Molecular and Functional Significance of Ca²⁺‐Activated Cl⁻ Channels in Pulmonary Arterial Smooth Muscle

Abstract

Increased peripheral resistance of small distal pulmonary arteries is a hallmark signature of pulmonary hypertension (PH) and is believed to be the consequence of enhanced vasoconstriction to agonists, thickening of the arterial wall due to remodeling, and increased thrombosis. The elevation in arterial tone in PH is attributable, at least in part, to smooth muscle cells of PH patients being more depolarized and displaying higher intracellular Ca 2+ levels than cells from normal subjects. It is now clear that downregulation of voltage-dependent K + channels (e.g., Kv1.5) and increased expression and activity of voltage-dependent (Cav1.2) and voltage-independent (e.g., canonical and vanilloid transient receptor potential [TRPC and TRPV]) Ca 2+ channels play an important role in the functional remodeling of pulmonary arteries in PH. This review focuses on an anion-permeable channel that is now considered a novel excitatory mechanism in the systemic and pulmonary circulations. It is permeable to Cl − and is activated by a rise in intracellular Ca 2+ concentration (Ca 2+-activated Cl − channel, or CaCC). The first section outlines the biophysical and pharmacological properties of the channel and ends with a description of the molecular candidate genes postulated to encode for CaCCs, with particular emphasis on the bestrophin and the newly discovered TMEM16 and anoctamin families of genes. The second section provides a review of the various sources of Ca 2+ activating CaCCs, which include stimulation by mobilization from intracellular Ca 2+ stores and Ca 2+ entry through voltage-dependent and voltage-independent Ca 2+ channels. The third and final section summarizes recent findings that suggest a potentially important role for CaCCs and the gene TMEM16A in PH.Keywords: calcium-activated chloride channels, pulmonary hypertension, TMEM16A, anoctamin, bestrophin. . They are widely distributed small-conductance channels located in the plasma membrane of many cell types and are hypothesized to serve important functions in sperm maturation, fluid transport across epithelial tissues, cardiac repolarization, olfactory transduction, photoreceptor stimulation, neuronal and skeletal muscle excitation, cardiac repolarization, and vascular smooth muscle excitability and contraction. For more information on the general properties and role of CaCCs in various cell types and tissues, the reader is invited to consult excellent reviews on this topic. [1][2][3][4][5][6][7][8][9][10][11] Byrne and Large 12 were the first to report the existence of a Ca 2+ -activated Cl − conductance in smooth muscle by describing its general properties in rat anococcygeus smooth muscle. Since this initial discovery, that group and others have described this conductance in a variety of smooth muscle cells, including vascular smooth muscle cells (VSMCs). 1,4,7 The identification in 2008 by three independent groups of the TMEM16, or anoctamin, family of genes as novel candidates for CaCCs 13-15 sparked a renewed interest in refining our understan...

Smooth Muscle Ion Channels and Regulation of Vascular Tone in Resistance Arteries and Arterioles

“…1), multiple members of five classes from the transient receptor potential (TRP) family of ion channels (Table 4, Fig. 1), two or more classes of Cl − channels (184, 303, 304, 584, 632, 829, 956, 957) and members of the epithelial Na + /acid-sensing channel (ENaC) (340–342, 460, 502, 510, 698, 697, 1465). This review will focus on VGCCs, K + channels, ER Ca 2+ release channels and TRP channels that contribute to myogenic and agonist-modulated tone in vascular SMCs of resistance arteries and arterioles.…”

“…This review will focus on VGCCs, K + channels, ER Ca 2+ release channels and TRP channels that contribute to myogenic and agonist-modulated tone in vascular SMCs of resistance arteries and arterioles. Chloride channels (184, 303, 304, 584, 632, 829, 956, 957) and ENaC (340–342, 460, 502, 510, 698, 697, 1465) that may contribute to myogenic- and agonist-induced tone of some resistance arteries and arterioles will not be discussed. Ion channel function in the pulmonary circulation also will not be reviewed in detail (937).…”

TMEM16A inhibition impedes capacitation and acquisition of hyperactivated motility in guinea pig sperm

“…These conditions are favorable to the sustained efflux of Cl − and influx of Ca 2+ into the cell. 33,43 Our results suggest that, during capacitation, Cl − could also promote sustained Ca 2+ uptake because the [Cl − ]i (∼58 mM) and the equilibrium potential for Cl − (−22 mV) were similar, and also that the inhibition of TMEM16A by T16Ainh-A01 alters both the equilibrium potential of Cl − and the plasma membrane potential such that Ca 2+ uptake decreases ( Figure 7B). It has also been suggested that the increased activity of an important complex of different transporters, CFTR/ SLC26A3/SLC26A6/sNHE (Na + /H + exchangers), could directly increase [Cl − ]i during capacitation, allowing HCO 3 − uptake and maintaining the high intracellular concentrations of cAMP needed for capacitation processes.…”

“…Several studies have compared the specificity of NFA with that of T16Ainh‐A01, and despite arguments against such specificity, both NFA, and T16Ainh‐A01 block Cl − currents produced by TMEM16A . Therefore, T16Ainh‐A01 and NFA are considered traditional and potent TMEM16A blockers and are frequently used to investigate the physiological function of these Cl − channels on tissues or cells . For these reasons, we analyzed the role of TMEM16A by blocking the activity of the channels using both T16Ainh‐A01 and NFA, assuming that blocking Cl − currents would affect the normal behavior of the sperm.…”

Permeation Mechanisms in the TMEM16B Calcium-Activated Chloride Channels

“…In olfactory and vomeronasal sensory neurons, CaCCs mediate a large component of transduction current [2–5] and in other neuronal cell types they can control excitability [6]. Moreover, they regulate the fluid transport in different types of epithelia [7] and modulate the activity of smooth muscles of the blood vessels [8,9]. …”