The phosphorylation of myosin light chain (MLC) maintains the contracted state of vascular smooth muscle. Dephosphorylation results in relaxation and is determined by the activity of myosin light chain phosphatase (MLCP), which is negatively regulated by Rho kinase.
We tested whether an increased Rho kinase activity, and hence a decreased contribution of MLCP, results in an increased contractility of small fourth-order mesenteric arteries (MA) during the early onset of angiotensin II (Ang II)-induced hypertension (Ang II-14d).
Calcium sensitivity was similar, but contractile tension in response to [Ca2+]ex (5 mmol/l) in endothelium-denuded and depolarized MA was greater, in Ang II-14d rats compared to sham-operated normotensive (SHAM) and Ang II-1d. The Rho kinase inhibitor Y-27632 caused a significantly greater inhibition of the contractile response to various agents (phenylephrine, norepinephrine, U46619 and K+) in MA of Ang II-14d compared to SHAM. Protein expression levels of the GDP/GTP exchange factor PDZ-RhoGEF, which co-immunoprecipitated with RhoA, were increased in MA of Ang II-14d compared to SHAM. RhoA translocation was greater in U46619 (1 μmol/l)-stimulated MA of Ang II-14d compared to SHAM. Expression levels of Rho kinase β were higher in MA of Ang II-14d. The MLCP inhibitor calyculin A (100 nmol/l) caused a greater contraction in MA of SHAM compared to Ang II-14d. Phosphorylation of the target subunit of MLCP (MYPT1) was enhanced in U46619-stimulated MA of Ang II-14d compared to SHAM.
This is the first study demonstrating enhanced PDZ-RhoGEF/RhoA/Rho kinase signaling during hypertension at the level of resistance-sized arteries. This enhanced signaling leads to increased MLCP phosphorylation, resulting in vascular hyper-reactivity.
Department of Physiology, Medical College of Georgia, Augusta, Georgia, USA
Received 4 January, 2007
Revised 12 March, 2007
Accepted 14 March, 2007
Correspondence to Dr Rob H. P. Hilgers, Department of Physiology, Medical College of Georgia, Augusta, GA 30912-3000, USA Tel: +1 706 721 3547; fax: +1 706 721 7299; e-mail: email@example.com