The mucosal lining of the internal anal sphincter inhibits contractility

Introduction: The internal anal sphincter, a ring of smooth muscle with a central role in maintaining faecal continence, is thought to contribute up to 85% of the anal resting pressure1. Our understanding of the physiology and pharmacology of the internal anal sphincter is still growing. We have recently shown that relaxation of the sphincter is caused by three neurotransmitters that are all gases, nitric oxide, carbon monoxide and hydrogen sulphide2. However, it is possible that the epithelial lining also plays a key role in controlling the sphincter smooth muscle, as has been reported in the lower urinary tract, including the bladder3. In the bladder, the urothelium releases a diffusible factor which inhibits contraction of the underlying bladder smooth muscle3. Our aim was to examine whether a similar mechanism operates to inhibit contractility of the smooth muscle of the internal anal sphincter. Materials and methods: Strips of internal anal sphincter were dissected from fresh porcine anal tissue. Half of the strips were left intact, whilst in the other half the mucosa was carefully removed by dissection (denuded). The tissue strips were then mounted and maintained under physiological conditions in tissue baths (370C, in oxygenated Krebs-bicarbonate solution). Drugs, including the endogenous neurotransmitter noradrenaline, or the agonist phenylephrine, were added to the tissue baths to induce contractions in the tissue strips. The effects of a number of inhibitors was also studied, to determine the involvement of particular neurotransmitters or their receptors. Specifically, carbon monoxide production, nitric oxide production, prostaglandins, and receptors for the non-classical neurotransmitters Adenosine Tri-Phosphate (ATP) and vasoactive intestinal peptide (VIP) were inhibited. All data were expressed as mean ± SEM. Contractions of intact strips and strips denuded of mucosa, in the presence or absence of inhibitors, were compared via Student's t test, and p<0.05 was considered significant. Results: In the presence of the mucosa contractions of the internal anal sphincter smooth muscle were significantly inhibited (Figure 1, p<0.001). The amount of inhibition caused by the mucosa was 75.8 ± 4.7% and 75.3 ± 7.2% for noradrenaline and phenylephrine respectively. This inhibitory effect of the mucosa was not prevented by blocking the receptors for ATP (with suramin) or blocking carbon monoxide production (with zinc protoporphyrin IX), and contractions remained significantly (p<0.001) inhibited compared to those in the absence of inhibitors (76.3 ± 8.0% vs 86.3 ± 6.4%) and (82.3 ± 5.7% vs 69.5 ± 10.5%). In addition, the inhibitory effect of the mucosa was not prevented by inhibition of prostaglandins or nitric oxide release, with contractions remaining significantly (p<0.001) inhibited in intact tissues in the presence of the inhibitors indomethacin and nitro-L-arginine (L-NNA) respectively. However, blockade of VIP receptors partly reduced the inhibitory effect of the mucosa, producing significantly less inhibition of contractions, 65.3 ± 5.68% versus 75.4 ± 4.7% (p<0.05). studied, to determine the involvement of particular neurotransmitters or their receptors. Specifically, carbon monoxide production, nitric oxide production, prostaglandins, and receptors for the non-classical neurotransmitters ATP and vasoactive intestinal peptide (VIP) were inhibited. All data were expressed as mean ± SEM. Contractions of intact strips and strips denuded of mucosa, in the presence or absence of inhibitors, were compared via Student's t test, and p<0.05 was considered significant. Results: In the presence of the mucosa contractions of the internal anal sphincter smooth muscle were significantly inhibited (Figure 1, p<0.001). The amount of inhibition caused by the mucosa was 75.8 ± 4.7% and 75.3 ± 7.2% for noradrenaline and phenylephrine respectively. This inhibitory effect of the mucosa was not prevented by blocking the receptors for ATP (with suramin) or blocking carbon monoxide production (with zinc protoporphyrin IX), and contractions remained significantly (p<0.001) inhibited compared to those in the absence of inhibitors (76.3 ± 8.0% vs 86.3 ± 6.4%) and (82.3 ± 5.7% vs 69.5 ± 10.5%). In addition, the inhibitory effect of the mucosa was not prevented by inhibition of prostaglandins or nitric oxide release, with contractions remaining significantly (p<0.001) inhibited in intact tissues in the presence of the inhibitors indomethacin and L-NNA respectively. However, blockade of VIP receptors partly reduced the inhibitory effect of the mucosa, producing significantly less inhibition of contractions, 65.3 ± 5.68% versus 75.4 ± 4.7% (p<0.05). Conclusions: The mucosal lining of the internal anal sphincter has an inhibitory effect on the underlying smooth muscle. The degree of inhibition of contraction was approximately 75%, which is greater than the inhibition of bladder contractions by the urothelium, which is normally around 55% 3. The inhibitory effect does not involve ATP, prostaglandins or the gaseous neurotransmitters nitric oxide or carbon monoxide. The peptide neurotransmitter vasoactive intestinal peptide appears to pay a minor role, although it is clear that other, as yet undetermined, mechanisms are involved. Identification of these inhibitory mechanisms within the internal anal sphincter will help further our understanding of the control of the faecal continence and may lead to novel targets for development of drugs to increase tone in the internal anal sphincter and thus treat faecal incontinence. Competing interest statement: none.