Pharmacological characterization of neurogenic responses of the sheep isolated internal anal sphincter

The aim of the study was to establish the nature of the neurogenic responses of the sheep isolated anal sphincter. Isolated strips of sheep internal anal sphincter develop intrinsic contractile tone following the application of stretch tension. On transmural stimulation (1–20 Hz, 10 V pulse strength, 0.5 ms pulse width, 1 s every 180 s) transient relaxations were observed. The amplitude of the relaxations were frequency-dependent reaching a maximal response at 10–20 Hz and were inhibited by tetrodotoxin (0.3 μM). Neither atropine (0.3 μM) nor phentolamine (1 μM) affected control responses. The nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME, 100 μM) and the selective inhibitor of soluble guanylyl cyclase ODQ, (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one) (1 μM) completely inhibited the neurogenic relaxations and uncovered contractions that were abolished by 1 μM phentolamine and 0.1 μM prazosin. The effect of L-NAME, but not that of ODQ, was partially reversed by the addition of L-arginine (1 mM). Sodium nitroprusside (10 nM–10 μM) caused concentration-dependent inhibition of myogenic tone and this effect was significantly reduced by ODQ. Calcium-free Krebs-Henseleit solution also reduced myogenic tone by 85%. Transmural electrical stimulation of the sheep isolated internal anal sphincter causes a transient relaxation of myogenic tone that appears to involve nitric oxide from non-adrenergic, non-cholinergic nerves and, to a lesser degree, noradrenaline from sympathetic nerves. The characteristics of the preparation compares well with that of human tissue and may prove to be a suitable animal based model for further studies. British Journal of Pharmacology (2000) 130, 489–494; doi:10.1038/sj.bjp.0703322