Roles of volume-regulatory anion channels, VSOR and Maxi-Cl, in apoptosis, cisplatin resistance, necrosis, ischemic cell death, stroke and myocardial infarction.

Two types of anion channels are directly activated by osmotic swelling and are involved in the regulatory volume decrease (RVD) in most types of mammalian cells, and they include the volume-sensitive outwardly rectifying anion channel (VSOR), also called the volume-regulated anion channel (VRAC), and the large-conductance maxi-anion channel (Maxi-Cl). In cardiomyocytes, a splice variant of cystic fibrosis transmembrane conductance regulator anion channel (cardiac CFTR) participates in the RVD mechanism under β-adrenergic stimulation. VSOR and Maxi-Cl are also involved in facilitation of the RVD process by releasing extracellular autocrine/paracrine signals, glutamate and ATP. Apoptotic cell death starts with cell shrinkage, called the apoptotic volume decrease (AVD), which is also caused by activation of VSOR. Since VSOR is implicated not only in the AVD induction but also in the uptake of an anti-cancer drug, cisplatin, downregulation of VSOR activity is causatively involved in acquisition of cisplatin resistance in cancer cells. Necrotic cell death exhibits persistent cell swelling, called the necrotic volume increase (NVI), which is coupled to RVD dysfunction due to impaired VSOR function. Acidotoxic and lactacidosis-induced necrotic cell death is induced both by glutamate release mediated by astroglial VSOR and Maxi-Cl and by exaggerated Cl- influx mediated by neuronal VSOR under prolonged depolarization caused by activation of ionotropic glutamate receptor (iGluR) cation channels. Both VSOR and Maxi-Cl are elaborately involved, in a manner as double-edged swords, in ischemia- and ischemia-reperfusion-induced apoptotic or necrotic cell death in cerebral and myocardial cells by mediating not only Cl- transport but also release of glutamate and/or ATP. Cardiac CFTR exerts a protective action against ischemia(-reperfusion)-induced cardiac injury, called myocardial infarction (MI), which is largely necrotic. Cardiac Maxi-Cl activity may participate in protection against ischemia(-reperfusion) injury by mediating ATP release.