Effects of catecholamines on DNA synthesis in vascular smooth muscle cells (VSMC) were investigated in a chemically defined medium that included insulin, transferrin, and sodium selenite. Smooth muscle-rich preparation was obtained from rat aortic media and VSMC were further purified by cell cloning. A clone that was positive for smooth muscle actin and was negative for the coagulation factor VIII was used in this study. The fetal calf serum-induced proliferation was enhanced by a-adrenergic and inhibited by 3-adrenergic stimulation. When cells of low passages were used, dose-response curves for norepinephrine were biphasic; when cells were subconfluent, norepinephrine stimulated DNA synthesis at as low as 1 n and was apparently ineffective at more than 100 nM. When cells were confluent, the effect of norepinephrine was inhibitory at lower concentrations(< 1 nM) and stimulatory at relatively higher concentrations. Cells of higher passages exhibited only inhibitory effects of the amine. Stimulatory and inhibitory effects on DNA synthesis were mediated through c-and fl2-adrenergic receptors, respectively. Thus, the a1-agonist phenylephrine was more potent than the a2-agonist clonidine in stimulating DNA synthesis. An a1-adrenergic antagonist, prazosin, was more effective than the a2-adrenergic antag-onist yohimbine in antagonizing the stimulatory effect of norepinephrine. f3-Adrenergic agonists inhibited DNA synthesis with IC50 values in the nanomolar range; the rank order of potency of agonists was isoproterenol> salbutamol(-)-epinephrine>>(-)-norepinephrine, consistent with $2-receptor specificity.(+)-Epinephrine or (+)-norepinephrine, the stereoisomers of the catecholamines, were ineffective. The inhibitory effects of norepi-nephrine were reversed by f3-adrenergic antagonists, with the rank order of potency of pindolol> butoxamine> atenolol, consistent with f32-receptor specificity. The dose-response curves of norepinephrine, therefore, seemed to be determined by a balance between a1-receptor-mediated stimulation and fl2-receptor-mediated inhibition of DNA synthesis. Minimum time required for exhibiting a1-adrenergic or/32-adrenergic effects was between 6 and 15 hr, suggesting that the G0 or G1 phase of the cell cycle might be the site of action. These results show that catecholamines dually modulate DNA synthesis in VSMC through specific adrenergic receptors.

Among major risk factors for atherosclerosis in humans, hypertension, psychological stress, and cigarette smoking are related to catecholamines(1-3). In fact, it has been shown experimentally that catecholamines aggravate atherosclerosis in animals and humans(4-6). Circulating catecholamines may affect the functions of many tissues. It is established that abnormal proliferation of VSMC is a key event in early stages of artherosclerosis(7). Therefore, it is possible that catecholamines have direct effects on the proliferation of VSMC. For this purpose, we have used pure VSMC, because the conventional primary culture of VSMC from rat aorta is often contaminated with endothelial cells whose DNA synthesis is known to be modulated by catecholamines(8), implying that data on the mixed population does not necessarily reflect those