The roles of serotonin, human urotensin II and calcium sensitivity in the regulation of pulmonary arterial tone

Pulmonary arterial hypertension (PAH) is a rare, progressive fatal disease characterised by persistent vasoconstriction and vascular remodelling of the small pulmonary arteries. 1) The endogenous peptide human urotensin II (hU-II) has recently been found to be a potent vasoconstrictor of the rat main pulmonary artery and the response was reported to be augmented in the chronically hypoxic rat model of PAH. 2) Increased basal Ca2+ sensitivity and enhanced contractile responses to 5-HT, have been reported to be augmented in the chronically hypoxic rat model of PAH. Additionally expression of the 5-HT transporter, 5-HTT, has been reported to be enhanced following exposure to chronic hypoxia and its over expression linked to PAH. These reports are controversial. Aims 1) To profile the vasoconstrictor and vasodilator responses to hU-II along the C57BL/6xCBA mouse and Wistar rat pulmonary arterial trees (extra/intra-lobar) and determine if the contractile activity of hU-II is augmented in rodent models of PAH. 2) To determine if basal Ca2+ sensitivity in the Wistar rat small pulmonary artery is augmented following exposure to chronic hypoxia. 3) To determine the influence of 5-HT and 5-HTT over-expression on Ca2+ sensitivity in Wistar rat, and C57BL/6xCBA mouse, small pulmonary arteries respectively. Results hU-II caused a potent but low efficacy contractile response in Wistar rat extra-lobar (Main pulmonary artery: pEC50; 8.64 +/- 0.17, Emax: 19 +/- 4% of the response to 50mM KC1 11=9, 1st branch; pEC50; 8.57 +/- 0.16, Emax: 9 +/- 2%, n=13) and 1st generation intra-lobar (pECso 8.54 +/- 0.19, Emax: 13 + 3%, n==14) vessels but had essentially no action on the smaller pulmonary arteries. hU-II failed to dilate any pre-constricted rat vessel tested and the contractile responses to hU-II were not augmented in rat models of PAH. Furthermore, hU-II had no effect on any mouse pulmonary vessel tested. 2) Basal Ca2+ sensitivity in beta-escin permeabilised, Wistar rat small pulmonary arteries was found to be slightly attenuated (pECso: Control; 6.33 +/- 0.02 vs CH; 6.24 +/- 0.02, n=10-11) following exposure to 2weeks chi'onic hypoxia, consistent with a decrease in Rho Kinase activity/Rho A expression. Neither 5-HT, nor 5-HTT over-expression augmented Ca2+ sensitivity. Conclusion hU-II can increase pulmonary vascular tone in the normal Wistar rat but the data was not consistent with a role in the pathogenesis of PAH in rodents. Altered Ca2+ sensitivity was not found to be responsible for the persistent elevated basal tone of the chronically hypoxic rat model of PAH. Furthermore 5-HT and 5-HTT overexpression did not exert their effects on vasomotor tone through altered Ca2+ sensitivity