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

Abstract

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 Ca 2ϩ 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.

Influence of Bovine Whey Protein Concentrate and Hydrolysate Preparation Methods on Motility in the Isolated Rat Distal Colon

“…We hypothesise that intact whey alters colonic motility and that hydrolysed whey will have a greater effect due to the higher levels of peptides present. Muscle contraction assays are in routine use in the pharmaceutical industry to identify active compounds involved in GI motility [13,14,15], but have not been widely used for food research. We used a rat model [16,17,18] because many dietary preclinical studies are done in this species and this would enable comparison with other studies.…”

Differential Cardiorespiratory and Sympathetic Reflex Responses to Microinjection of Neuromedin U in Rat Rostral Ventrolateral Medulla

“…The nature of NMU1 and NMU2 and their ability to increase or decrease neuronal excitability is controversial. Agonist binding to NMU1 and NMU2 is reported to activate both G q/11 and G i/0 (Brighton et al, 2004(Brighton et al, , 2008 . By contrast, other studies suggest that 1) NMU1 and NMU2 couple only to G q/11 (Raddatz et al, 2000;Shan et al, 2000); 2) activation of NMU2 increases neuronal excitability, suggesting an excitatory role for NMU2 (Qiu et al, 2003); or 3) NMU1 activation inhibits Fig.…”

Uterotonic Neuromedin U Receptor 2 and Its Ligands Are Upregulated by Inflammation in Mice and Humans, and Elicit Preterm Birth

“…NmU exerts its actions by binding to two G protein-coupled receptors: neuromedin U receptor 1 (NmU-R1; FM-3 or GPR66) and NmU-R2 (FM-4) [8–12]. Both NmU receptors are coupled to G i and G q/11 -phospholipase C β [13].…”