AJP - Cell Physiology, Vol 263, Issue 5 C1111-C1117, Copyright © 1992 by American Physiological Society
Single-channel behavior of a purified epithelial Na+ channel subunit that binds amiloride
S. Sariban-Sohraby, M. Abramow and R. S. Fisher
Laboratoire de Physiologie, Universite Libre de Bruxelles, Belgium.
The apical membrane of high electrical resistance epithelia, which is
selectively permeable to Na+, plays an essential role in the maintenance of
salt balance. Na+ entry from the apical fluid into the cells is mediated by
amiloride-blockable Na(+)-specific channels. The channel protein, purified
from both amphibian and mammalian sources, is composed of several subunits,
only one of which the 150-kDa polypeptide, specifically binds the Na+
transport inhibitor amiloride. The goal of the present study was to
investigate whether the isolated amiloride-binding subunit of the channel
could conduct Na+. The patch-clamp technique was used to study the 150-kDa
polypeptide incorporated into a lipid bilayer formed on the tip of a glass
pipette. Unitary conductance jumps averaged 4.8 pS at 100 mM Na2HPO4. Open
times ranged from 24 ms to several seconds. The channel spent most of the
time in the closed state. Channel conductance and gating were independent
of voltage between -60 and +100 mV. Amiloride (0.1 microM) decreased the
mean open time of the channel by 98%. We conclude that the 150-kDa subunit
of the amiloride-blockable Na+ channel conducts current and may be
sufficient for the Na+ transport function of the whole channel.
