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Vascular remodeling resulting from altered pulmonary arterial smooth muscle cell (PASMC) growth is a contributing factor to the pathogenesis of hypoxic pulmonary hypertension. PASMC growth requires an alkaline shift in intracellular pH (pHi) and we previously showed that PASMCs isolated from mice exposed to chronic hypoxia exhibited increased Na+/H+ exchanger (NHE) expression and activity, which resulted in increased pHi. However, the mechanism by which hypoxia caused these changes was unknown. In this study we tested the hypothesis that hypoxia-induced changes in PASMC pH homeostasis are mediated by the transcriptional regulator hypoxia-inducible factor 1 (HIF-1). Consistent with previous results, increased NHE isoform 1 (NHEI) mRNA and protein, enhanced NHE activity, and an alkaline shift in pHi were observed in PASMCs isolated from wild-type mice exposed to chronic hypoxia (3 wk at 10% O2). In contrast, these changes were absent in PASMCs isolated from chronically hypoxic mice with partial deficiency for HIF-1. Exposure of PASMCs to hypoxia ex vivo (48 h at 4% O2) or overexpression of HIF-1 in the absence of hypoxia also increased NHE1 mRNA and protein expression. Our results indicate that full expression of HIF-1 is essential for hypoxic induction of NHE1 expression and changes in PASMC pH homeostasis and suggest a novel mechanism by which HIF-1 mediates pulmonary vascular remodeling during the pathogenesis of hypoxic pulmonary hypertension. [ABSTRACT FROM AUTHOR]

Chronic hypoxia (CH), caused by many lung diseases, results in pulmonary hypertension due, in part, to increased muscularity of small pulmonary vessels. Pulmonary arterial smooth muscle cell (PASMC) proliferation in response to growth factors requires increased intracellular pH (pHi) mediated by activation of Na+/H+ exchange (NHE); however, the effect of CH on PASMC pHi homeostasis is unknown. Thus we measured basal pHi and NHE activity and expression in PASMCs isolated from mice exposed to normoxia or CH (3 wk/10% O2). pHi was measured using the pH-sensitive fluorescent dye BCECF-AM. NHE activity was determined from Na+-dependent recovery from NH4-induced acidosis, and NHE expression was determined by RTPCR and immunoblot. PASMCs from chronically hypoxic mice exhibited elevated basal pHi and increased NHE activity. NHE1 was the predominate isoform present in mouse PASMCs, and both gene and protein expression of NHE1 was increased following exposure to CH. Our findings indicate that exposure to CH caused increased pHi, NHE activity, and NHE1 expression, changes that may contribute to the development of pulmonary hypertension, in part, via pH-dependent induction of PASMC proliferation. [ABSTRACT FROM AUTHOR]