Identification and pharmacological characterization of native, functional human urotensin‐II receptors in rhabdomyosarcoma cell lines

In an effort to identify endogenous, native mammalian urotensin‐II (U‐II) receptors (UT), a diverse range of human, primate and rodent cell lines (49 in total) were screened for the presence of detectable [125I]hU‐II binding sites. UT mRNA (Northern blot, PCR) and protein (immunocytochemistry) were evident in human skeletal muscle tissue and cells. [125I]hU‐II bound to a homogenous population of high‐affinity, saturable (Kd 67.0±11.8 pM, Bmax 9687±843 sites cell−1) receptors in the skeletal muscle (rhabdomyosarcoma) cell line SJRH30. Radiolabel was characteristically slow to dissociate (15% dissociation 90 min). A lower density of high‐affinity U‐II binding sites was also evident in the rhabdomyosarcoma cell line TE671 (1667±165 sites cell−1, Kd 74±8 pM). Consistent with the profile recorded in human recombinant UT‐HEK293 cells, [125I]hU‐II binding to SJRH30 cells was selectively displaced by both mammalian and fish U‐II isopeptides (Kis 0.5±0.1–1.2±0.3 nM) and related analogues (hU‐II[4‐11]>[Cys5,10]Acm hU‐II; Kis 0.4±0.1 and 864±193 nM, respectively). U‐II receptor activation was functionally coupled to phospholipase C‐mediated [Ca2+]i mobilization (EC50 6.9±2.2 nM) in SJRH30 cells. The present study is the first to identify the presence of ‘endogenous’ U‐II receptors in SJRH30 and TE671 cells. SJRH30 cells, in particular, might prove to be of utility for (a) investigating the pharmacological properties of hU‐II and related small molecule antagonists at native human UT and (b) delineating the role of this neuropeptide in the (patho)physiological regulation of mammalian neuromuscular function.