Upregulation of Na+/Ca2+ exchanger contributes to the enhanced Ca2+ entry in pulmonary artery smooth muscle cells from patients with idiopathic pulmonary arterial hypertension

A rise in cytosolic Ca2+ concentration ([Ca2+](cyt)) in pulmonary artery smooth muscle cells ( PASMC) is a trigger for pulmonary vasoconstriction and a stimulus for PASMC proliferation and migration. Multiple mechanisms are involved in regulating [Ca2+](cyt) in human PASMC. The resting [Ca2+](cyt) and Ca2+ entry are both increased in PASMC from patients with idiopathic pulmonary arterial hypertension ( IPAH), which is believed to be a critical mechanism for sustained pulmonary vasoconstriction and excessive pulmonary vascular remodeling in these patients. Here we report that protein expression of NCX1, an NCX family member of Na2+/Ca2+ exchanger proteins is upregulated in PASMC from IPAH patients compared with PASMC from normal subjects and patients with other cardiopulmonary diseases. The Na+/Ca2+ exchanger operates in a forward ( Ca2+ exit) and reverse ( Ca2+ entry) mode. By activating the reverse mode of Na+/Ca2+ exchange, removal of extracellular Na+ caused a rapid increase in [Ca2+] cyt, which was significantly enhanced in IPAH PASMC compared with normal PASMC. Furthermore, passive depletion of intracellular Ca2+ stores using cyclopiazonic acid (10 mu M) not only caused a rise in [Ca2+] cyt due to Ca2+ influx through store-operated Ca2+ channels but also mediated a rise in [Ca2+] cyt via the reverse mode of Na+/Ca2+ exchange. The upregulated NCX1 in IPAH PASMC led to an enhanced Ca2+ entry via the reverse mode of Na+/ Ca2+ exchange, but did not accelerate Ca2+ extrusion via the forward mode of Na+/Ca2+ exchange. These observations indicate that the upregulated NCX1 and enhanced Ca2+ entry via the reverse mode of Na+/ Ca2+ exchange are an additional mechanism responsible for the elevated [ Ca2+] cyt in PASMC from IPAH patients.