Received for publication October 16, 2007.
Revised January 4, 2008.
Accepted for publication January 4, 2008.

Paradoxical behaviour 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, pro-nociception and circardian rhythm. Two GPCRs 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, whilst NmU binds essentially irreversibly, preventing signaling to repetitive applications of NmU. It is unclear, however, 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. Further, 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 desensitisation, highlighting a major discrepancy between NmU binding in single-cells and in intact tissues. The reason for this discrepancy is presently unknown.

Key words: GPCR, binding, calcium, neuromedin U, regulation, smooth muscle