Interactions of Divalent Cations and Guanine Nucleotides at α₂‐Noradrenergic Receptor Binding Sites in Bovine Brain Mechanisms

Guanine nucleotides selectively decrease binding of the agonist ligands [³H]clonidine and [³H]epinephrine to α₂‐noradrenergic receptors in calf cortex membranes. In the presence of 1.0 mM‐calcium,‐magnesium, ormanganese, inhibition by GTP of [³H]clonidine binding is reversed, so that low concentrations of GTP increase [³H]clonidine binding, whereas high GTP concentrations cause a secondary decrease. In the presence of divalent cations, low concentrations of guanyl‐5′‐yl imidodiphosphate [Gpp(NH)p], unlike GTP, do not increase binding. Differences between effects of GTP and Gpp(NH)p in the presence of divalent cations are also observed with [³H]epinephrine binding to rat and calf cortex α₂‐receptors. In reversing the inhibition of α₂‐agonist binding by GTP, manganese, with an ED₅₀ of 21 μM, is much more potent and effective than magnesium or calcium. Manganese by itself increases [³H]clonidine binding by 20% an effect which is irreversible, while the interactive effects of manganese and GTP are reversible. Divalent cations also antagonize the sodium‐induced inhibition of α₂‐agonist binding in calf cortex membranes, and manganese has similar ED₅₀ values in antagonizing both sodium and GTP.