Interactions of divalent cations and guanine nucleotides at alpha 2-noradrenergic receptor binding sites in bovine brain mechanisms.

Abstract

Abstract: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.