Magnesium reduces affinities of antagonists at rat cortex α 2-adrenergic receptors labeled with 3H-clonidine: Evidence for heterogeneity of α 2-receptor conformations with respect to antagonists

3H-clonidine labeled two binding sites in rat cortex membranes with apparent K D values of about 1.0 and 5.9 nM. These sites appeared analogous to “super-high” (SH) and “high” (H) affinity states of the α 2-receptor described in human platelets. 10 mM magnesium increased the number of SH receptors by 30% whereas 100 μM GTP reduced SH 3receptor number by 45% with no significant change in the K D of 3H-clonidine at α 2(SH) sites. In drug competition studies using 1.0 nM 3H-clonidine, 100 μM GTP reduced the affinity of clonidine and increased the affinity of yohimbine, whereas 10 mM magnesium increased the affinity of clonidine and reduced the affinity of yohimbine. The effect of magnesium on the affinity of several antagonists at cortex 3H-clonidine sites ranged from none (phentolamine) to a 6-fold reduction (piperoxan). These data indicate that different states of the α 2-receptor exhibit different affinities for some antagonists.