Downregulation of Epithelial Sodium Channel (ENaC) by CFTR Co-expressed in Xenopus Oocytes is Independent of Cl− Conductance

Defective regulatory interactions between the cystic fibrosis conductance regulator (CFTR) and the epithelial sodium channel (ENaC) have been implicated in the elevated Na+ transport rates across cystic fibrosis airway epithelium. It has recently been proposed that ENaC downregulation by CFTR depends on the ability of CFTR to conduct Cl− into the cell and is negligible when Cl− flows out of the cell. To study the mechanisms of this downregulation we have measured amiloride-inhibitable Na+ current (Iamil) in oocytes co-expressing rat ENaC and human wild-type CFTR. In oocytes voltage-clamped to −60 mV, stimulating CFTR with 1 mm IBMX reduced Iamil by up to 80%, demonstrating that ENaC is inhibited when Cl− is conducted out of the cell. Decreasing the level of CFTR stimulation in a single oocyte, decreased both the degree of Iamil downregulation and the CFTR-mediated plasma membrane Cl− conductance, suggesting a direct correlation. However, Iamil downregulation was not affected when Cl− flux across oocyte membrane was minimized by holding the oocyte membrane potential near the Cl− reversal potential (67% ± 10% inhibition at −20 mV compared to 79% ± 4% at −60 mV) demonstrating that Iamil downregulation was independent of the amount of current flow through CFTR. Studies with the Ca2+-sensitive photoprotein aequorin showed that Ca2+ is not involved in Iamil downregulation by CFTR, although Ca2+ injection into the cytoplasm did inhibit Iamil. These results demonstrate that downregulation of ENaC by CFTR depends on the degree of CFTR stimulation, but does not involve Ca2+ and is independent of the direction and magnitude of Cl− transport across the plasma membrane.