Transient high-affinity binding of agonists to α 1-adrenergic receptors of intact liver cells

• Published in: FEBS letters Vol. 187; no. 2; pp. 205 - 210
• Main Authors: Schwarz, Kurt R., Lanier, Stephen M., Carter, Edward A., Graham, Robert M., Homey, Charles J.
• Format: Journal Article
• Language: English
• Published: England Elsevier B.V 05-08-1985
• Subjects: Adrenergic alpha-Agonists - metabolism; Animals; Binding, Competitive; Conformational change; Epinephrine - metabolism; Female; Glycogenolysis; In Vitro Techniques; Isolated hepatocyte; Kinetics; Liver - metabolism; Nonequilibrium binding; Norepinephrine - metabolism; Phosphorylases - metabolism; Prazosin - metabolism; Protein Conformation; Rats; Rats, Inbred Strains; Receptor affinity state; Receptors, Adrenergic, alpha - metabolism; α 1-Adrenergic receptor; Glycogenolysis; Nonequilibrium binding; Receptor affinity state; Conformational change; α 1-Adrenergic receptor; Isolated hepatocyte
• ISSN: 0014-5793; 1873-3468
• DOI: 10.1016/0014-5793(85)81243-6

At α
1-adrenergic receptors in isolated rat liver parenchymal cells, (−)-epinephrine is potent in eliciting a maximal increase in glycogenolysis (
K
act = 24 nM). This contrasts with a 100-fold lower affinity for the agonist at α
1-adrenergic receptors of intact hepatocytes determined from equilibrium competition assays with the α
1-adrenergic antagonist [
3H]prazosin. We demonstrate here that agonists bind to α
1-adrenergic receptors of intact liver cells initially with a markedly higher affinity than under equilibrium conditions. When incubations are performed for 15 s at 37°C, the affinity is more than 100-fold higher than that obtained in equilibrium (45 min) assays (IC
50 = 28 ± 3 vs 5300 + 400 nM for (−)-epinephrine and 32 ± 3 vs 6100 ± 500 nM for (−)-norepinephrine). When incubations are performed at 4°C (150 min), high-affinity binding similar to that obtained In short-term incubations can also be demonstrated. In contrast, antagonist compete with similar affinities in 15 s and 45 min assays, and their dissociation constants are not affected by changes in the incubation temperature. These results indicate that agonists bind to native α-adrenergic receptors transiently with high affinity. The conversion of receptors to a state of predominantly low affinity for agonists, which occurs rapidly and irreversibly with increasing incubation at 37°C, is inhibited at low incubation temperatures. It is suggested that the high-affinity configuration of the α
1-adrenergic receptor for agonists observed in nonequilibrium experiments or at reduced incubation temperatures represents the physiologically relevant state of the α
1-adrenergic receptor.