A ternary complex model explains the agonist-specific binding properties of the adenylate cyclase-coupled beta-adrenergic receptor.

A mutation-induced activated state of the beta 2-adrenergic receptor. Extending the ternary complex model.

Receptor-effector coupling by G proteins

Specific uncoupling by islet-activating protein, pertussis toxin, of negative signal transduction via alpha-adrenergic, cholinergic, and opiate receptors in neuroblastoma x glioma hybrid cells.

Purification and properties of the inhibitory guanine nucleotide-binding regulatory component of adenylate cyclase.

The alpha adrenergic receptors of rabbit platelets can be identified by using [3H]-dihydroergocryptine. The binding of [3H]dihydroergocryptine to platelet lysates reaches equilibrium in 30 min and is slowly reversible. The maximal number of binding sites is 280 ± 25 fmoles/mg, and the dissociation constant was determined to be 2.26 ± 0.48 nM. The specificity of the binding sites for receptor agonists and antagonists is consistent with that of an alpha adrenergic response in inhibiting prostaglandin E1-stimulated adenylate cyclase in the same lysate preparation and is thus in agreement with the classical definition of alpha adrenergic receptors in mediating physiological responses. Divalent cations such as magnesium and manganese and monovalent cations such as sodium markedly influence the binding affinity of agonists but have little or no effect on the binding of antagonists. Calcium produces no change in binding. Neither magnesium nor sodium alters the number of receptor sites. Magnesium (1.25 mM) increases the binding affinity of (-)-epinephrine by a factor of 4. In contrast, sodium chloride (100 mM) strikingly decreases the binding affinity of agonists (more than 10-fold). The effect of sodium chloride on agonists is apparent with concentrations as low as 10 mM, and shows a dose-dependent response. LiCl and KCl are only 44% and 19% as effective, respectively, as sodium chloride. The magnitude of the shift in binding affinity of alpha adrenergic agents produced by sodium chloride is directly related to the intrinsic activity of those agents for inhibition of prostaglandin E1-stimulated adenylate cyclase. The results suggest that physiological concentrations of magnesium chloride and sodium chloride may selectively regulate the binding affinity of agonists.

Agonist-specific effects of monovalent and divalent cations on adenylate cyclase-coupled alpha adrenergic receptors in rabbit platelets.

The effect of GTP on the binding affinity of alpha-adrenergic receptors for alpha-adrenergic agents was studied in human platelet lysates using direct ligand binding methods with [3H]dihydroergocryptine. GTP at a concentration of 0.1 mM markedly decreased the binding affinity of the agonist (-)epinephrine for the receptors (more than 10-fold) but had no effect on the binding of antagonists. The half maximal effect of GTP on epinephrine binding occurred at a concentration of 4 µM. Gpp(NH)p was as effective as GTP at the same concentration, whereas GDP was only 80% as effective. Other nucleotides such as ATP and ITP were less effective. The extent of the GTP-induced reduction in the affinity of alpha-adrenergic agents for the receptors was directly related to the intrinsic activity of these agents for inhibition of PGE1-stimulated adenylate cyclse. The effect of GTP appears to depend on the concurrent presence of Mg++. In the absence of Mg++, GTP caused only a slight decrease in the agonist binding affinity. When Mg++ was present without GTP, the binding affinity of the agonist (-)epinephrine was increased by 5-fold. GTP in the presence of Mg++ induces a state of diminished affinity of the receptor for the agonist which is lower than that induced by the nucleotide in the absence of MgCl2. The relationship between GTP and MgCl2 in the regulation of platelet alpha-adrenergic receptors which are inhibitory to adenylate cyclase activity appears to be analogous to their role in regulating beta-adrenergic receptors which are stimulatory for the enzyme in other tissues.

Agonist-specific effects of guanine nucleotides on alpha-adrenergic receptors in human platelets.

Subclassification of alpha-adrenergic receptors by direct binding studies.

VARIOUS pharmacologically active agonists capable of altering cell function have been shown to interact primarily with specific receptors on the outer cell membrane1. Evidence has accumulated from several systems to suggest that there may be a dynamic and inverse relationship between the number or density of receptors on the membrane surface and the degree of receptor occupancy by agonist2,3. We present here experiments which demonstrate in rat hearts that chronic withdrawal of adrenergic stimulation produced by guanethidine treatment in vivo results in increased cardiac β-adrenergic receptor density. Further, in isolated, perfused hearts obtained from guanethidine-treated animals, there is an increased cyclic AMP (cAMP) accumulation in response to the β-adrenergic agonist isoprenaline, suggesting that the increased numbers of receptors may be related to the increased biochemical response.

Bie Shung Tsai

Papers

Agonist-specific effects of monovalent and divalent cations on adenylate cyclase-coupled alpha adrenergic receptors in rabbit platelets.

Agonist-specific effects of guanine nucleotides on alpha-adrenergic receptors in human platelets.

Subclassification of alpha-adrenergic receptors by direct binding studies.

Chronic guanethidine treatment increases cardiac beta-adrenergic receptors.

Preferential uncoupling by manganese of alpha adrenergic receptor mediated inhibition of adenylate cyclase in human platelets.