Orphan-receptor ligand human urotensin II: receptor localization in human tissues and comparison of vasoconstrictor responses with endothelin-1

Abstract

We have determined the distribution of receptors for human urotensin-II (U-II) in human and rat CNS and peripheral tissues. In rat, [(125)I]-U-II binding density was highest in the abducens nucleus of brainstem (139.6+/-14 amol mm(-2)). Moderate levels were detected in dorsal horn of spinal cord and lower levels in aorta (22. 5+/-6 amol mm(-2)). In human tissues density was highest in skeletal muscle and cerebral cortex ( approximately 30 amol mm(-2)), with lower levels (<15 amol mm(-2)) in kidney cortex and left ventricle. Little binding was identified in atria, conducting system of the heart and lung parenchyma. Receptor density was less in human coronary artery smooth muscle (14.6+/-3 amol mm(-2), n=10) than rat aorta with no significant difference between normal and atherosclerotic vessels. In human skeletal muscle [(125)I]-U-II bound to a single receptor population with K(D)=0.24+/-0.17 nM and B(max)=1.97+/-1.1 fmol mg(-1) protein (n=4). U-II contracted human coronary, mammary and radial arteries, saphenous and umbilical veins with sub-nanomolar EC(50) values. U-II was 50 times more potent in arteries and <10 times more potent in veins than endothelin-1 (ET-1). The maximum response to U-II ( approximately 20% of control KCl) was significantly less than to ET-1 ( approximately 80% KCl). In contrast, in rat aorta, U-II and ET-1 were equipotent with similar maximum responses. This is the first report of high affinity receptors for [(125)I]-U-II in human CNS and peripheral tissues. This peptide produces potent, low efficacy, vasoconstriction in human arteries and veins. These data suggest a potential role for U-II in human physiology.

Figure 1

Deduced amino acid sequence of human U-II indicating the position of the iodinated tyrosine residue. Shaded residues are those that are conserved across all known species homologues of the peptide.

Figure 2

Relative densities of [¹²⁵I]-U-II binding determined autoradiographically in 20-μm sections of rat and human tissues. Data are mean±s.e.mean. n-values are given in parentheses.

Figure 3

Autoradiographical localization of [¹²⁵I]-U-II binding to 20-μm coronal sections (Bregma −10.3) of rat brain showing (A) total binding to abducens nuclei and (B) non-specific binding in adjacent section. Scale bar=2 mm.

Figure 4

Example of response to U-II in consecutive 4-mm rings of rat thoracic aorta: effect of distance of aortic ring from carotid bifurcation on maximum U-II response.

Figure 5

(A) Concentration-response curves to ET-1 and U-II in endothelium-denuded rat thoracic aorta. Data points are mean±s.e.mean, n=6. (B) Concentration-response curves to ET-1 (n=9) and U-II (n=6) in endothelium-denuded human coronary artery as an example of the relative response of the two peptides in human arteries and veins. Data points are mean±s.e.mean.