Human urotensin-II (U-II) is an 11-amino-acid cyclic peptide that activates a Gq-coupled receptor named UT. Little is known about the desensitisation profile of this receptor as peptide binding is essentially irreversible. In the present study, we have examined the effects of U-II and carbachol on Ca2+ signalling in SJCRH30 rhabdomyosarcoma (receptor density, Bmax ~0.1 pmol/mg protein) and human embroynic kidney (HEK)hUT (B max ~1.4 pmol/mg protein) cells expressing native and recombinant UT, respectively. In SJCRH30, HEKhUT and human peripheral blood mononuclear cells induced to express native UT by treatment with 2 μg/ml lipopolysaccharide (LPS), we have measured the effects of U-II treatment on UT mRNA. In SJCRH30 cells, primary stimulation with carbachol (250 μM) did not affect a secondary challenge with U-II (1 μM) and primary challenge with U-II did not affect a secondary challenge with carbachol. In contrast, in HEKhUT cells, primary stimulation with carbachol (250 μM) reduced a secondary challenge to U-II (1 μM) by 84% and primary challenge with U-II reduced a secondary challenge to carbachol by 76%. Pre-treatment of SJCRH30 cells with U-II reduced UT mRNA after 6 h and this returned to basal after 24 h. In recombinant HEKhUT cells, UT mRNA expression increased following a 6 h treatment with 1 μM U-II. U-II treatment of naïve un-stimulated peripheral blood mononuclear cells was without effect. However, when UT expression is up-regulated following 15 h of LPS treatment, a 6 h U-II challenge reduced UT mRNA by 66%. In summary, we report differences in the desensitisation profiles of native and recombinant U-II receptors. Design and interpretation of functional experiments are hampered by irreversibility of U-II binding. ; Metadata ; Full text of this item is not currently available on the LRA.