Evidence for the appearance of an uncoupled form of the β-adrenergic receptor distinct from the internalized receptor

Agonist treatment of C6‐glioma cells induces two altered states in β‐adrenergic receptors, a low affinity for the hydrophilic antagonist CGP‐12177 and a low affinity for agonists like isoproterenol. We present evidence that, in cells not treated to inhibit receptor internalization, the two properties occur with a different time course, the low affinity for isoproterenol preceding that for CGP‐12177. In that the low affinity for CGP‐12177 is due to the internalization of the receptor, the results indicate that uncoupling of the receptor, indicated by the low affinity for isoproterenol, occurs while the receptor is, still located on the cell surface. Removal of the agonist leads to reappearance of the receptor to the plasma membrane followed by loss of the uncoupled state.