The overlooked background of gastric MMC III: the signalling pathways of motilin receptors inducing canine left gastric artery relaxation

Abstract Background: In diabetic patients with gastroparesis, the gastric blood supply is often decreased and delayed gastric emptying associating MMCIII absence is the main symptom. Under physiological conditions, motilin has been shown to induce a sustained increase in left gastric artery (LGA) blood flow and initiate MMC phase III simultaneously. The study aimed to elucidating the signal transduction pathways of motilin receptors (MLNRs) in the relaxation of LGA.Methods: MLNR expression in the LGA was analysed by immunohistochemistry. Motilin-induced relaxation of the LGA was tested in a multi-wire myograph system. Effects of inhibitors or blockers in the signal transduction pathway were observed.Results: Immunohistochemical and immunofluorescence staining showed that the MLNRs were on the membranes of endothelial cells. Motilin relaxed U46619 pre-contracted canine LGA rings in a concentration-dependent manner, with an EC50 value of 9.010 ± 0.789 × 10−8 M. Motilin’s effect was inhibited by denuded endothelium but not by muscarinic receptor inhibitors. The effect was selectively and competitively inhibited by Phe-cyclo[Lys-Tyr(3-tBu)-Ala-]•trifluoroacetate (GM-109; MLNR antagonist) and completely or partially inhibited by inhibitors of the G protein–phospholipase C–inositol trisphosphate (G pr–PLC–IP3) and nitric oxide synthase–nitric oxide–soluble guanylyl cyclase (NOS–NO–sGC) signal transduction pathway, inhibitors of cyclooxygenase and myoendothelial gap junction, blockers of the potassium channel and low/free Ca2+ Krebs solutions, but not by inhibitors of protein kinase C, protein kinase A or L-type voltage-operated Ca2+ channel.Conclusions: MLNRs were on the membranes of endothelial cells of canine LGA. The main intracellular signal transduction pathway was motilin–MLNR–G pr–PLC–IP3–NOS–NO–sGC–cGMP. These results may provide a new theoretical basis for research on diabetic gastroparesis.