Cellular and molecular physiology of volume-sensitive anion channels

Maintenance of a constant cell volume in the face of osmotic stress is an evolutionarily ancient homeostatic process. Over the last two decades physiologists have gained an impressive understanding of the ''volume-sensitive'' channels, cotransporters, exchangers, metabolic pathways, and genes that are responsible for modulating intracellular solute content and cell volume. This review focuses on one part of this story, the characteristics and osmoregulatory functions of volume-sensitive anion channels. Three distinct types of swelling-activated anion channels have been observed and studied extensively in animal cells. These channels include 1) CIC-2, which is a member of the CIC family of voltage-gated anion channels, 2) an outwardly rectifying intermediate conductance channel, and 3) a large-conductance or ''maxi'' channel. In addition to these three channels, several other less well-characterized anion channels have been observed. This review discusses the electrophysiological and molecular biological characteristics and regulation of these channels. The possible roles different types of anion channels might play in cell volume homeostasis are also discussed.