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% O-2 for 21 days was 293.9 +/- 25.2 nM (vs. 153.6 +/- 28.7 nM in normoxia). Resting [Ca 21]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).