Abstract
Neuromedin U (NmU) is a smooth muscle contracting peptide. Recently, two G-protein-coupled receptors for NmU (NmU1R and NmU2R) have been cloned having approximately 50% homology. They have distinct patterns of expression suggesting they may have different biological functions. This study provides a comprehensive characterization of both NmU receptors expressed in human embryonic kidney 293 cells. [<sup>125</sup>I]hNmU binding to the recombinant NmU receptors was rapid, saturable, of high affinity and to a single population of binding sites. Exposure of these cells to NmU isopeptides resulted in an increase in intracellular [Ca<sup>2+</sup>]<sub>i</sub> release (EC<sub>50</sub> value of 0.50 ± 0.10 nmol/l) and inositol phosphate formation (EC<sub>50</sub> 1.6 ± 0.2 and 1.50 ± 0.4 nmol/l for NmU1R and NmU2R respectively). Furthermore, hNmU inhibited forskolin (3 µmol/l)-stimulated accumulation of cAMP in intact HEK-293 cells expressing either NmU1R or NmU2R. The inhibitory effect was significant for the cells expressing NmU2R with IC<sub>50</sub> value of 0.80 ± 0.21 nmol/l. In summary, both NmU1R and NmU2R in HEK-293 cells have similar signaling capability.
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