Formation of the High-affinity Agonist State of the Alpha 1-adrenergic Receptor at Cold Temperatures Does Not Require a G-protein

Two methods were employed to uncouple hepatic alpha 1-adrenergic receptors from their associated G-protein (termed Gp) in order to determine whether locking of the alpha 1-receptor in a high-affinity agonist state at cold temperatures (2 degrees C) represents formation of a ternary complex. Uncoupling is defined as the inability to observe the GppNHp-sensitive, high-affinity agonist state of the receptor in [3H]prazosin competition binding studies performed at 25 degrees C. The first method for achieving uncoupling involved brief alkalinization and resulted in greater than 95% loss of several G-proteins. The second method involved proteolytic cleavage of either part or all of the alpha 1-receptor coupling domain from the binding domain. Following either treatment, receptors were converted to the high-affinity agonist state at 2 degrees C. Thus, while formation of the high-affinity state of the receptor at higher temperatures may require Gp, formation of this state at 2 degrees C does not require Gp or even the entire alpha 1-adrenergic receptor.