l-NAME pre-treatment partially inhibits the agmatine-evoked depression of the electrically induced twitch contraction of isolated rat vas deferens

The effect of the putative endogenous ligand for α2-adrenoceptors and imidazoline receptors agmatine was studied in sympathetic neurotransmission in the rat epididymal vas deferens. Tissues were obtained from Nϖ-nitro-l-arginine methyl ester (l-NAME)-treated or normal animals and were contracted by electrical stimulation or by exogenous adenosine 5′-triphosphate (ATP). In the electrically stimulated epididymal end, agmatine produced an inhibitory effect on twitch contraction that was partially reversed in l-NAME-treated animals, whereas the inhibition produced by clonidine was not affected by l-NAME treatment. The nitric oxide (NO)-donor S-nitroso-N-acetyl-penicillamine (SNAP) also inhibited twitch contraction. Neither agmatine nor SNAP interfered with the responses induced by exogenous ATP in the epididymal end. Removal of the epithelium of the preparation did not modify the agmatine response. We conclude that a nitrergic pathway activated by agmatine plays a role in its inhibitory effect in rat vas deferens, but it remains to be investigated whether it results from a direct action on the enzyme NO-synthase or a receptor-mediated mechanism.