Hypoxic pulmonary vasoconstriction (HPV) is a widely conserved, homeostatic, vasomotor response of resistance pulmonary arteries to alveolar hypoxia. HPV mediates ventilation-perfusion matching and, by reducing shunt fraction, optimizes systemic PO₂. HPV is intrinsic to the lung, and, although modulated by the endothelium, the core mechanism resides within the smooth muscle cell (SMC). Acute hypoxia induces pulmonary vasoconstriction mediated by O₂-sensitive Kv channels (Pozeg et al. 2003) and voltage gated calcium channels (VGCCs) (Moudgil et al. 2005), whilst chronic hypoxia causes structural changes of the pulmonary vasculature including arterial hypertrophy, but the mechanism remains unclear. It is widely accepted that intracellular Ca²⁺ serves as an important signal in regulating contraction and proliferation of pulmonary artery smooth muscle cells (Ward et al. 2004). In this study, we investigated the plasticity of calcium handling ion channels and pumps in human pulmonary artery smooth muscle cells (Promocell) under chronic hypoxia stress using real time, quantitative PCR (ABI 7900) with calcium imaging and patch clamping methods. Chronic hypoxia was induced in a hypoxia incubator with 1% O₂, 5% CO₂, balance N₂. We determined the expression levels of a number of ion channels including: TRPC3, TRPC4, TRPC5, TRPC6, TRPC7, P2X1, P2X4, P2X7, P2Y1, P2Y4, P2Y6, NCX (Na-Ca exchanger), NCKX (Na-K-Ca exchanger), VGCC α1C, VGCCα1G, VGCCα1H subunits and TASK2 channels compared with GADPH. We found mRNA levels for most of the channels examined were upregulated after 60 h exposure to 1% O₂ compared with cells growing in room air O₂ tension (with 5% CO₂). Noticeably, there were 127-fold increases in P2X1 receptor expression, 32-fold increases in TASK2 channel expression and 27-fold increases in TRPC5 channels expression. In a functional, follow up study, we measured intracellular calcium with conventional imaging methods, using Oregon Green (Molecular Probes). Our data showed that 100 μM ATP induced a dramatic cytoplasmic calcium increase, which was partially quenched by 100 μM suramin. 10 μM αβmethylene ATP also elicited an intracellular calcium rise. Nifedipine, 10 μM and 20μM, were used in our experiments and they exhibited a similar blockage effect on intracellular calcium elevation, suggesting that VGCCs were not the only resource contributing to the calcium rise. Whilst further studies to measure the protein levels of ion channels regulated during chronic hypoxia stress are required, these results suggest that P2 receptors might play an important role in HPV under chronic hypoxia.