Inhibition of the
type 5 phosphodiesterase and inhibition of
Rho kinase are both effective in reducing
pulmonary hypertension (PH). Here we investigate whether
Rho kinase inhibition is involved in the beneficial effect of the
type 5 phosphodiesterase inhibitor
sildenafil on PH. Chronic
hypoxia-induced PH in rats is associated with an increase in RhoA activity in pulmonary artery that was maximal after 2 days (10.7+/-0.9-fold increase, n=6, P<0.001). The activity of
Rho kinase assessed by measuring the level of
myosin phosphatase target subunit 1 (MYPT1) phosphorylation was also increased (5.7+/-0.8-fold over control, n=8). Chronic
fasudil (30 mg kg(-1) day(-1); 14 days) and
sildenafil (25 mg kg(-1) day(-1); 14 days) treatments reduced PH and pulmonary cardiovascular remodelling, and inhibited the MYPT1 phosphorylation in pulmonary artery from hypoxic rats by 82.3+/-3% (n=4) and by 76.6+/-2% (n=4), respectively. The inhibitory effect of
sildenafil (10 microM) on MYPT1 phosphorylation was demonstrated by the loss of actin stress fibres in vascular smooth muscle cells. However, in vitro
kinase assays indicated that
sildenafil had no direct inhibitory action on
Rho kinase activity.
Sildenafil treatment induced increased RhoA phosphorylation and association to its cytosolic inhibitory
protein,
guanine dissociation inhibitor (GDI) in pulmonary artery.We propose that
sildenafil inhibits RhoA/
Rho kinase-dependent functions in pulmonary artery through enhanced RhoA phosphorylation and cytosolic sequestration by GDI. The inhibition of intracellular events downstream of RhoA thus participates in the beneficial effect of
sildenafil on PH.