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CELLULAR AND MOLECULAR

Paradoxical Behavior of Neuromedin U in Isolated Smooth Muscle Cells and Intact Tissue

Department of Cell Physiology and Pharmacology, University of Leicester, Leicester, United Kingdom (P.J.B., G.B.W.); Screening and Compound Profiling (A.W.) and Neurology and Gastrointestinal Centre of Excellence for Drug Discovery (N.B.D.), GlaxoSmithKline, New Frontiers Science Park, Harlow, United Kingdom

Neuromedin U (NmU) is a neuropeptide showing high levels of structural conservation across different species. Since its discovery in 1985, NmU has been implicated in numerous physiological roles, including smooth muscle contraction, energy homeostasis, stress, intestinal ion transport, pronociception, and circadian rhythm. Two G-protein-coupled receptors have been identified for NmU and cloned from humans, rats, and mice. Recombinantly expressed NmU receptors couple to both G_q/11 and G_i G-proteins, and NmU binds essentially irreversibly, preventing signaling to repetitive applications of NmU. However, it is unclear whether these properties reflect those of endogenously expressed NmU receptors or how these properties influence the functional consequences of NmU receptor signaling. Here, we have explored the signaling by rat NmU receptors expressed endogenously in cultured rat colonic smooth muscle cells and explore the functional consequence of this signaling by investigating the NmU-mediated contraction of ex vivo rat colonic smooth muscle preparations. We demonstrate that endogenous rat NmU receptors couple to both G_q/11 and G_i G-proteins. Furthermore, we show complex patterns of Ca²⁺ signaling, including oscillations, and provide evidence of essentially irreversible binding of NmU to smooth muscle cells. Challenge of either circular or longitudinal rat isolated colonic smooth muscle preparations with NmU resulted in robust contractions. Stimulation was direct, and paradoxically, repetitive applications of NmU mediated repetitive contractions with no evidence of desensitization, highlighting a major discrepancy in the behavior of NmU in single cells and in intact tissues. The reason for this discrepancy is presently unknown.

Address correspondence to: Dr. Gary B. Willars, Department of Cell Physiology and Pharmacology, Medical Sciences Building, University of Leicester, University Road, Leicester LE1 9HN, UK. E-mail: gbw2{at}le.ac.uk