Influence of gastrointestinal neuropeptides on the anal canal

Octreotide acts on anorectal physiology: a dynamic study in healthy subjects

BACKGROUND

Somatostatin is localized in the intestinal and pelvic nerves of the anorectum and it seems to act as an important neurotransmitter. However, previous analyses of octreotide (a somatostatin analog) effects on anal function showed conflicting results. By use of a dynamic model in healthy subjects, with comparison to the myogenic effect of glucagon, the aim of our study was to further investigate the pharmacologic targets of octreotide.

METHODS

This was a placebo-controlled, randomized, double-blinded crossover study performed in 12 healthy volunteers who received octreotide, glucagon, or placebo intravenously on separate days. During each sequence, several pressure steps in 3 different protocols of rectal isobaric distension were applied with an electronic barostat. Manometric responses of the anal canal, adaptative volumes, and perception scores of the rectum were recorded.

RESULTS

During both phasic and stepwise distensions, a significant drug effect was encountered at the anal level. Compared with placebo, octreotide significantly increased pressures at both upper and lower levels of the anal canal. In contrast, glucagon decreased pressures at the upper part of the anal canal. Octreotide significantly decreased rectal volumes to phasic distension, but glucagon did not induce any change on rectal adaptation. In addition, neither drug modified perception scores.

CONCLUSION

This study suggests that octreotide acts on reflex arcs and rectal myenteric neurons rather than on anal myogenic targets that respond to glucagon.

Effects of cholinergic agents on anorectal physiology

BACKGROUND

Despite their potential therapeutic benefit, the effects of cholinergic agents on anal function have been poorly investigated.

AIM

To analyse the effects of neostigmine and atropine on anorectal responses to rectal isobaric distension.

METHODS

This was a placebo-controlled, randomized, double-blind crossover study, performed in 12 healthy volunteers who received intravenously, on 3 separate days, neostigmine, atropine or the placebo. During each day of the experiment, seven pressure steps (ranging from 1 to 31 mmHg) in three different protocols of rectal isobaric distension (phasic, stepwise and tonic) were applied using an electronic barostat. Manometric responses of the anal canal, adaptative volumes and perception scores of the rectum were recorded.

RESULTS

During stepwise distension, a significant drug effect was encountered at the anal level. No drug effect was observed on the other investigated parameters (rectal volumes and rectal perception scores) or for the other modes of distension. Compared to placebo, neostigmine significantly decreased pressures at the upper level of the anal canal for both recto anal inhibitory reflex and mean resting pressures. In contrast, atropine significantly increased pressures at the lower part of the anal canal but did not modify upper anal pressures.

CONCLUSION

The present study suggests that cholinergic effects result more from an indirect action on intermediate neurotransmitters and rectal myenteric neurons, than from a direct action on anal targets.

Peptidergic innervation of the internal anal sphincter in Hirschsprung's disease

The pathophysiology of the impaired sphincter function in Hirschsprung's disease is still unclear. The peptidergic innervation of the aganglionic large intestine is known to be disturbed. The present study analyzes the peptidergic innervation of the aganglionic internal anal sphincter (IAS) in comparison with that of the circular layer of ganglionic and aganglionic large intestine. Immunoreactivity for the following substances was analyzed: vasoactive intestinal polypeptide (VIP), substance P (SP), met-enkephalin (ENK), calcitonin gene-related peptide (CGRP), somatostatin (SOM), and neuropeptide Y (NPY). All patients were operated upon with Soave's endorectal pull-through technique and a posterior partial myectomy of the IAS. For comparison, specimens of resected IAS from adult patients operated upon for rectal cancer as well as autopsy specimens from a 2-year-old child were analyzed. Differences in the density of nerve fibers between the ganglionic and aganglionic large intestine were in accordance with previous studies. In sections of normoganglionic IAS moderately dense networks of nerve fibers immunoreactive for NPY, SOM, and VIP were observed. The occurrence of NPY and SOM was somewhat more frequent here compared to the colonic circular muscle coat, whereas the opposite was seen for VIP. In aganglionic IAS abundant nerve fibers immunoreactive for NPY, SOM, and VIP were observed. Only a few SP-, CGRP-, and ENK-immunoreactive fibers were found in normal and aganglionic IAS. It is concluded that there were moderate differences in the peptidergic innervation of the aganglionic IAS as compared to the normal ganglionic IAS and the circular muscle coat of the ganglionic and aganglionic large intestine.

Effect of octreotide on anal pressure and rectal compliance

PURPOSE

The somatostatin analog, octreotide, has previously been found to influence rectal sensation and may also influence anal resting pressure.

