TMEM16 (transmembrane protein 16) proteins, which possess eight putative transmembrane domains with intracellular NH₂- and COOH-terminal tails, are thought to comprise a Cl⁻ channel family. The function of TMEM16F, a member of the TMEM16 family, has been greatly controversial. In the present study, we performed whole cell patch-clamp recordings to investigate the function of human TMEM16F. In TMEM16F-transfected HEK293T cells but not TMEM16K- and mock-transfected cells, activation of membrane currents with strong outward rectification was found to be induced by application of a Ca²⁺ ionophore, ionomycin, or by an increase in the intracellular free Ca²⁺ concentration. The free Ca²⁺ concentration for half-maximal activation of TMEM16F currents was 9.6 μM, which is distinctly higher than that for TMEM16A/B currents. The outwardly rectifying current-voltage relationship for TMEM16F currents was not changed by an increase in the intracellular Ca²⁺ level, in contrast to TMEM16A/B currents. The Ca²⁺-activated TMEM16F currents were anion selective, because replacing Cl⁻ with aspartate⁻ in the bathing solution without changing cation concentrations caused a positive shift of the reversal potential. The anion selectivity sequence of the TMEM16F channel was I⁻ > Br⁻ > Cl⁻ > F⁻ > aspartate⁻. Niflumic acid, a Ca²⁺-activated Cl⁻ channel blocker, inhibited the TMEM16F-dependent Cl⁻ currents. Neither overexpression nor knockdown of TMEM16F affected volume-sensitive outwardly rectifying Cl⁻ channel (VSOR) currents activated by osmotic swelling or apoptotic stimulation. These results demonstrate that human TMEM16F is an essential component of a Ca²⁺-activated Cl⁻ channel with a Ca²⁺ sensitivity that is distinct from that of TMEM16A/B and that it is not related to VSOR activity.

There are a number of mammalian anion channel types associated with cell volume changes. These channel types are classified into two groups: volume-activated anion channels (VAACs) and volume-correlated anion channels (VCACs). VAACs can be directly activated by cell swelling and include the volume-sensitive outwardly rectifying anion channel (VSOR), which is also called the volume-regulated anion channel; the maxi-anion channel (MAC or Maxi-Cl); and the voltage-gated anion channel, chloride channel (ClC)-2. VCACs can be facultatively implicated in, although not directly activated by, cell volume changes and include the cAMP-activated cystic fibrosis transmembrane conductance regulator (CFTR) anion channel, the Ca²⁺-activated Cl⁻ channel (CaCC), and the acid-sensitive (or acid-stimulated) outwardly rectifying anion channel. This article describes the phenotypical properties and activation mechanisms of both groups of anion channels, including accumulating pieces of information on the basis of recent molecular understanding. To that end, this review also highlights the molecular identities of both anion channel groups; in addition to the molecular identities of ClC-2 and CFTR, those of CaCC, VSOR, and Maxi-Cl were recently identified by applying genome-wide approaches. In the last section of this review, the most up-to-date information on the pharmacological properties of both anion channel groups, especially their half-maximal inhibitory concentrations (IC₅₀ values) and voltage-dependent blocking, is summarized particularly from the standpoint of pharmacological distinctions among them. Future physiologic and pharmacological studies are definitely warranted for therapeutic targeting of dysfunction of VAACs and VCACs.

Cell swelling activates or upregulates a number of anion channels. Of the volume‐activated or ‐regulated anion channels (VAACs or VRACs), the volume‐sensitive outwardly rectifying anion channel (VSOR) is most prominently activated and ubiquitously expressed. This channel is known to be involved in a variety of physiological processes including cell volume regulation, cell proliferation, differentiation and cell migration as well as cell turnover involving apoptosis. Recent studies have shown that VSOR activity is also involved in a number of pathophysiological processes including the acquisition of cisplatin resistance by cancer cells, ischaemia–reperfusion‐induced death of cardiomyocytes and hippocampal neurons, glial necrosis under lactacidosis as well as neuronal necrosis under excitotoxicity. Moreover, VSOR serves as the pathway for glutamate release from astrocytes under ischaemic conditions and when stimulated by bradykinin, an initial mediator of inflammation. So far, many signalling molecules including the EGF receptor, PI3K, Src, PLCγ and Rho/Rho kinase have been implicated in the regulation of VSOR activity. However, our pharmacological studies suggest that these signals are not essential components of the swelling‐induced VSOR activation mechanism even though some of these signals may play permissive or modulatory roles. Molecular identification of VSOR is required to address the question of how cells sense volume expansion and activate VSOR.

Arginine vasopressin (AVP) neurons in the hypothalamus are osmosensory neurons that respond to increased or decreased plasma osmolarity by releasing more or less AVP, respectively, from their axon terminals. Here, we found that, in contrast, hypo-osmotic stress enhanced somatodendritic AVP secretion from isolated rat AVP neurons, and this somatodendritic release depended on actin depolymerization. In AVP neurons identified by transgenic expression of green fluorescent protein, hypo-osmotic stimulation led to activation of anion currents and a slow regulatory volume decrease (RVD). Bath application of AVP increased the volume-sensitive anion current and accelerated RVD; these effects were abolished by inhibition of adenylate cyclase or by a specific antagonist of the V₂-type vasopressin receptor. The V₂ receptor antagonist slowed the RVD rate of AVP neurons even in the absence of exogenous AVP when the volume of bath solution was reduced. Reverse transcription polymerase chain reaction and immunostaining both indicated that the V₂ receptor was present in AVP neurons. We conclude that somatodendritic release of AVP under hypo-osmotic conditions acts through the V₂ receptor as an autocrine signal to enhance volume-sensitive anion channel activity and thereby facilitate cell volume regulation.