Regulation of ion channels in the cortical collecting duct

Properties of ion channels of the cortical collecting duct (CCD) and their relevance for the macroscopic conductances are discussed. Although the regulation of the Na+ conductance by various hormones is widely studied, the relevant Na+ channel appears to be extremely difficult to be detected in the intact preparation. Three different K+ channels with slope conductances of about 30, 140 and 80 pS (excised, 145 mmol/l KCL and 145 mmol/l NaCl on both sides of the membrane) have been found in principal cells of the CCD so far. The first two channels are located in the luminal, the latter one in the basolateral membrane. The two luminal channels are highly sensitive to the cytosolic pH and are also inhibited by cytosolic ATP. The small luminal K+ channel, highly active on the cell, is most likely responsible for K+ secretion. The large luminal K+ channel is involved in the volume regulation. The basolateral K+ channel, again highly active on the cell, is probably responsible for the recirculation of K+ across this membrane. The physiological role of the observed Cl- channels is still unknown.