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

Abstract

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.