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Membrane Transporters, Ion Channels, and Pumps

Orai1, a critical component of store-operated Ca²⁺ entry, is functionally associated with Na⁺/Ca²⁺ exchanger and plasma membrane Ca²⁺ pump in proliferating human arterial myocytes

Department of Physiology, University of Maryland School of Medicine, Baltimore, Maryland

Submitted 30 June 2009 ; accepted in final form 7 August 2009

Ca²⁺ entry through store-operated channels (SOCs) in the plasma membrane plays an important role in regulation of vascular smooth muscle contraction, tone, and cell proliferation. The C-type transient receptor potential (TRPC) channels have been proposed as major candidates for SOCs in vascular smooth muscle. Recently, two families of transmembrane proteins, Orai [also known as Ca²⁺ release-activated Ca²⁺ channel modulator (CRACM)] and stromal interacting molecule 1 (STIM1), were shown to be essential for the activation of SOCs mainly in nonexcitable cells. Here, using small interfering RNA, we show that Orai1 plays an essential role in activating store-operated Ca²⁺ entry (SOCE) in primary cultured proliferating human aortic smooth muscle cells (hASMCs), whereas Orai2 and Orai3 do not contribute to SOCE. Knockdown of Orai1 protein expression significantly attenuated SOCE. Moreover, inhibition of Orai1 downregulated expression of Na⁺/Ca²⁺ exchanger type 1 (NCX1) and plasma membrane Ca²⁺ pump isoform 1 (PMCA1). The rate of cytosolic free Ca²⁺ concentration decay after Ca²⁺ transients in Ca²⁺-free medium was also greatly decreased under these conditions. This reduction of Ca²⁺ extrusion, presumably via NCX1 and PMCA1, may be a compensation for the reduced SOCE. Immunocytochemical observations indicate that Orai1 and NCX1 are clustered in plasma membrane microdomains. Cell proliferation was attenuated in hASMCs with disrupted Orai1 expression and reduced SOCE. Thus Orai1 appears to be a critical component of SOCE in proliferating vascular smooth muscle cells, and may therefore be a key player during vascular growth and remodeling.

C-type transient receptor potential proteins; sarcoplasmic reticulum Ca²⁺ stores; proliferation