Cation Channels in the Apical Membrane of Collecting Duct Prinicipal Cell Epithelium in Culture

The apical cell membrane of cultured monolayers of collecting duct principal cells was investigated with the patch-clamp technique to study single ion channels. A broad spectrum of channel events was observed which complicated the analysis. Although principal cells absorb mainly Na+ ions and although most patches must have contained an Na+ conductance as evidenced by asymmetric clamp currents in the presence of amiloride, single Na+ channel events could not be identified with certainty, possibly because single-channel conductance was too low (≤5pS). Instead, non-selective cation channels of 21.2 ± 4.0 pS were frequently observed. They were impermeable to choline and anions but did not discriminate between Na+ and K+. These channels, however, do not appear to participate in active Na+ absorption. Different types of K+ channels were observed: a high-conductance Ca2+-activated K+ channel and a bursting low-conductance K+ channel. Since the former channel has been denied a role in K+ secretion/ absorption in native collecting ducts, the latter might be involved. In addition three types of CΓ channels have been observed which will be described separately. At least one of those, a 30-pS outwardly rectifying Cl- channel appears to allow small amounts of Cl- ions to be absorbed across principal cells.