Tyrosine kinase inhibitors are promising for the treatment of severe pulmonary hypertension. Their therapeutic effects are postulated to be due to inhibition of cell growth-related kinases and attenuation of vascular remodeling. Their potential vasodilatory activities have not been explored. Vasorelaxant effects of the tyrosine kinase inhibitors imatinib, sorafenib, and nilotinib were examined in isolated pulmonary arterial rings from normal and pulmonary hypertensive rats. Phosphorylation of myosin light chain phosphatase and myosin light chain was assessed by Western blots. Acute hemodynamic effects of imatinib were tested in the pulmonary hypertensive rats. In normal pulmonary arteries, imatinib reversed serotoninand U46619-induced contractions in a concentration-dependent and endothelium-independent manner. Sorafenib and nilotinib relaxed U46619-induced contraction. Imatinib inhibited activation of myosin phosphatase induced by U46619 in normal pulmonary arteries. All three tyrosine kinase inhibitors concentration-dependently and completely reversed the spontaneous contraction of hypertensive pulmonary arterial rings unmasked by inhibition of nitric oxide synthase. Acute intravenous administration of imatinib reduced high right ventricular systolic pressure in pulmonary hypertensive rats, with little effect on left ventricular systolic pressure and cardiac output. We conclude that tyrosine kinase inhibitors have potent pulmonary vasodilatory activity, which could contribute to their long-term beneficial effect against pulmonary hypertension. Vascular smooth muscle relaxation mediated via activation of myosin light chain phosphatase (Ca 21 desensitization) appears to play a role in the imatinib-induced pulmonary vasodilation.Keywords: tyrosine kinase inhibitors; pulmonary hypertension; vasodilation; SU5416; Ca 21 sensitization Pulmonary hypertension (PH) is characterized by progressive narrowing of small pulmonary arteries and arterioles, which results in increased pulmonary vascular resistance and right ventricular pressure overload. Despite recent advances in treatment severe PH remains debilitating and fatal (1). Major factors that contribute to the complex pathogenesis of pulmonary arterial narrowing are sustained vasoconstriction and fixed vascular remodeling.Tyrosine kinase inhibitors (TKIs), such as imatinib, nilotinib, and sorafenib, are approved for the treatment of patients with malignant diseases (2, 3). Imatinib, an inhibitor of platelet-derived growth factor receptor (PDGFR), has also been reported as promising in the treatment of patients with severe PH (4). In addition, chronic treatment with imatinib or the tyrosine/serine/ threonine kinase inhibitor sorafenib attenuates PH in rodent models (5-7). The therapeutic effects of these TKIs against PH are attributed to inhibition of cell growth-related factors and subsequent suppression of vascular remodeling (5, 7). It is possible, however, that TKIs also induce vascular smooth muscle cell relaxation and vasodilation because cell growth-related kin...

We have previously reported that vasoconstrictor sensitivity to KCl (a receptor-independent and voltage-gated Ca influx-mediated vasoconstrictor) is augmented in the chronically hypoxic hypertensive rat pulmonary circulation probably through increased Rho kinase-mediated Ca sensitization. However, the upstream mechanism by which the RhoA/Rho kinase signaling pathway is activated is unknown. This study examined if endogenous endothelin-1 (ET-1) and serotonin (5-HT) play roles in the Rho kinase-mediated augmented vasoconstrictor response to KCl and the activation of RhoA in chronically hypoxic hypertensive rat pulmonary arteries. The augmented KCl vasoconstriction in hypertensive lungs was reduced by the ETA receptor antagonist BQ123, while a dual ETA/B antagonist had no further effects. A combination of BQ123 and a 5-HT1B/1D receptor antagonist, GR127935, was more effective than either agent alone. The combined antagonists also reduced augmented contractile sensitivity to KCl in hypertensive intrapulmonary arteries. Membrane-to-cytosol ratio of RhoA expression in hypertensive arteries was greater than that in normotensive arteries and was reduced by BQ123 and GR127935. These results suggest that stimulation of ETA and 5-HT1B/1D receptors by endogenous ET-1 and 5-HT, respectively, is involved in RhoA/Rho kinase-mediated increased Ca sensitization in the chronically hypoxic hypertensive rat pulmonary circulation.

Rho kinase-mediated vasoconstriction rather than fixed arterial wall thickening is responsible for increased pulmonary vascular resistance and pulmonary hypertension in chronically hypoxic and monocrotaline-injected rats. In the absence of vascular tone, the medial and adventitial thickening in these models has only minimal impact on the cross-sectional area of the pulmonary arterial bed. In contrast, increased pulmonary vascular resistance in left-pneumonectomized plus monocrotaline-injected rats and VEGF receptor blocker-injected plus chronic hypoxia rats is attributable to both Rho kinase-mediated vasoconstriction and formation of lumen obliterating lesions in small pulmonary arteries. The upstream signals responsible for activation of RhoA/Rho kinase signaling in hypertensive pulmonary arteries and whether or not they differ in different forms of pulmonary hypertension are unclear. The RhoA/Rho kinase pathway is a convergence point of several different vasoconstrictor signals, including those mediated by G protein-coupled receptors, receptor tyrosine kinases, and integrin clustering. Both isoforms of Rho kinase can also be constitutively activated by cleavage, and cleaved Rho kinase 1 has been detected in the hypertensive lungs of left-pneumonectomized plus monocrotaline-injected rats. That such diverse stimuli can lead to activation of Rho kinase, which may cause hypercontraction of smooth muscle by promoting both actomyosin interaction and remodeling of the cytoskeleton, may explain why in various rat models of pulmonary hypertension Rho kinase inhibitors are more effective pulmonary vasodilators than conventional agents such as nitric oxide, prostacyclin, and nifedipine. We suspect the same will be true in at least some forms of human pulmonary arterial hypertension.