We have previously shown human urotensin-II (hU-II) to be a potent but low efficacy vasoconstrictor of the rat main pulmonary artery (MacLean et al., 2000). To determine whether the pressor response of hU-II is enhanced in pulmonary arterial hypertension (PAH), the action of hU-II on the rat pulmonary arterial circulation was examined in control rats and rats exposed to 2 weeks chronic hypoxia (CH).

45 day old male Wistar rats (200-250g) were studied. Some rats were exposed to 2 weeks of chronic hypobaric hypoxia (10%) as described previously [CH rats] (MacLean et al., 2000). The following vessels were studied in vitro using an isometric wire myograph: Extra-lobar; the main pulmonary artery (2-4mm I.D.) (referred to as A-class vessels) and the first branch pulmonary artery (1-2mm I.D.) (B-class). Intra-lobar; the 1^st generation intra-lobar, pulmonary artery (0.75 -1.25mm I.D.) (C-class) and the 2^nd generation intra-lobar, pulmonary artery (150-300µm I.D.) (D-class). Vessels were set up to a pressure equivalent to ~16mmHg (controls) and ~36mmHg (hypoxics). Cumulative concentration response curves to hU-II (10^-12-10^-6M) were constructed and data (± sem) analysed with an unpaired Student's test. In vivo studies: 60 day old male Wistar rats (300-350g) were anaesthetised (nitrous oxide:oxygen (1:1), 1% halothane), ventilated and the ascending aorta and right ventricle catheterised as described previously (Keegan et al., 2001). Following a period of stabilisation, hU-II (0.01-100nmol.kg^-1) was cumulatively administered via the femoral vein. Right ventricular and aortic pressures (RVP and AOP respectively) were recorded using an Elcomatic pressure transducer connected to an MP100 data acquisition system. Data were analysed using ANOVA followed by Dunnets post-test.

In vitro, hU-II had a potent, but low efficacy, constrictor action on control A-class (Emax: 20 ± 4% (of the response to 50mM KCl); pEC₅₀ 8.63 ± 0.18, n=8 rats), negligible effect in B-class (Emax: 8 ± 2%; pEC₅₀ 8.71 ± 0.29, n=8) and C-class vessels (Emax: 11 ± 3%; pEC₅₀ 8.38 ± 0.22, n=7) and essentially no action on the D-class vessels (Emax <2%, n=8). In CH rats, the maximum response to hU-II in A-class (Emax: 4 ± 2%; pEC₅₀ 8.59 ± 0.37, n=6) and C-class (Emax: 2 ± 0.5%, n=6) vessels was significantly attenuated (P< 0.05), whilst the B-class and D-class responses were essentially unchanged (B:Emax: 9 ± 3%; pEC₅₀ 8.55 ± 0.23, n=6; D: Emax: 5 ± 2%; pEC₅₀ 8.29 ± 0.43, n=7). In vivo, in control rats, hU-II induced a small steady increase in mean RVP from 10.2 ± 0.5mmHg (pre-hU-II) to a maximum of 14.0 ± 1.2mmHg (100nmol.kg^-1 hU-II, P<0.01). Mean AOP was not significantly affected. In CH rats, mean RVP was doubled to 22.1 ± 1.7mmHg. hU-II did not induce a f^sturther increase in RVP and had no effect on AOP.

The results indicate that pulmonary constrictor responses to exogenous hU-II are not enhanced in CH rats under these experimental conditions. We have previously shown that in A class arteries (from control and CH rats) set up in organ baths under 1.5g tension, responses to hU-II in A Class vessels can be elevated in CH rat vessels and responses are also elevated by increased vascular tone in control rats (MacLean et al., 2000). Therefore applied basal tension and/or the in vitro experimental system used can influence responses to hU-II. An examination of selective hU-II antagonists in vivo is urgently required to determine the role of endogenous hU-II in the development of PAH in this model.

MacLean et al., (2000). Br. J. Pharmacol., 130, 201-204.
Keegan et al., (2001). Circ. Res., 89, 1231-1239.