Sa2046 Effect of RM-131 on Circular Smooth Muscle Cells in Human and Mouse Colon and on Colonic Intraluminal Pressure in Conscious Mice

Abstract

Background and Aims: RM-131 is a synthetic ghrelin agonist that has been found to significantly accelerate gastric emptying, intestinal transit and colonic filling. The mechanism(s) through which smooth muscle cells of gastrointestinal tract respond to RM-131 is not known. We studied the effect of RM-131 on circular smooth muscle cells of human and mouse colon in vitro and on colonic intraluminal pressure in conscious mice. Methods: SJL/J mice and human colonic tissue were used in this study. Normal human colon was obtained from surgical excess tissue. The use of mice was approved by Mayo Institutional Animal Care and Use Committee and the use of human biopsy specimens was approved by Institutional Review Board. Muscle strips without the mucosa from mouse colon and human colon strips were placed in a recording chamber for intracellular recording. The recording chamber was superfused with oxygenized normal Krebs solution at 37°C. A bipolar stimulating electrode was placed on both sides of the muscle strip for electric field stimulation to elicit inhibitory junction potentials (IJPs). To test the effect of RM-131 on colonic intraluminal pressure in conscious mice, a miniaturized pressure transducer catheter was introduced into the colon such that the middle of the pressure sensor was 2.5 cm proximal to the anus. Results: RM-131 (0.25 μM) significantly hyperpolarized the resting membrane potential for human colonic circular smooth muscle cells by 2.8±0.7 mV (P<0.01, n=9) and in mouse preparations by 3.4±0.4 mV (P<0.01, n=6). RM-131 had no significant effect on either the fast IJP or the slow IJP. Ghrelin (1 μM) also hyperpolarized the resting membrane potential of circular smooth muscle cells in all cells recorded from human preparations by 3.5±1.4 mV (P=0.06, n=3) and in mouse preparations by 3.8±0.8 mV (P<0.01, n=7). Ghrelin also had no significant effect on either the fast IJP or slow IJP. For intraluminal pressure recordings, RM-131 or control saline was applied via intraperitoneal injection. Intraluminal pressure changes with RM-131 or saline were normalized with their pre-control. In the RM-131 treated group, intraluminal pressure was 0.57±0.05 (n=8) which was significantly less compared to the control group (0.82±0.08, n=8), suggesting that RM131 significantly reduced colonic tone. Conclusions: Our results suggest that RM-131 decreases colonic tone by hyperpolarizing the resting membrane potential of circular smooth muscle cells and decreases the excitability of colonic circular smooth muscle cells. These effects of RM-131 are likely the basis for the decrease in colonic intraluminal pressure. The previously reported increase in upper GI contractility together with a decrease in colonic intraluminal pressure position RM-131 for the treatment of functional gastrointestinal disorders.