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

1 In segments of rat vena cava preincubated with [H-3]-noradrenaline and su perfused with physiological salt solution, the influence of agmatine on the electrically evoked [H-3]-noradrenaline release, the EP3 prostaglandin rec eptor-mediated and the alpha(2D)-adrenoceptor-mediated inhibition of evoked [H-3]-noradrenaline release was investigated. 2 Agmatine (0.1-10 mu M) by itself was without effect on evoked [H-3]-norad renaline release. In the presence of 10 mu M agmatine, the prostaglandin E- 2(PGE(2))-induced EP3-receptor-mediated inhibition of [H-3]-noradrenaline r elease was not modified, whereas the alpha(2D)-adrenoceptor-mediated inhibi tion of [H-3]-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 [H-3]-noradren aline release. 3 Agmatine concentration-dependently inhibited the binding of [H-3]-clonidi ne and [H-3]-rauwolscine to rat brain cortex membranes (K-i values 6 mu M a nd 12 mu M, respectively). In addition, 30 and 100 mu M agmatine increased the rate of association and decreased the rate of dissociation of [H-3]-clo nidine resulting in an increased affinity of the radioligand for the alpha( 2D)-adrenoceptors. 4 [C-14]-agmatine labelled specific binding sites on rat brain cortex membr anes. In competition experiments. [C-14]-agmatine was inhibited from bindin g to its specific recognition sites by unlabelled agmatine, but not by rauw olscine and moxonidine. 5 In conclusion, the present data indicate that agmatine both acts as an an tagonist at the ligand recognition site of the alpha(2D)-adrenoceptor and e nhances the effects of alpha(2)-adrenoceptor agonists probably by binding t o an allosteric binding site of the alpha(2D)-adrenoceptor which seems to b e labelled by [C-14]-agmatine.