1. The conduction and gating properties of K(+)‐conducting channels were studied in isolated turtle colon cells in an attempt to identify the single channels responsible for specific components of the macroscopic conductance of the basolateral membrane. Three types of Ca(2+)‐activated channel were identified, two of which were selective for K+ over Na+ and a third which was selective for monovalent cations over anions, but did not discriminate between K+ and Na+. 2. One of the K(+)‐selective channels was a large‐conductance ‘maxi’ K+ channel. A second was characterized by a lower conductance and pronounced inward rectification. 3. The inward‐rectifying K+ channel was selectively blocked by diphenylamine‐2‐carboxylate (DPC). Neither the maxi K+ channel nor a previously identified K(+)‐selective channel thought to be activated by cell swelling was affected by this compound. DPC also blocked the non‐selective cation channel. 4. An inward‐rectifying, DPC‐sensitive current was prominent in whole cell‐recordings, and DPC blocked basolateral K+ currents in colonic cell layers apically permeabilized with amphotericin‐B. In addition, the compound blocked active Na+ absorption. 5. The selective block of a class of epithelial K+ channels by DPC may be a useful tool for determining the contribution of this specific subpopulation to macroscopic conductance and transepithelial salt transport.
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