Endothelin-1 mediates hypoxia-induced inhibition of voltage-gated [K.sup.+] 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.sup.+] ([K.sub.v]) channels may contribute to the development of pulmonary hypertension by increasing intracellular calcium concentration ([[Ca.sup.2+]][sub.i]). The peptide endothelin-1 (ET-1) has been implicated in the development of pulmonary hypertension and acutely decreases [K.sub.v] channel activity. ET-1 also activates several transcription factors, although whether ET-1 alters [K.sub.v] channel expression is unclear. The hypoxic induction of ET-1 is regulated by the transcription factor hypoxia-inducible factor-1 (HIF1), which we demonstrated to regulate hypoxia-induced decreases in [K.sub.v] channel activity. In this study, we tested the hypothesis that HIF-l-dependent increases in ET-1 lead to decreased [K.sub.v] channel expression and subsequent elevation in [[Ca.sup.2+]][sub.i]. Resting [[Ca.sup.2+]][sub.i] and [K.sub.v] channel expression were measured in cells exposed to control (18% 02, 5% C[O.sub.2]) and hypoxic (4% [O.sub.2], 5% C[O.sub.2]) conditions. Hypoxia caused a decrease in expression of [K.sub.v]l.5 and [K.sub.v]2.1 and a significant increase in resting [[Ca.sup.2+]][sub.i]. The increase in [[Ca.sup.2+]][sub.i] was reduced by nifedipine, an inhibitor of voltage-dependent calcium channels, and removal of extracellular calcium. Treatment with BQ123, an ET-1 receptor inhibitor, prevented the hypoxia-induced decrease in [K.sub.v] channel expression and blunted the hypoxia-induced increase in [[Ca.sup.2+]][sub.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 [K.sub.v] channel expression and rise in [[Ca.sup.2+]][sub.i] during chronic hypoxia may be the result of HIF-1-dependent induction of ET-1. hypoxia-inducible factor-l; pulmonary hypertension