Vascular smooth muscle hypertrophy in resistance vessels may play an important role in the ethiology of hypertension. [1][2][3][4][5] Mechanical strain plays a pivotal role in the development of vascular hypertrophy in hypertension.6) However, its exact mechanisms remain unknown.Mitogen-activated protein kinases (MAPKs), members of a family of serine/threonine-specific protein kinases, 7) are believed to be involved in the pathway of cell proliferation and thus in vascular structural remodeling.8-10) Angiotensin II and endothelins stimulate MAPKs, leading to stimulation of protein synthesis. [11][12][13][14] We have previously reported that in isolated perfused rat aortae, increases in perfusion pressure cause a pressure-dependent increase in the activity of MAPKs.15) The MAPK activation was inhibited by the angiotensin receptor antagonist losartan and the angiotensinconverting enzyme inhibitor captopril. These findings suggest that pressure loading of the vascular wall per se can activate MAPKs in the vasculature and that the MAPK activation is mediated at least partly via the vascular angiotensin system.We also demonstrated that the pressure loading-induced MAPK activation still occurred in endothelium-denuded aortae.15) Endothelial and vascular smooth muscle cell (VSMC) integrity is thought to be a crucial factor in the maintenance of the structural properties of the vascular wall. 16,17) Thus VSMC may be, at least in part, involved in pressure-induced MAPK activation. Mechanical stress causes MAPK activation in VSMC. 18,19) In VSMC, angiotensin and endothelin systems are present and regulate cell growth. [20][21][22][23][24] The purpose of the present study was to determine, using cultured rat VSMC, whether cyclic stretching causes increases in p42 and p44 MAPK activity and whether angiotensin and endothelin systems in VSMC are involved in stretch-induced MAPK activation.
MATERIALS AND METHODSCell Culture Rat aortic VSMC were isolated from the thoracic aortae of Wistar rats (200-250 g) using the method of Ross 25) as described previously. 26) In brief, stripped aortae were placed on polyethylene dishes (diameter 60 mm). Dulbecco's modified Eagle's medium (DMEM) containing 20% fetal calf serum, penicillin 100 U/ml, and streptomycin 100 mg/ml was added to the dishes to cover the aortae. The dishes were kept at 37°C in a humidified atmosphere of 95% air and 5% CO 2 . The cells growing from the explants reached confluence after about 14 d, were harvested by brief exposure to Hanks' medium supplemented with 0.25% trypsin and 0.02% ethylenediaminetetraacetic acid (EDTA)-sodium salt, and transferred to fresh dishes. The properties of the cells cultured showed a typical hills-and-valleys growth pattern and a-actin molecule upon immunohistochemical analysis using mouse anti-a-actin (Zymed Laboratories, Inc.), indicating that the cells were grown from VSMC.Primary cultures of VSMC were plated in 60-mm culture dishes (1ϫ10 5 cells/dish) containing DMEM with 10% fetal calf serum. The cells used in this study were in pas...