Dual interaction of agmatine with the rat alpha(2D)-adrenoceptor: competitive antagonism and allosteric activation.

In segments of rat vena cava preincubated with [(3)H]-noradrenaline and superfused with physiological salt solution, the influence of agmatine on the electrically evoked [(3)H]-noradrenaline release, the EP(3) prostaglandin receptor-mediated and the alpha(2D)-adrenoceptor-mediated inhibition of evoked [(3)H]-noradrenaline release was investigated. Agmatine (0.1-10 microM) by itself was without effect on evoked [(3)H]-noradrenaline release. In the presence of 10 microM agmatine, the prostaglandin E(2)(PGE(2))-induced EP(3)-receptor-mediated inhibition of [(3)H]-noradrenaline release was not modified, whereas the alpha(2D)-adrenoceptor-mediated inhibition of [(3)H]-noradrenaline release induced by noradrenaline, moxonidine or clonidine was more pronounced than in the absence of agmatine. However, 1 mM agmatine antagonized the moxonidine-induced inhibition of [(3)H]-noradrenaline release. Agmatine concentration-dependently inhibited the binding of [(3)H]-clonidine and [(3)H]-rauwolscine to rat brain cortex membranes (K(i) values 6 microM and 12 microM, respectively). In addition, 30 and 100 microM agmatine increased the rate of association and decreased the rate of dissociation of [(3)H]-clonidine resulting in an increased affinity of the radioligand for the alpha(2D)-adrenoceptors. [(14)C]-agmatine labelled specific binding sites on rat brain cortex membranes. In competition experiments. [(14)C]-agmatine was inhibited from binding to its specific recognition sites by unlabelled agmatine, but not by rauwolscine and moxonidine. In conclusion, the present data indicate that agmatine both acts as an antagonist at the ligand recognition site of the alpha(2D)-adrenoceptor and enhances the effects of alpha(2)-adrenoceptor agonists probably by binding to an allosteric binding site of the alpha(2D)-adrenoceptor which seems to be labelled by [(14)C]-agmatine.