ALPHA-NORADRENERGIC RECEPTORS IN BRAIN MEMBRANES - SODIUM, MAGNESIUM AND GUANYL NUCLEOTIDES MODULATE AGONIST BINDING

Binding of [3H]clonidine to alpha noradrenergic receptors in rat brain is inhibited by monovalent cations (Na+>Li+>K+), stimulated by magnesium ion and inhibited by guanyl nucleotides. In the presence of 1 mM EDTA the receptors bind tritiated clonidine in a noncooperative fashion at a single site with a Ka(association constant) of 0.12 nM-1. In the presence of magnesium the affinity of the receptors increases by a factor of two (Ka=0.23 nM-1). The increase of affinity is attributed to a two-fold decrease in the dissociation rate constant. In the presence of sodium ions the concentration of binding sites is not changed but Scatchard plots are now curvilinear indicating either heterogeneity of the receptors or negative cooperativity in ligand binding. This effect of sodium ions is not influenced by the presence of magnesium. The conversion into the sodium-liganded state is rapid; it is complete within 60 s at 30° C. The effects of the guanyl nucleotides on clonidine binding are complex: In the presence saturating concentrations of sodium ions they cannot inhibit clonidine binding except when free magnesium (>1 mM) is present. Without added sodium and in the presence of 1 mM EDTA the rank order of potencies is: GDP≧GTP>Gpp(NH)p. In the presence of 10 mM magnesium the rank order is reversed: Gpp(NH)p ≫ GTP≧GDP. The apparent affinity of the nucleotides for inhibition of clonidine binding is also changed by magnesium. The affinity of Gpp(NH)p increases about 100-fold by addition of magnesium ion. © 1979 Springer-Verlag.