Chronic hypoxia elevates intracellular pH and activates Na ⁺/H⁺ exchange in pulmonary arterial smooth muscle cells

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 (pH_i) mediated by activation of Na⁺/H⁺ exchange (NHE); however, the effect of CH on PASMC pH_i homeostasis is unknown. Thus we measured basal pH_i and NHE activity and expression in PASMCs isolated from mice exposed to normoxia or CH (3 wk/10% O₂). pH_i 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 RT-PCR and immunoblot. PASMCs from chronically hypoxic mice exhibited elevated basal pH_i 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 pH_i, NHE activity, and NHE1 expression, changes that may contribute to the development of pulmonary hypertension, in part, via pH-dependent induction of PASMC proliferation.