Q. After prolonged exposure to isoproterenol, that binds to b-adrenergic receptors and activates adenylate cyclase, cells become refractory and cease responding. To investigate this phenomenon, you treat cells for a variety of times with isoproterenol, wash it out, and next assay for b-adrenergic receptors by measuring binding of dihydroalprenolol, that is hydrophobic, and CGP-12177, which is hydrophilic. You find that CGP-12177 binding decreases in parallel to adenylate cyclase activity while dihydroalprenolol binding remains high. When you lyse isoproterenol-treated (desensitized) cells and fractionate membrane-enclosed vesicles by centrifugation through sucrose-density gradients, you find out 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 extra population of vesicles as well.

a. Give an description for the differences in binding between CGP-12177 and dihydroalprenolol;

b. What do you think may be the basis for isoproterenol-induced desensitization in these cells?