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

Abstract

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 Na⁺/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 μM) 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.