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.