METHODS

We studied the effect of octreotide on anal resting pressure and rectal compliance in eight healthy patients. Octreotide was administered intravenously as a bolus injection in doses of 100 and 10 micrograms or as infusion of 250 micrograms/hour on separate days and compared with placebo.

RESULTS

Within one minute after a bolus injection of 100 micrograms of octreotide, anal resting pressure increased from 56 +/- 12 to 96 +/- 16 cm H2O (P < 0.005). Octrotide had no effect on rectal sensitivity or compliance measurements. Octreotide counteracted rectoanal reflex by increasing anal pressure almost to the level found with an empty rectum.

CONCLUSION

Somatostatin thus seems to contribute to the regulation of rectoanal reflex.

Physiology and pathophysiology of anal function

A review of the Danish contributions to the increased understanding of anorectal physiology and pathophysiology during the last 25 years is presented. In this period there has been a vast international increase in interest in anorectal physiology and pathophysiology, with much improvement in the understanding and treatment of anorectal functional disorders. The application of new sophisticated techniques to anorectal physiology research continues to improve our knowledge of anorectal function.

'Reversible chemical sphincterotomy' by local application of glyceryl trinitrate

Nitric oxide has emerged as one of the most important neurotransmitters mediating internal anal sphincter relaxation. The effect of glyceryl trinitrate, a nitric oxide donor, on anal tone was examined. Maximum resting pressure, predominantly a function of the smooth muscle internal anal sphincter, was measured before and 20 min after application of 0.2 per cent glyceryl trinitrate ointment in ten patients. Pressure decreased by a mean of 27 per cent (95 per cent confidence interval 18-36 per cent) (P = 0.0004) after administration of the drug. A further 20 patients were then randomized to either active or placebo ointment. Anal pressure was significantly decreased (P = 0.002) in those who received 0.2 per cent glyceryl trinitrate, but there was no significant reduction in the control patients. Mild headache occurred in two patients who were given the active preparation and in one who received placebo. Manometry was repeated 9h after application of glyceryl trinitrate and showed a sustained decrease in pressure in two patients. Topical glyceryl trinitrate may have a role in the treatment of anal fissure, haemorrhoids, certain types of constipation and anal pain. It may also reduce injury to the internal sphincter during peranal operations.

Anorectal function

A review in a historic perspective of the present knowledge of anorectal physiology is presented. The techniques used in the anorectal physiology laboratory are discussed. Application of new sophisticated techniques to anorectal physiology research in recent years continue to improve our knowledge of anorectal function. Anal continence and defecation depend on both the anal sphincter and the rectum. The assessment of patients with functional anorectal diseases should include a more complete physiologic evaluation of the anorectum than used previously.

The internal anal sphincter: mechanisms of control and its role in maintaining anal continence

The human IAS has particular structural and functional characteristics. This smooth muscle constantly generates rhythmic electrical slow waves, but no action potentials. The slow waves are linked to calcium fluxes and both are essential for mechanical activity, i.e. the ASPW. The IAS is pharmacologically characterized by the presence of alpha excitatory and beta inhibitory adrenergic receptors. Cholinergic drugs have an indirect effect through the release of an inhibitory neurotransmitter, very probably VIP, from NANC nerves. The myogenic activity of the IAS is enhanced by its extrinsic sympathetic innervation. Thus, at rest, the IAS is in a state of partial tetanus and contributes approximately 55% of the MABP. Because the IAS ring cannot be completely closed, the anal mucosa and the haemorrhoidal plexuses fill the gap. By compressing these tissues, the IAS perfectly closes the anal canal to retain not only solids but also fluid stool and gas. Acute rectal distension and rectal activity, mainly through intramural pathways, induce reflex IAS relaxation, permitting the rectal contents to be sampled by receptors in the upper anal canal while continence is temporarily maintained by EAS activity and by expansion of the haemorrhoidal cushions. There is a correlation between the volume of rectal distension and the parameters of IAS relaxation. At maximal IAS relaxation, ASPW are absent, indicating the completeness of the inhibition. Although this RAIR is not essential for defecation, insufficient relaxation may be implicated in constipation. Hyperactivity of the IAS resulting in a high MABP and AUSPW has been considered both as a cause and as an effect in haemorrhoids and anal fissure. Continence for fluids and gas is impaired if IAS activity is decreased (i.e. a low MABP), either by direct trauma or by damage of its sympathetic innervation. Severe faecal incontinence will develop when the contractility of both the IAS and the EAS is affected.

Influence of peptides on anorectal function

Immunohistochemical and immunochemical studies on biopsies from the human rectum and anal canal have shown several regulatory peptides present in the muscle layers and the mucosa, suggesting a regulatory action on defecation. This view has been supported by studies of anorectal function in man during administration of different peptides. The physiological implications of these observations remain obscure.