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Adenbanjo AO, Ambache N, Verney J. 
“The Inhibitory Transmission To The Internal Anal Sphincter”. 
Brit.J.Pharmacol.. 1976;56(3):392P-393P.
Abstract
Inhibition of this sphincter by sacral nerves is atropine-resistant. Our results show that this sacral inhibitory post-ganglionic transmission resembles that present in the retractor penis. It is likely that the other atropine-resistant sacral inhibitory effects described by Langly and Anderson (1895b) are similarly mediated.

PROCEDURE: The anal canal is excised by a wide perineal incision. The anus is slit and a 1 - 2 cm width of mucosa, adjoining the skin, is dissectd off the undelying sphincters. A plane of separation is found between the pink-brown external sphincter (skeletal) and the white smooth muscle band of the internal sphincter. Strips from > 50 rabbit, pig, dog, cat and guinea-pig sphincters were suspended in Krebs-Henseleit (35░C) and stimulated transmurally (1-20) pulses at 1 min intervals; supramaximal voltage; 0.2 ms 10 Hz).

RESULTS: Transmural stimulation elicited repeateable inhibitions unaffectd by pentolinium (93 mcM) but abolished by tetrodotoxin (0.63 mcM). Pure relaxations were obtained without │rebound▓ in most preparations; only a few had an additonal (adrenergic) motor component which disappeared after guanethidine, 10 mcM, or phentolamine, 5.3 mcM. That acetylcholine is not the inhibitory parasympathetic transmitter was shown by : (a) its opposite, contractile effect, except once when, in high doses (277 mcM), acetylcholine, llike nicotine, produced a pentolinium-susceptivle inhibition. (b) Persistence of these neurogenic inhibitions in atropine (0.029-29 mcM); and (c) failure of physostigmine (7.7 mcM) to potentiate the atropine-resistant inhibitions. The inhibitory transmission is not adrenergic or tryptaminergic, persisting after propranolol (6.8 mcM) or butoxamine (6.6 mcM) +/- phentolamine (5.3 mcM) or phenoxybenzamine (5.8 mcM); and after guanethidine (10 mcM), bretylium (24 mcM) or LSD (125 nM). Transmission by ATP, histamine or prostoglandins (E and F), was excluded.
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