Sphincteric and rectal electrical field stimulation produce different responses in the sheep isolated internal anal sphincter

Thomas Eames1, Sarah Rayment1, Yvette Henry2, Hans-Jurgen Gruss1, Austin Acheson1, John Scholefield1, Vincent Wilson1

1University of Nottingham, Nottingham, UK, 2Norgine International Ltd, Harefield, UK

Activation of intrinsic nerves of the internal anal sphincter (IAS) is known to cause the release of inhibitory transmitters, chiefly nitric oxide, that cause a reduction in myogenic tone (Bhardwaj et al., 2000). Relatively little is known about the neural regulation between rectal tissue and IAS that is involved in the rectoanal inhibitory reflex (RAIR). Using longitudinal strips of cat IAS connected to rectal tissue, Penninckx and colleagues (1982) demonstrated that electrical field stimulation (EFS) of rectal tissue caused an atropine-sensitive reduction in anal sphincter tone. We have examined whether a similar nervous link between rectum and internal anal sphincter is present in sheep, as a model for man.

Sheep rectal and sphincter tissue was obtained from a local abattoir and opened on the ventral side. The mucosa and external anal sphincter were removed. The IAS was prepared and vertically attached to an isometric force transducer and a fixed support, with a strip of rectum attached but orientation at 90°. The sphincter was placed under 2 g wt. resting tension and allowed to develop myogenic tone while warmed at 37°C. A stimulator was then attached to the electrode and the sphincter subjected to runs of EFS (10Hz) for 30s at 0.3ms 150V. This stimulation protocol was repeated with the electrodes placed 3cm along the rectum and the IAS tone monitored.

IAS developed 3-5 g wt. myogenic tone. Sphincteric EFS produced a relaxation of the IAS that was significantly reduced (p<0.05, student’s t-test) by a combination of L-NAME and bretylium (inhibits nitrergic and adrenergic responses) or 0.1μM TTX in 3/4 preparations (Table 1). In contrast, rectal EFS (3 cm from the rectal-anal verge) caused a contraction of the IAS that was unaffected by the combination of L-NAME and bretylium. The contraction was abolished either by 0.1μM TTX (n=4) or 1μM atropine (Table 1).

Table1: The effect of sphincteric and rectal stimulation on sheep IAS myogenic tone. Responses are shown as the mean change in tone (g wt.) ± s.e.m.

	Sphincteric EFS	Rectal EFS
Control	-1.00±0.24 (n=18)	1.00±0.2 (n=18)
100μM L-NAME & 10μM Bretylium	-0.24±0.1 (n=17)	1.31±0.37 (n=11)
1μM Atropine	n.d.	0 (n=4)

We have demonstrated a neural link between the rectum and IAS of sheep. Although cholinergic nerves appear to be involved in rectal activation of the sheep IAS, unlike the cat (Penninckx et al., 1982) this transmitter causes a contraction of the smooth muscle. The precise relationship between the response of the IAS to rectal EFS and the RAIR is unclear, but further studies are warranted.

Bhardwaj R, et al., (2000). Gut 46, 861-868

Penninckx FM, et al., (1982) Scand J Gastroenterol Suppl,. 71: 147-9