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Transient high‐affinity binding of agonists to α 1 ‐adrenergic receptors of intact liver cells
Author(s) -
Schwarz Kurt R.,
Lanier Stephen M.,
Carter Edward A.,
Graham Robert M.,
Homey Charles J.
Publication year - 1985
Publication title -
febs letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.593
H-Index - 257
eISSN - 1873-3468
pISSN - 0014-5793
DOI - 10.1016/0014-5793(85)81243-6
Subject(s) - receptor , chemistry , adrenergic receptor , alpha 1b adrenergic receptor , alpha 1a adrenergic receptor , biophysics , adrenergic , microbiology and biotechnology , endocrinology , medicine , biochemistry , biology , beta 3 adrenergic receptor
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 [ 3 H]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.