Figure 5.

Various sources of Ca²⁺ activating the Ca²⁺-activated Cl⁻ channel (CaCC) ANO1 in vascular smooth muscle cells. A, The classical mode of activation of CaCCs by stimulation of a G_q-coupled receptor (G_qPCR) with a constricting agonist (e.g., 5-hydroxytryptamine, angiotensin II, endothelin), leading to an elevation of intracellular Ca²⁺ concentration and contraction, is illustrated in this panel by the circled numbers: (1) binding of an agonist to a G_qPCR; (2) breakdown of membrane-bound phosphatidyl-inositol by G_q-stimulated phospholipase C (PLC), leading to the production of the second messengers inositol trisphosphate (InsP₃) and diacylglycerol (DAG); (3) while DAG stimulates protein kinase C (PKC), which targets many proteins involved in contraction and cell proliferation, InsP₃ diffuses toward the sarcoplasmic reticulum (SR), where it binds to and activates a Ca²⁺-permeable receptor-operated channel (InsP₃R), triggering Ca²⁺ release in the cytoplasm; (4) Ca²⁺ diffuses in the cytoplasm and activates contraction and ANO1; (5) opening of ANO1 leads to Cl⁻ efflux and membrane depolarization due to an outwardly directed electrochemical gradient for Cl⁻, which stimulates voltage-gated L-type Ca²⁺ channels encoded by the gene Ca_V1.2; (6) Ca²⁺ entry through Ca_V1.2 would in turn promote actomyosin bridge cycling and further stimulate ANO1, establishing a positive-feedback loop sustaining membrane depolarization and Ca²⁺ entry; (7) there is evidence for stimulation of ANO1 in some vascular myocytes by Ca²⁺ entry through Ca_V1.2 triggering SR Ca²⁺ release from ryanodine receptors (RyR) in a process called Ca²⁺-induced Ca²⁺ release (CICR), which can provide an additional stimulus for ANO1 activation by Ca_V1.2; (8) spontaneous Ca²⁺ release by RyR in the SR, giving rise to “spontaneous transient inward currents,” or STICs, produced by transient openings of ANO1. B, Activation of ANO1 by voltage-independent Ca²⁺ entry pathways: (1) Ca²⁺ unbinding from the SR Ca²⁺ sensor protein STIM1 (stromal interacting molecule 1) by a decrease in the concentration of Ca²⁺ in the SR due to spontaneous Ca²⁺ leakage or mediated by an agonist leads to clustering of portions of the SR with the plasma membrane; (2) STIM1 then physically interacts with a complex composed of one or more members of the canonical transient receptor potential (TRPC) and/or Orai families of cation channels to trigger store-operated Ca²⁺ entry (SOCE); (3) Ca²⁺ entry via SOCE can then stimulate ANO1; (4) ANO1 can also be stimulated by Ca²⁺ influx through a receptor-operated channel stimulated by an agonist (ROCE); (5) although hypothetical, it is possible that ANO1 could be stimulated locally by Ca²⁺ influx through TRPV4 (vanilloid transient receptor potential) channels physically interacting with the anion channel.