Nonselective cation transport in native esophageal epithelia

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Abstract
Rabbit esophageal epithelia actively transport Na+ in a manner similar to that observed in classic electrically tight Na+-absorbing epithelia, such as frog skin. However, the nature of the apical entry step is poorly understood. To address this issue, we examined the electrophysiological and biochemical nature of this channel. Western blotting experiments with epithelial Na+ channel (ENaC) subunit-specific antibodies revealed the presence of all three ENaC subunits in both native and immortalized esophageal epithelial cells. The amino acid sequence of the rabbit α-ENaC cloned from native rabbit esophageal epithelia was not significantly different from that of other published α-ENaC homologs. To characterize the electrophysiological properties of this native apical channel, we utilized nystatin permeabilization to eliminate the electrical contribution of the basolateral membrane in isolated native epithelia mounted in Ussing-type chambers. We find that the previously described apical Na+ channel is nonselective for monovalent cations (Li+, Na+, and K+). Moreover, this channel was not blocked by millimolar concentrations of amiloride. These findings document the presence of a nonselective cation channel in a native Na+ transporting epithelia, a finding that hereto has been thought to be limited to artificial culture conditions. Moreover, our data are consistent with a potential role of ENaC subunits in the formation of a native nonselective cation channel.