L-type Ca²⁺ channels, resting [Ca²⁺](i), and ET-1-induced responses in chronically hypoxic pulmonary myocytes

In the lung, chronic hypoxia (CH) causes pulmonary arterial smooth muscle cell (PASMC) depolarization, elevated endothelin-1 (ET-1), and vasoconstriction. We determined whether, during CH, depolarization-driven activation of L-type Ca²⁺ channels contributes to 1) maintenance of resting intracellular Ca²⁺ concentration ([Ca²⁺](i)), 2) increased [Ca²⁺](i) in response to ET-1 (10^-8 M), and 3) ET-1-induced contraction. Using indo 1 microfluorescence, we determined that resting [Ca²⁺](i) in PASMCs from intrapulmonary arteries of rats exposed to 10% O₂ for 21 days was 293.9 ± 25.2 nM (vs. 153.6 ± 28.7 nM in normoxia). Resting [Ca²⁺](i) was decreased after extracellular Ca²⁺ removal but not with nifedipine (10^-6 M), an L-type Ca²⁺ channel antagonist. After CH, the ET-1-induced increase in [Ca²⁺](i) was reduced and was abolished after extracellular Ca²⁺ removal or nifedipine. Removal of extracellular Ca²⁺ reduced ET-1-induced tension; however, nifedipine had only a slight effect. These data indicate that maintenance of resting [Ca²⁺](i) in PASMCs from chronically hypoxic rats does not require activation of L-type Ca²⁺ channels and suggest that ET-1-induced contraction occurs by a mechanism primarily independent of changes in [Ca²⁺](i).