Angiotensin II can cause hypertension through enhanced vasoconstriction of renal vasculature. One proposed mechanism for reduction of angiotensin II-induced hypertension is through inhibition of the mitogen-activated protein kinase kinase (MEK)/ extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase cascade. MEK/ERK has been shown to phosphorylate the regulatory subunit of myosin light chain at identical positions as myosin light chain kinase. There are multiple mechanisms proposed regarding angiotensin II-mediated ERK activation. We hypothesized that renal microvascular smooth muscle cells (RVSMCs) signal through a unique pathway compared with thoracic aorta smooth muscle cells (TASMCs), which is involved in blood pressure regulation. Use of epidermal growth factor (EGF) and platelet derived growth factor (PDGF) receptor-specific inhibitors 4-(3-chloroanilino)-6,7-dimethoxyquinazoline (AG1478) and 6,7-dimethoxy-3-phenylquinoxaline (AG1296), respectively, demonstrates that angiotensin II activates ERK in TASMCs, but not RVSMCs, through transactivation of EGF and PDGF receptors. In addition, inhibition of Src with its specific inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo [3,4-d]pyrimidine (PP2) abolishes angiotensin II-, but not EGF-or PDGF-, mediated phosphorylation of ERK in RVSMCs, yet it has no effect in TASMCs. The physiological significance of transactivation was examined in vivo using anesthetized Wistar-Kyoto rats with 15 mg/kg 2Ј-amino-3Ј-methoxyflavone (PD98059), an MEK inhibitor, as well as 20 mg/kg AG1478 and 1.5 mg/kg AG1296 in an acute model of angiotensin II-mediated increase in blood pressure. None of the inhibitors had an effect on basal blood pressure, and only PD98059 reduced angiotensin II-mediated increase in blood pressure. Moreover, in RVSMCs, but not TASMCs, angiotensin II localizes phosphorylated ERK to actin filaments. In conclusion, angiotensin II signals through a unique mechanism in the renal vascular bed that may contribute to hypertension.Extracellular signal-regulated kinases ERK1 and ERK2 (herein referred to as ERK) are involved in smooth muscle cell contraction (Touyz et al., 1999), attenuation of vascular relaxation (Touyz et al., 2002a), and blood pressure control (Muthalif et al., 2000a,b;Hu et al., 2007); however, this is not a universal mechanism (Watts et al., 1998;Touyz et al., 2002a). In addition, the molecular mechanism underlying the role of ERK in control of vascular contraction is not completely understood; currently there are two proposed mechanisms. One pathway is an ERK-mediated phosphorylation of the 20-kDa myosin light chain regulatory subunit (MLC20) at the same position as myosin light chain kinase (D'Angelo and Adam, 2002;Roberts, 2004). A second pathway involves ERK-mediated phosphorylation of caldesmon (Adam and Hathaway, 1993;D'Angelo et al., 1999). Phosphorylation of This work was supported by National Institutes of Health Grants HL074940 (Georgetown University; to Dr. Pedro A. Jose) and Grant DK52612 (to B.T.A.) a...