrophobic, and CGP-12177, which is hydrophilic. You find that CGP-12177 binding decreases in parallel to adenylate cyclase activity whereas dihydroalprenolol binding remains high. When you lyse isoproterenol-treated (desensitized) cells and fractionate membrane-enclosed vesicles by centrifugation through sucrose-density gradients, you find that CGP-12177 binds only to vesicles derived from the plasma membrane (as indicated by the presence of the marker enzyme, 5'nucleotidase) whereas dihydroalprenolol binds to an additional population of vesicles as well.
a. Give an explanation for the differences in binding between CGP-12177 and dihydroalprenolol;
b. What do you think might be the basis for isoproterenol-induced desensitization in these cells?