Formation of the high‐affinity agonist state of the α₁‐adrenergic receptor at cold temperatures does not require a G‐protein

Abstract

Two methods were employed to uncouple hepatic cq-adrenergic receptors from their associated G-protein (termed Gv) in order to determine whether locking of the ~l-receptor in a high-affinity agonist state at cold temperatures (2°C) represents formation of a ternary complex. Uncoupling is defined as the inability to observe the GppNHp-sensitive, highaffinity agonist state of the receptor in pH]prazosin competition binding studies performed at 25°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 ~-receptor coupling domain from the binding domain. Following either treatment, receptors were converted to the high-affinity agonist state at 2°C. Thus, while formation of the high-affinity state of the receptor at higher temperatures may require Gp, formation of this state at 2°C does not require Gp or even the entire ~-adrenergic receptor.

Reduced high‐affinity α1‐adrenoceptors in liver of senescent rats: implications of assessment at various temperatures

“…There was no difference in the ratio of high-and low-affinity receptors measured either at 40C or 250C, with all receptors converting to the low-affinity state at 370C. Our results differ somewhat from another report published by Lynch et al (1988) Lynch et al (1985), a mixed population of high-and low-affinity receptors in binding studies conducted at 250C. We measured the same mixed population at 40C.…”

“…In contrast, changes with age in the xl-receptor may or may not result in altered al control of glycogenolysis. Lynch et al (1988) reported that noradrenaline-stimulated phosphorylase activity in isolated hepatocytes is unchanged with age. This is consistent with our finding that adrenaline-stimulated PI hydrolysis in liver slices prelabelled with [3H]-myoinositol is also unchanged with age (unpublished observations).…”

Guanine nucleotide-binding protein-coupled and -uncoupled states of opioid receptors and their relevance to the determination of subtypes.

“…pINHlppG promotes the dissociation of receptor-G-protein-agonist complexes (18). Similarly, exposure of membrane preparations to a high pH can inactivate G proteins, leaving P-, a,-, and aradrenergic receptors intact but lacking high-affinity agonist binding (19,20) Fig. 2A and Table 1).…”

Membrane Microviscosity Modulates μ‐Opioid Receptor Conformational Transitions and Agonist Efficacy

“…2B), indicating a conversion of the receptor to a higher‐affinity conformation. Similarly, low temperature‐induced conversion of the α 1 ‐adrenergic receptor to a single agonist high‐affinity state was not dependent on the presence of a G protein (Lynch et al, 1988). Rigidification of the membrane bilayer or low temperature may create a condition in the lipid surrounding the receptor where increased lateral pressure (Yeagle, 1991) directly alters the equilibrium between low‐ and high‐affinity receptor conformations, leading to the stabilization of a G protein‐independent agonist high‐affinity state.…”