resulting in a prolonged action potential refractory period (Burdyga and Wray, 2005). Finally, because of their capability to also turn on spontaneous transient inward currents (STICs), which counteracts the STOC, Ca2+ sparks serve as a membrane potential stabilizer in smooth muscle from airway, portal vein, and corpora cavernosa (Hogg et al., 1993; Van Helden, 1993; Janssen and Sims, 1994; ZhuGe et al., 1998; Karkanis et al., 2003; Bao et al., 2008). The critical role of Ca2+ sparks and STOCs in smooth muscle contractility is heightened by the findings that uncoupling or suppressing the coupling between them leads to hypertension, urinary incontinence, and retinopathy (Brenner et al., 2000; Plüger et al., 2000; Amberg et al., 2003; Meredith et al., 2004; McGahon et al., 2007).
