Electrolyte Transport Through a Cation-Selective ion Channel in Large Intestinal Enterocytes of Xenopus Laevis

Electrogenic ion transport through the colon epithelium of the African clawed toad (Xenopus laevis) was investigated with electrophysiological methods in vitro. Interest was focused on a previously described phenomenon, that removal of Ca²⁺ from the mucosal Ringer’s solution increases electrogenic sodium absorption. Our results clearly show that Ca²⁺ removal reveals an apical ion channel that is not a specific Na⁺ channel, but a non-selective cation channel with an ‘apparent’ ion selectivity of the order K⁺>Na⁺=Rb⁺>Cs⁺>Li⁺. This Ca²⁺-sensitive current increased linearly with the mucosal pH, and could be inhibited by other divalent cations (Mg²⁺, Ba²⁺) and the organic ion channel blockers quinidine and verapamil. The mucosal Ca²⁺ concentration that induced a half-maximal inhibition of the Ca²⁺-sensitive current was about 1 μmoll⁻¹ and was independent of the mucosal pH. Owing to the high Ca²⁺ sensitivity, a regulation of the channel conductivity by extracellular Ca²⁺ is ruled out. It is concluded that this channel, which is almost identical to similar channels found in amphibian skin and bladder, acts as a pathway for cation absorbing or secreting processes. Possibly the binding of extracellular Ca²⁺ is related to selectivity changes of the Ca²⁺-sensitive ion channel.