Investigation of the role of adrenergic and non‐nitrergic, non‐adrenergic neurotransmission in the sheep isolated internal anal sphincter

Abstract

Nitric oxide is widely established as an important neurotransmitter in the control of anal sphincter tone; although, a number of other transmitters have also been tentatively implicated. Whilst alpha-adrenoceptor antagonists reduce anal sphincter pressure in man, the role of noradrenaline as a possible transmitter is poorly characterised. We have investigated the contribution of these transmitters to neurogenic relaxations, and evaluated the possible role of a non-nitrergic, non-adrenergic transmitter. The magnitude and duration of neurogenic responses were examined by measuring responses to electrical field stimulation (EFS) in segments of sheep internal anal sphincter following the development of spontaneous myogenic tone. Neurogenic relaxations induced by EFS were significantly reduced in the presence of N(G)-nitro-L-arginine methyl ester (L-NAME) suggesting major involvement of nitric oxide as a neurotransmitter. The duration of neurogenic relaxations was inversely related to the frequency of EFS, with contractile responses often manifest at higher frequencies. The duration of relaxations at high frequencies of EFS was increased by bretylium (adrenergic neurone blocker) and prazosin (alpha(1)-adrenoceptor antagonist). At higher frequencies of EFS, 60% of preparations also produced a residual non-nitrergic, non-adrenergic, apamin-sensitive relaxation which was unaffected by vasoactive intestinal polypeptide (VIP) and inhibitors of purinergic responses [suramin, pyridoxal-phosphate-6-azophenyl 2',4' disulfonic acid (PPADS) and alpha,beta-methylene adenosine triphosphate (ATP)]. However, MRS2179 (P2Y(1) receptor antagonist) showed a modest inhibitory effect. We conclude that endogenous noradrenaline acts via postjunctional alpha(1)-adrenoceptors to antagonize neurogenic relaxations that are largely mediated by nitric oxide. Our results indicate the involvement of a non-nitrergic, non-adrenergic, apamin-sensitive transmitter which is inhibited by MRS2179, suggesting a possible role for purines.

Three Gaseous Neurotransmitters, Nitric oxide, Carbon Monoxide, and Hydrogen Sulfide, Are Involved in the Neurogenic Relaxation Responses of the Porcine Internal Anal Sphincter

“…The non-nitrergic relaxation induced by EFS in the presence of L-NNA was also investigated. Neither the VIP receptor antagonist PheLeu-VIP nor the COX1/2 inhibitor indomethacin significantly reduce the non-nitrergic relaxation ruling out a role for VIP or prostaglandins in relaxation of the pig IAS, a conclusion also reached by Acheson et al 19 in the sheep IAS. However the guanylate cyclase inhibitor ODQ 20 did have a significant effect, reducing…”

Control of Motility in the Internal Anal Sphincter

“…Excitatory motor innervation to the IAS of non-rodent species is sympathetic 4,44,[92][93][94] whereas in the intestine sympathetic nerves serve a neuromodulatory role. 95,96 Postganglionic sympathetic neurons originate in the inferior mesenteric ganglion or pelvic plexus and travel to the IAS via lumbar colonic nerves, hypogastric nerves, or branches of the pelvic plexus.…”

“…Nitric oxide (NO) plays a prominent role in inhibitory NMT in the IAS of all species studied including man. 15,45,93,[123][124][125] Although the IAS lacks a myenteric plexus there are numerous varicose nicotinamide adenine dinucleotide phosphate/nNOS neurons located within the circular muscle layer. 15,16 Since first proposed in 1996, a number of studies have provided evidence that ICC-IM participate in nitrergic NMT in GI muscles.…”

Dual effects of α2‐adrenoceptors in modulating myogenic tone in sheep isolated internal anal sphincter

“…Sheep internal anal sphincter (IAS) was isolated by dissection from sheep anus (obtained from a local abattoir), as previously described …”

“…Tissue used for contractile studies was stored overnight at 4 °C in modified Krebs–Henseleit solution and prepared for isometric tension recording the following day as previously described . Segments were placed in a 20‐mL isolated organ bath containing modified Krebs–Henseleit solution maintained at 37 °C and gassed with 95% O 2 /5% CO 2 .…”