Mobilization of sarcoplasmic reticulum stores by hypoxia leads to consequent activation of capacitative Ca2+ entry in isolated canine pulmonary arterial smooth muscle cells

Capacitative Ca2+ entry (CCE) has been speculated to contribute to Ca2+ influx during hypoxic pulmonary vasoconstriction (HPV). The aim of the present study was to directly test if acute hypoxia causes intracellular Ca2+ concentration ([Ca2+](i)) rises through CCE in canine pulmonary artery smooth muscle cells (PASMCs). In PASMCs loaded with fura-2, hypoxia produced a transient rise in [Ca2+](i) in Ca2+-free solution, indicating Ca2+ release from the intracellular Ca2+ stores. Subsequent addition of 2 mm Ca2+ in hypoxia elicited a sustained rise in [Ca2+](i), which was partially inhibited by 10 mu m nisoldipine. The dihydropyridine-insensitive rise in [Ca2+](i) was due to increased Ca2+ influx, because it was abolished in Ca2+-free solution and hypoxia was shown to significantly enhance the rate of Mn2+ quench of fura-2 fluorescence. The dibyropyridine-insensitive rise in [Ca2+](i) and the increased rate of Mn2+ quench of fura-2 fluorescence were inhibited by 50 mu m SKF 96365 and 500 mu m Ni2+, but not by 100 mu m La3+ or 100 mu m Gd3+, exhibiting pharmacological properties characteristic of CCE. In addition, predepletion of the intracellular Ca2+ stores inhibited the rise in [Ca2+](i) induced by hypoxia. These results provide the first direct evidence that acute hypoxia, by causing Ca2+ release from the intracellular stores, activates CCE in isolated canine PASMCs, which may contribute to HPV.