Abstract

Catecholamines may influence vascular smooth muscle cell (SMC) growth and vascular hypertrophic diseases. We previously demonstrated that stimulation of α₁-adrenoceptors (AR) causes hypertrophy of vascular SMCs in vitro and in situ. Here, we used adult rat aorta SMCs that express α_1D- and α_1B-ARs (but not α_1A-ARs) in vitro to examine the mechanisms and α₁-AR subtypes involved. Norepinephrine (NE) increased protein synthesis and content in a time- and dose-dependent manner. To identify the responsible α₁-AR subtype, we first documented the selectivity of two α₁-AR subtype antagonists, BMY 7378 (α_1D-AR antagonist) and chloroethylclonidine (CEC; α_1B-AR antagonist), using Rat-1 fibroblasts stably transfected with the three different rodent α₁-AR cDNAs. NE dose-dependently increased protein synthesis in each cell line. In α_1D fibroblasts, BMY 7378 inhibited growth and protected α_1D-ARs from CEC alkylation while having little blocking or protecting effect on the growth induced by stimulation of fibroblasts that express α_1A- or α_1B-ARs. In rat aorta SMCs, pretreatment with CEC in the presence of BMY 7378 to protect α_1D-ARs had no effect on NE-induced protein synthesis. BMY 7378 inhibited the SMC growth response with a pK _b of 8.4. NE caused rapid and transient p42-p44 mitogen-activated protein kinase (MAPK) activation that was α_1D-AR dependent. Furthermore, NE caused tyrosine phosphorylation of multiple cellular proteins, phosphorylation of Raf-1, and stimulation of c-fos mRNA expression in aorta SMCs. The selective MAPK kinase inhibitor PD 98059 inhibited NE-induced protein synthesis and MAPK activation with IC₅₀values of 2.3 and 1.6 μm, respectively. These data demonstrate that SMC growth induced by NE is mediated by α_1D-ARs that couple to activation of the MAPK cascade.