Diversity of Cl− Channels

Cl − channels are expressed as selective anion pores, have been found in a variety of physiologic preparations and are ubiquitously distributed, including in oocytes. Cl − channels allow the passive diffusion of negatively charged ions along electrochemical gradients or the transport of positive charges, and they conduct other anions, such as HCO 3 − , I − , SCN − and NO 3 − . The physiological roles of Cl − channels in the plasma membrane and in the vesicular membrane of intracellular organelles were vigorously investigated in the 1980s. Measurement of Cl − current in various cells revealed that Cl − channel gates were regulated by membrane voltage, cell volume, extracellular ligands, intracellular ions (such as Ca 2+ , H + or anions) or phosphorylation of residues by various protein kinases. Therefore, one method of classification of Cl − channels was based on their functions. To summarize the diversity of function, the following list may conducted on the basis of phenotype in vivo: (i) ligand-gated transmission in the post-synaptic membrane, (ii) stabilization of resting membrane potential in (skeletal) muscle, (iii) depolarization of smooth-muscle cells or possibly of retinal pigment epithelium, (iv) cell-volume regulation in various cells, (v) fluid transport in epithelia and (v) neutralization of H + ions in lysosomal vesicles.