H. Chabot et al., Downregulation of epithelial sodium channel (ENaC) by CFTR co-expressed inXenopus oocytes is independent of Cl- conductance, J MEMBR BIO, 169(3), 1999, pp. 175-188
Defective regulatory interactions between the cystic fibrosis conductance r
egulator (CFTR) and the epithelial sodium channel (ENaC) have been implicat
ed in the elevated Na+ transport rates across cystic fibrosis airway epithe
lium. It has recently been proposed that ENaC downregulation by CFTR depend
s on the ability of CFTR to conduct Cl- into the cell and is negligible whe
n Cl- flows out of the cell. To study the mechanisms of this downregulation
we have measured amiloride-inhibitable Na+ current (I-amil) 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 I-amil by up to 80%, demon
strating that ENaC is inhibited when Cl- is conducted out of the cell. Decr
easing the level of CFTR stimulation in a single oocyte, decreased both the
degree of I-amil downregulation and the CFTR-mediated plasma membrane Cl-
conductance, suggesting a direct correlation. However, I-amil downregulatio
n was not affected when Cl- flux across oocyte membrane was minimized by ho
lding the oocyte membrane potential near the Cl- reversal potential (67% +/
- 10% inhibition at -20 mV compared to 79% +/- 4% at -60 mV) demonstrating
that I-amil downregulation was independent of the amount of current flow th
rough CFTR. Studies with the Ca2+-sensitive photoprotein aequorin showed th
at Ca2+ is not involved in I-amil downregulation by CFTR, although Ca2+ inj
ection into the cytoplasm did inhibit I-amil. These results demonstrate tha
t downregulation of ENaC by CFTR depends on the degree of CFTR stimulation,
but does not involve Ca2+ and is independent of the direction and magnitud
e of Cl- transport across the plasma membrane.