Fig. (2) Mechanism of acute hypoxic vasoconstriction in human pulmonary artery. ROS= reactive oxygen species, SR= sarcoplasmic reticulum, TRPC=transient receptor potential channels. Redox hypothesis: Hypoxia decreases the ROS level and cause vasoconstriction through inhibition of K⁺ channels. Energy hypothesis: hypoxia shifts the energy production cycle and produces more AMP that increases the intracellular Ca²⁺ through SR. ROS hypothesis: hypoxia augments the production of ROS that cause HPV through SR induced release of Ca²⁺.