Chloride conductance pathways in heart.

Nonelectrogenic movement of Cl- is believed to be responsible for the active accumulation of intracellular Cl- in cardiac muscle. The electro-neutral pathways underlying this nonpassive distribution of Cl- are believed to include Cl(-)-HCO3- exchange, Na(+)-dependent cotransport (operating as Na(+)-Cl- and Na(+)-K(+)-2Cl- cotransport), and K(+)-Cl- cotransport. The electrogenic movement of Cl- in cardiac muscle is particularly interesting from a historical perspective. Until recently, there was some doubt as to whether Cl- carried any current in the heart. Early microelectrode experiments indicated that a Cl- conductance probably played an important role in regulating action potential duration and resting membrane potential. Subsequent voltage-clamp experiments identified a repolarizing, transient outward current that was believed to be conducted by Cl-, yet further investigation suggested that this transient outward current was more likely a K+ current, not a Cl- current. This left some doubt as to whether Cl- played any role in regulating membrane potential in cardiac muscle. More recent studies, however, have identified a highly selective Cl- conductance that is regulated by intracellular adenosine 3',5'-cyclic monophosphate, and it appears that this Cl- current may play an important role in the regulation of action potential duration and resting membrane potential.