Existence and nature of the chloride pump.

Seven widely documented mechanisms of chloride transport across plasma membranes are: anion-coupled antiport, sodium symport, sodium-potassium-chloride symport, potassium chloride symport, proton-coupled symport, an electrochemical coupling process and chloride channels. No direct genetic evidence has yet been provided for primary active chloride transport despite numerous reports of cellular Cl(-)- stimulated ATPases coexisting, in the same tissue, with uphill chloride transport that could not be accounted for by the four common chloride transport processes. Cl(-)-stimulated ATPases are a common property of practically all biological cells with the major location being of mitochondrial origin. It also appears that plasma membranes are sites of Cl(-)-stimulated ATPase activity. Recent studies of Cl(-)-stimulated ATPase activity and chloride transport in the same membrane system, including liposomes, suggest a mediation by the ATPase in net movement of chloride up its electrochemical gradient across plasma membranes. Further studies, especially from a molecular biological perspective, are required to confirm a direct transport role to plasma membrane-localized Cl(-)-stimulated ATPases.