Intricate interaction between store-operated calcium entry and calcium-activated chloride channels in pulmonary artery smooth muscle cells

Abstract

Ca2+-activated Cl- channels (ClCa) represent an important excitatory mechanism in vascular smooth muscle cells. Active accumulation of Cl- by several classes of anion transporters results in an equilibrium potential for this ion about 30 mV more positive than the resting potential. Stimulation of ClCa channels leads to membrane depolarization, which enhances Ca 2+ entry through voltage-gated Ca2+ channels and leads to vasoconstriction. ClCa channels can be activated by distinct sources of Ca 2+ that include (1) mobilization from intracellular Ca2+ stores (ryanodine or inositol 1,4,5-trisphosphate [InsP3]) and (2) Ca 2+ entry through voltage-gated Ca2+ channels or reverse-mode Na+/Ca2+ exchange. The present study was undertaken to determine whether Ca2+ influx triggered by store depletion (store-operated calcium entry, SOCE) activates ClCa channels in rabbit pulmonary artery (PA) smooth muscle. Classical store depletion protocols involving block of sarcoplasmic reticular Ca2+ reuptake with thapsigargin (TG; 1 mM) or cyclopiazonic acid (CPA; 30 mM) led to a consistent nifedipine-insensitive contraction of intact PA rings and rise in intracellular Ca2+ concentration in single PA myocytes that required the presence of extracellular Ca2+. In patch clamp experiments, TG or CPA activated a timeindependent nonselective cation current (ISOC) that (1) reversed between -10 and 0 mV; (2) displayed the typical "N"-shaped current-voltage relationship; and (3) was sensitive to the (ISOC) blocker by SKF-96365 (50 mM). In double-pulse protocol experiments, the amplitude of ISOC was varied by altering membrane potential during an initial step that was followed by a second constant step to +90 mV to register Ca2+-activated Cl- current, ICl(Ca). The niflumic acid-sensitive time-dependentICl(Ca) at +90 mV increased in proportion to the magnitude of the preceding hyperpolarizing step, an effect attributed to graded membrane potential-dependent Ca2+ entry through ISOC and confirmed in dual patch clamp and Fluo-5 experiments to record membrane current and free intracellular Ca2+ concentration simultaneously. Reverse-transcription polymerase chain reaction (RT-PCR) experiments confirmed the expression of several molecular determinants of SOCE, including transient receptor potential canonical (TRPC) 1, TRPC4, and TRPC6; stromal interacting molecule (STIM) 1 and 2; and Orai1 and 2, as well as the novel and probable molecular candidates thought to encode for ClCa channels transmembrane protein 16A (TMEM16A) Anoctamin 1 (ANO1) and B (ANO2). Our preliminary investigation provides new evidence for a Ca2+ entry pathway consistent with store-operated Ca2+ entry signaling that can activate Ca 2+-activated Cl- channels in rabbit PA myocytes. We hypothesize that this mechanism may be important in the regulation of membrane potential, Ca 2+ influx, and tone in these cells under physiological and pathophysiological conditions. © Humana Press, a part of Springer Science+ Business Media, LLC 2010.