MITOGENIC EFFECTS OF ATP ON VASCULAR SMOOTH-MUSCLE CELLS VS OTHER GROWTH-FACTORS AND SYMPATHETIC COTRANSMITTERS

The sympathetic nervous system has been shown to exert a trophic influence on vascular smooth muscle cells (VSMC). Therefore, we studied the growth-regulating effects of the sympathetic cotransmitters ATP, neuropeptide Y (NPY), and norepinephrine (NE). ATP in concentrations of 1-100 muM greatly increased the incorporation of [H-3]thymidine in VSMC from rat aorta and vena cava. ATP also increased cell number and total protein content. The maximal effect on [H-3]thymidine incorporation was greater than for epidermal growth factor (20 ng/ml) or insulin (1 mug/ml) and approximately one-half that of 10% fetal calf serum. The potency series of other nucleotides and analogues of ATP was ATP > beta,gamma-methyleneATP (AMP-PCP) > ADP > adenosine > alpha,beta-methyleneATP (AMP-CPP) > 2-methylthioATP, indicating involvement of a P2 receptor; however, it does not meet proposed pharmacological criteria of either the P2x or P2y subclass. Several proposed P2 receptor antagonists were without effect. The effect of ATP could be mediated by a ''nucleotide receptor,'' since UTP also stimulated [H-3]thymidine incorporation. In our model, there was a strong correlation between the mitogenic effects of ATP, AMP-CPP, AMP-PCP, and UTP and their ability to stimulate influx of extracellular Ca2+ (Ca(o)2+). Moreover, the mitogenic effect of ATP was increased by high concentrations of Ca(o)2+. Taken together with data showing the lack of involvement of several other second-messenger systems, this indicates a critical role for Ca(o)2+ in mediating the mitogenic effects of ATP. Amiloride, known to inhibit the action of several growth factors, also inhibited ATP-induced mitogenesis. As previously shown, NE had mitogenic effects, although the catecholamine was weaker than ATP. NPY had a small but significant mitogenic effect. In conclusion, extracellular ATP may be an important sympathetic mediator of vascular smooth muscle growth. Because the nucleotide may derive from many sources, including sympathetic nerves as well as platelets, endothelial cells, and damaged smooth muscle cells, ATP may play a role in human disease, e.g., atherosclerosis and hypertension.