Effects of antihypertensive clonidine congeners on alpha-adrenergic receptors

Evidence in the literature suggests that the antihypertensive effects of clonidine stem from its action on alpha-2 adrenergic receptors. In order to examine this possibility we tested the effects of 13 congeners of clonidine on the binding of [3H]WB-4101 and [3H]clonidine to calf frontal cortex homogenates; [3H]WB-4101 served as a label for alpha-1 receptors while [3H]clonidine served to label alpha-2 adrenergic receptors. All the substituted imidazolines were two to three orders more potent in inhibiting the binding of [3H]clonidine than they were against [3H]WB-4101. There was a strong correlation between the antihypertensive doses of these congeners and their concentrations required to inhibit the binding of [3H]clonidine. The results are compatible with the concept that the antihypertensive action of clonidine is more likely due to interactions with alpha-2 adrenergic receptors than with alpha-1 receptors.