Guanyl triphosphate and diphosphate nucleotides decrease the binding of the agonist ligands (±)-[3H]epinephrine and (-)-[ 3H]norepinephrine to α-noradrenergic receptors in calf frontal cerebral cortex membranes, with IC50 values (concentrations which reduced specific binding by 50%) of 1 to 10 μM and the potency order GTP = guanyl-5'-yl imidodiphosphate > GDP. Corresponding adenyl nucleotides have a similar effect, but are more than 100 times weaker. Guanosine, 5'-GMP, adenosine, and 5'-AMP have no effect on (±)-[3H]epinephrine binding at concentrations up to 0.1 mM. Guanyl and adenyl nucleotides do not decrease the binding of the α-receptor antagonist ligand [3H]WB-4101 or the mixed agonist-antagonist ligand [3H]dihydroergokryptine at concentrations up to 1.0 mM. Ten micromolar GTP has no effect on the affinities of α-receptor agonists and antagonists at [3H]WB-4101 and [3H]dihydroergokryptine binding sites or on the affinities of antagonists at (±)-[3H]epinephrine binding sites, but selectively lowers the potencies of agonist inhibitors of (±)-[3H]epinephrine binding by 4- to 5-fold. In (±)-[3H]epinephrine saturation experiments, 1.0 μM GTP and 10 μM GTP lower the affinity of the ligand 2 and 6 times, respectively, without altering the Bmax. GTP increases the rate of association at 25°C of (±)-[3H]epinephrine and (-)-[3H]norepinephrine binding to α-receptors and also greatly increases the rate of dissociation at 25°C of the 3H-catecholamine ligands, both when added during initial labeling of the receptors and when added at the onset of dissociation. Guanyl-5'-yl imidodiphosphate and GTP are equipotent in accelerating (±)-[3H]epinephrine dissociation. The effect of GTP on (±)-[3H]epinephrine dissociation seemed more striking than its effect on (±)-[3H]epinephrine association and may cause the observed decrease in (±)-[3H]epinephrine affinity at α-receptors. Both NaC1 and GTP lower 3H-catecholamine α-receptor binding, but the effectiveness of one agent is unaltered by the presence of the other. Thus, the effects of monovalent cations and guanyl nucleotides on α-receptor agonist ligand binding appear to be mutually independent. Dissociation of 3H-catecholamines from α-receptors in calf cerebellum is more rapid at 25°C than in frontal cortex and is greatly accelerated by 10 μM GTP. However, GTP and guanyl-5'-yl imidodiphosphate have no effect on (±)-[3H]epinephrine binding at 4°C to β2-noradrenergic receptors in calf cerebellar membranes, and these nucleotides also have no effect on the potency of β-agonists in competing for the binding of (-)-[3H]dihydroalprenolol at 25°C to these cerebellar β2-receptors.
|Original language||English (US)|
|Number of pages||9|
|Journal||Journal of Biological Chemistry|
|State||Published - Dec 1 1978|
ASJC Scopus subject areas
- Molecular Biology
- Cell Biology