Functional ion channels in human pulmonary artery smooth muscle cells: Voltage-dependent cation channels.

The activity of voltage-gated ion channels is critical for the maintenance of cellular membrane potential and generation of action potentials. In turn, membrane potential regulates cellular ion homeostasis, triggering the opening and closing of ion channels in the plasma membrane and, thus, enabling ion transport across the membrane. Such transmembrane ion fluxes are important for excitation-contraction coupling in pulmonary artery smooth muscle cells (PASMC). Families of voltage-dependent cation channels known to be present in PASMC include voltage-gated K(+) (Kv) channels, voltage-dependent Ca(2+)-activated K(+) (Kca) channels, L- and T- type voltage-dependent Ca(2+) channels, voltage-gated Na(+) channels and voltage-gated proton channels. When cells are dialyzed with Ca(2+)-free K(+)- solutions, depolarization elicits four components of 4-aminopyridine (4-AP)-sensitive Kvcurrents based on the kinetics of current activation and inactivation. In cell-attached membrane patches, depolarization elicits a wide range of single-channel K(+) currents, with conductances ranging between 6 and 290 pS. Macroscopic 4-AP-sensitive Kv currents and iberiotoxin-sensitive Kca currents are also observed. Transcripts of (a) two Na(+) channel α-subunit genes (SCN5A and SCN6A), (b) six Ca(2+) channel α-subunit genes (α(1A), α(1B), α(1x), α(1D), α(1E) and α(1G)) and many regulatory subunits (α(2)δ(1), β(1-4), and γ(6)), (c) 22 Kv channel α-subunit genes (Kv1.1 - Kv1.7, Kv1.10, Kv2.1, Kv3.1, Kv3.3, Kv3.4, Kv4.1, Kv4.2, Kv5.1, Kv 6.1-Kv6.3, Kv9.1, Kv9.3, Kv10.1 and Kv11.1) and three Kv channel β-subunit genes (Kv(β1-3) and (d) four Kca channel α-subunit genes (Sloα1 and SK2-SK4) and four Kca channel (β-subunit genes (Kca(β1-4) have been detected in PASMC. Tetrodotoxin-sensitive and rapidly inactivating Na(+) currents have been recorded with properties similar to those in cardiac myocytes. In the presence of 20 mM external Ca(2+), membrane depolarization from a holding potential of -100 mV elicits a rapidly inactivating T-type Ca(2+) current, while depolarization from a holding potential of -70 mV elicits a slowly inactivating dihydropyridine-sensitive L-type Ca(2+) current. This review will focus on describing the electrophysiological properties and molecular identities of these voltage-dependent cation channels in PASMC and their contribution to the regulation of pulmonary vascular function and its potential role in the pathogenesis of pulmonary vascular disease.