Revisiting the mechanism of hypoxic pulmonary vasoconstriction using isolated perfused/ventilated mouse lung

Hypoxic Pulmonary Vasoconstriction (HPV) is an important physiological mechanism of the lungs that matches perfusion to ventilation thus maximizing O₂ saturation of the venous blood within the lungs. This study emphasizes on principal pathways in the initiation and modulation of hypoxic pulmonary vasoconstriction with a primary focus on the role of Ca²⁺ signaling and Ca²⁺ influx pathways in hypoxic pulmonary vasoconstriction. We used an ex vivo model, isolated perfused/ventilated mouse lung to evaluate hypoxic pulmonary vasoconstriction. Alveolar hypoxia (utilizing a mini ventilator) rapidly and reversibly increased pulmonary arterial pressure due to hypoxic pulmonary vasoconstriction in the isolated perfused/ventilated lung. By applying specific inhibitors for different membrane receptors and ion channels through intrapulmonary perfusion solution in isolated lung, we were able to define the targeted receptors and channels that regulate hypoxic pulmonary vasoconstriction. We show that extracellular Ca²⁺ or Ca²⁺ influx through various Ca²⁺-permeable channels in the plasma membrane is required for hypoxic pulmonary vasoconstriction. Removal of extracellular Ca²⁺ abolished hypoxic pulmonary vasoconstriction, while blockade of L-type voltage-dependent Ca²⁺ channels (with nifedipine), non-selective cation channels (with 30 µM SKF-96365), and TRPC6/TRPV1 channels (with 1 µM SAR-7334 and 30 µM capsazepine, respectively) significantly and reversibly inhibited hypoxic pulmonary vasoconstriction. Furthermore, blockers of Ca²⁺-sensing receptors (by 30 µM NPS2143, an allosteric Ca²⁺-sensing receptors inhibitor) and Notch (by 30 µM DAPT, a γ-secretase inhibitor) also attenuated hypoxic pulmonary vasoconstriction. These data indicate that Ca²⁺ influx in pulmonary arterial smooth muscle cells through voltage-dependent, receptor-operated, and store-operated Ca²⁺ entry pathways all contribute to initiation of hypoxic pulmonary vasoconstriction. The extracellular Ca²⁺-mediated activation of Ca²⁺-sensing receptors and the cell-cell interaction via Notch ligands and receptors contribute to the regulation of hypoxic pulmonary vasoconstriction.