, and pyridoxalphosphate-6-azophenyl-2,-4-disulfonic acid (100 M) as well as apyrase (5 units/ml) attenuated hypoxia-and ATP-induced and DNA synthesis, indicating activation and a functional role of purinoceptors under hypoxic conditions. ATP-induced DNA synthesis was augmented by hypoxia in an additive fashion, whereas ATP and hypoxia synergistically increased growth factor-induced DNA synthesis, again suggesting that ATP and hypoxia utilize similar signaling pathways to induce proliferation. Indeed, we found that ATP (100 M) and hypoxia (3% O 2 ) induced expression and activation of Egr-1 transcription factor, and both stimuli acted, in part, through a G␣ i /ERK1/2-dependent signaling pathway. Suramin, Cibacron blue 3GA, and apyrase attenuated hypoxia-induced ERK1/2 activation and Egr-1 expression. We conclude that hypoxia induces ATP release from endothelial cells and fibroblasts and that the activation of P2 purinoceptors is involved in the regulation of DNA synthesis by fibroblasts under hypoxic conditions. Hypoxia has been shown to act as a direct proliferative stimulus for fibroblasts in a variety of organs. This capability of fibroblasts is unusual, at least among mesenchymally derived cells, and appears to be important in normal development, wound healing, and fibrosis as well as in the vascular changes that characterize hypoxic pulmonary hypertension (1-3). With regard to pulmonary artery adventitial fibroblasts, we have shown that among the resident vascular wall cells they exhibit the earliest and most dramatic responses to hypoxic exposure in vivo (4). In tissue culture we have also demonstrated that hypoxia in the absence of exogenous mitogens induces proliferation of pulmonary artery fibroblasts as well as some subpopulations of aortic adventitial fibroblasts. This response is due in large part to G␣ i/o -mediated activation of a complex network of mitogen-activated protein kinases, whose specific contributions to hypoxia-induced proliferation differ from those of serum-induced growth signals (5). It remains unclear, however, whether either activation or augmentation of the hypoxiainduced growth response is due at least in part to autocrine/ paracrine responses to factors secreted by fibroblasts during hypoxia, which act through G-protein-coupled pathways.One factor that could contribute to such an autocrine loop is ATP. Purines and pyrimidines (mainly ATP, ADP, adenosine, and UTP) have widespread and specific extracellular signaling actions in the regulation of a variety of functions in many tissues and appear to have key roles in development, proliferation, differentiation, and release of hormones, neurotransmitters, and cytokines (6 -10). It is also becoming evident that alterations in the physiology of purinergic signaling may result in the development of a variety of pathologies including disorders of the immune system, neurodegenerative, and vascular diseases (7). Extracellular ATP can, in fact, stimulate the growth and proliferation of vascular smooth muscle cells (SMC), 1 and this response...
