Low affinity of beta-adrenergic receptors for agonists on intact cells is not due to receptor sequestration

The low affinity of beta-adrenergic receptors for agonists described on intact cells at 37 degrees C has usually been interpreted in terms of reduced accessibility of agonists (which are usually hydrophilic) for sequestered receptors. We challenged this hypothesis by eliminating the plasma membrane barrier with low doses of the detergent digitonin. In human mononuclear leukocytes (MNL) permeabilized with digitonin, sequestered receptors became accessible to hydrophilic ligands such as agonists, but the affinity was still low. Then we investigated the relationship between low affinity agonist binding and sequestration using concanavalin A, which blocks sequestration. Even when sequestration was blocked, the affinity of the beta-adrenergic receptors for agonists was low. We conclude that: (a) low affinity agonist binding is independent of receptor sequestration; (b) the receptors which undergo conformational change are those that are sequestered; (c) the low affinity appears before sequestration occurs. This receptor conformational change could be the first step in agonist-induced desensitization.