Decreased sensitivity of muscarinic but not 5-hydroxytryptamine receptors of the internal anal sphincter in neurogenic faecal incontinence.

Previous studies of the internal anal sphincter in patients with neurogenic faecal incontinence have indicated an abnormality of the adrenergic innervation, but little is known about the responsiveness of other receptors in the internal and sphincter in this condition. In this study the in vitro sensitivity to carbachol and 5-hydroxytryptamine (5-HT) of muscle strips from patients with neurogenic incontinence (n = 6 and n = 7) and from control patients (n = 9 and n = 10) was examined. Preparations of internal and sphincter from patients with incontinence were less sensitive to the relaxant actions of carbachol than preparations from the control group. The pD2 value for carbachol (i.e. the negative logarithm of the concentration for half-maximal response) was significantly greater in the controls than in the incontinent group (mean(s.e.m.) 6.03(0.15) versus 5.43(0.24), P < 0.05). There was no significant difference in the contractile responses to 5-HT, which had pD2 values of 6.93(0.13) and 6.63(0.27) for the control and incontinent groups respectively. The unaffected state of the 5-HT receptor and the subsensitivity of the muscarinic receptor are discussed in relation to intrinsic neural control of the internal and sphincter in neurogenic faecal incontinence.