Endothelin-1 mediates hypoxia-induced inhibition of voltage-gated K+ channel expression in pulmonary arterial myocytes

Prolonged exposure to decreased oxygen tension causes contraction and proliferation of pulmonary arterial smooth muscle cells (PASMCs) and pulmonary hypertension. Hypoxia-induced inhibition of voltage-gated K+ (K-v) channels may contribute to the development of pulmonary hypertension by increasing intracellular calcium concentration ([Ca2+](i)). The peptide endothelin-1 (ET-1) has been implicated in the development of pulmonary hypertension and acutely decreases Kv channel activity. ET-1 also activates several transcription factors, although whether ET-1 alters K-v channel expression is unclear. The hypoxic induction of ET-1 is regulated by the transcription factor hypoxia-inducible factor-1 (HIF-1), which we demonstrated to regulate hypoxia-induced decreases in K-v channel activity. In this study, we tested the hypothesis that HIF-1-dependent increases in ET-1 lead to decreased Kv channel expression and subsequent elevation in [Ca2+](i). Resting [Ca2+](i) and Kv channel expression were measured in cells exposed to control (18% O-2, 5% CO2) and hypoxic (4% O-2, 5% CO2) conditions. Hypoxia caused a decrease in expression of K(v)1.5 and K(v)2.1 and a significant increase in resting [Ca2+](i). The increase in [Ca2+](i) was reduced by nifedipine, an inhibitor of voltage- dependent calcium channels, and removal of extracellular calcium. Treatment with BQ-123, an ET-1 receptor inhibitor, prevented the hypoxia-induced decrease in Kv channel expression and blunted the hypoxia-induced increase in [Ca2+] i in PASMCs, whereas ET-1 mimicked the effects of hypoxia. Both hypoxia and overexpression of HIF-1 under normoxic conditions increased ET-1 expression. These results suggest that the inhibition of Kv channel expression and rise in [Ca2+] i during chronic hypoxia may be the result of HIF-1-dependent induction of ET-1.