Is there a Cl- pump?

Abstract

Three universally accepted mechanisms of Cl- transport across plasma membranes exist and they are 1) anion-coupled antiport, 2) cation-coupled symport, and 3) coupling to primary active ion transport through electrical and/or chemical processes. No unequivocal direct evidence has been provided for primary active Cl- transport (Cl- pump) despite numerous reports of cellular Cl- -stimulated adenosinetriphosphatase (ATPases) and of Cl- transport that cannot be accounted for by the three well-documented Cl- transport processes. It has been demonstrated that Cl- -stimulated ATPase activity is localized to both mitochondrial and microsomal aspects of the cellular apparatus. However, one group ascribes microsomal localization of Cl- -stimulated ATPase activity to mitochondrial contamination of that membrane fraction. Therefore, no Cl- pump could ever exist naturally in any plasma membrane. The other group simply states that there is plasma membrane localization of Cl- -stimulated ATPase activity that could function as a Cl- pump. Both arguments are logically advanced and their conclusions are consistent with their respective premises. Resolution to the question Is there a Cl- pump? rests with each reader's critique and objective evaluation.