Hypoxic pulmonary vasoconstriction (HPV) is a physiological response to localized alveolar hypoxia that is intrinsic to the pulmonary circulation. By hypoxia-induced contraction of pulmonary arterial smooth muscle cells (PASMCs), pulmonary capillary blood flow is redirected to alveolar areas of high oxygen partial pressure (pO2), thus maintaining the ventilation-perfusion ratio(1). Although the principle of HPV was recognized decades ago the underlying pathway still remains elusive. Voltage gated K+-channels (Kv-channels) are known to be redox-sensitive and crucial for mediating membrane potential in PASMCs – thereby controlling Ca2+-entry and subsequently vascular tone. Here, we investigated whether acute hypoxia alters the redox state of isolated PASMCs from mice and if this impacts Kv-channel activity, membrane potential and intracellular Ca2+- level. Kv-currents and membrane potential in response to hypoxia or the oxidizing agent H2O2 (124 nM) were investigated by whole cell patch clamp experiments (voltage and current clamp, resp.) on freshly isolated murine PASMCs. Simultaneously, pO2 was recorded using an optical oxygen sensor and intracellular Ca2+ was measured by ratiometric FURA-2 imaging. The redox state was monitored by Raman spectroscopy using an excitation wavelength that was in resonance with the hemeproteins (532 nm). Raman-spectra values for reduced myoglobin (1607 cm-1) and cytochrome c (1622 cm-1) both increased upon exposure of PASMCs to hypoxia (4 % O2) and returned to the initial value upon reverting to normoxia (21 % O2), thus indicating a shift in the cells’ redox state. In accordance, Kv-currents were dose-dependently inhibited by hypoxia, resulting in a depolarization from -37,5 ± 2 mV (before) to -28.1 ± 2 mV after exposure to hypoxia (Mean ± SEM; n = 23; p = < 0,001; paired t-test) – thereby triggering a rise in intracellular Ca2+. The oxidizing agent H2O2 (124 nM) mimicked this effect under normoxic conditions by elevating the membrane potential of the PASMCs from -37.4 ± 3 mV (before) to -25,2 ± 4 mV (Mean ± SEM; n = 5, p = 0,005, paired t-test). In conclusion, hypoxia induced a shift in the redox state of PASMCs, thereby mediating the inhibition of Kv-channels. The resulting depolarization and Ca2+-entry may account for the contraction of PASMCs – thus initiating HPV.