Relaxation Of The Lower Esophageal Sphincter In Human: Role Of Vasoactive Intestinal Peptide

Objective:The lower esophageal sphincter(LES)is a thickened region of the circular muscle layer of the distal esophagus, in human extending over an axial distance of 2–3 cm. Liebermann-Meffer proposed that the muscular equivalent of LES consists of sling like muscle bundles on the greater curvature side formed by long oblique gastric fibers and a thickened inner circular muscle layer configured as semicircular or clasp fibres adjacent to the lesser curvature. These separate muscle groups create a zone of high pressure (15-30mmHg) and maintain sphincter closure. In vivo studies using manometric catheter with three paired holes in the left and right orientations within LES have shown a significant radial asymmetry in the distribution of the high-pressure zone. The highest pressures have been recorded in the left lateral direction of LES region which was sling fibers. Apparently, the two fibres have different functions in LES pressure. The function of LES is mediated by the excitatory and inhibitory nerves, humoral factors, as well as the muscles. By now the transmitters of the nonadrenergic noncholinergic(NANC) nerve is still to be at the research phase, Nitric oxide(NO) is thought of a nervetransmitter, and Vasoactive intestinal peptide (VIP) is still in dispute. VIP have been reported to significantly decreases the pressure of LES in animal experiments. The aims of this study is to test the hypothesis that VIP control the nerve-mediated relaxation of the LES and hopefully to further study the differences between the sling fibre and the clasp fibre in LES, to provide theoretical bases for the clinical treatment of esophageal motility disorders.Methods: Gastroesophageal junction including some esophagus and some gastric fundus were obtained from 30 patients undergoing esophagectomy for cancer in our hospital between March 2010 and August 2010 were selected in this study. At esophagectomy, an esophagogastric segment from 3cm above the LES and including a 3cm cuff of the stomach was carefully excised and placed into Krebs solution equilibrated with 95%O2 -5%CO2 and maintained at pH 7.40±0.05. The sling and clasp fiber strips were separated in Kreb's solution. The tissue was freed from the surrouding fascia, stretched to its situ length, and the cut along the greater curvature of the stomach. The mucosa was then gently removed to expose the LES clasp and sling regions. Muscle strips, 2mm wide and 10mm long, were obtained from the long axis of the clasp and oblique sling muscles. The strips were mounted individually in water-jacketed tissue baths containing 10 ml of Krebs solution continuously bubbled with at 37°C.A silk thread was tied to each end of the strips, one end of the strip was secured to an electrode holder and the other end to an isometric force transducer supported on a rack-and-pinion clamp to facilitate accurate length adjustment of the muscle strips. EFS(0.5 ms square wave pulses;50V;4s train; frequencies 1-32Hz)was delivered by a stimulator through two parallel platinum wire electrodes which were placed in parallel with either side of the tissue strips. There was a distance of 3 mm from electrodes to the strips. Tension signals from each strip were amplified with an amplifier, and then recorded using a computer with software MedLab 6.0. Before the experiment, the muscle strips were hung with 0.2g tension for a 1quarter equilibration period, and the length was then measured and defined as the initial length (L0). Strips were then slowly stretched four times at increments of 25% of L0 with 15min between each stretch. At a study length of 200%L0, EFS of the sling muscle resulted in an initial contraction in sling strips and relaxation in clasp strips.Then the muscle strips in Krebs solution balance 1 hour to form a equilibration tension. The responses of muscle strips stimulated by EFS were observed after adding VIP(10–28)(5μmol/L),distilled-water. Concentration-response curves to exogenous VIP(10-9-10-6mol/L) were produced in the strips which did not accept EFS.Results: 1 Effect of exogenous VIP on the clasp and sling fibers Exogenous VIP(10-9～10-6mol/L) may caused the concentration-dependent relaxation of the sling and clasp, the two fibres had statistical difference between these four concentration gradients(P<0.05).2 Effect of VIP(10-28) to exogenous VIP The VIP antagonist VIP(10-28) proved to be a inconsistent inhibitor of exogenetic VIP-induced LES relaxation.3 Effects of VIP(10-28) to the clasp and sling fibers after EFS VIP(10-28) caused a inconsistent increasing tension on either clasp or sling fibre after the activation of EFS.Conclusions: The experiment studies the differences between the VIP on clasp fibre and on sling fibre, and explore whether VIP belongs to the LES NANC nervetransmitters. We found that the exogenetic VIP could induce the LES clasp and sling fibres a concentration-dependent relaxation. There was a stastical significance between the different effects of human LES clasp and sling fibres to VIP in the same concentration. The LES could release VIP after EFS in NANC conditions.