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J. Gen. Physiol.,
Volume 111, Number 6, June 1, 1998 825-846
§§§
From the * Hospital for Sick Children, Division of Respiratory Research, Toronto, Ontario M5G 1X8, Canada; and the The epithelial Na+ channel (ENaC), composed of three subunits ( Department of
Biochemistry, and § Department of Paediatrics, University of Toronto, Toronto, Ontario, Canada
, 
, and 
), is expressed in several epithelia and plays a critical role in salt and water balance and in the regulation of blood pressure. Little is
known, however, about the electrophysiological properties of this cloned channel when expressed in epithelial cells. Using whole-cell and single channel current recording techniques, we have now characterized the rat
ENaC (rENaC) stably transfected and expressed in Madin-Darby canine kidney (MDCK) cells. Under whole-cell patch-clamp configuration, the rENaC-expressing MDCK cells exhibited greater whole cell Na+ current at
143 mV (1,466.2 ± 297.5 pA) than did untransfected cells (47.6 ± 10.7 pA). This conductance was completely and reversibly inhibited by 10 µM amiloride, with a Ki of 20 nM at a membrane potential of 103 mV; the
amiloride inhibition was slightly voltage dependent. Amiloride-sensitive whole-cell current of MDCK cells expressing or subunits alone was 115.2 ± 41.4 pA and 52.1 ± 24.5 pA at 143 mV, respectively, similar to the
whole-cell Na+ current of untransfected cells. Relaxation analysis of the amiloride-sensitive current after voltage
steps suggested that the channels were activated by membrane hyperpolarization. Ion selectivity sequence of the Na+ conductance was Li+ > Na+ >> K+ = N-methyl-D-glucamine+ (NMDG+). Using excised outside-out patches,
amiloride-sensitive single channel conductance, likely responsible for the macroscopic Na+ channel current, was
found to be ~5 and 8 pS when Na+ and Li+ were used as a charge carrier, respectively. K+ conductance through
the channel was undetectable. The channel activity, defined as a product of the number of active channel (n) and
open probability (Po), was increased by membrane hyperpolarization. Both whole-cell Na+ current and conductance were saturated with increased extracellular Na+ concentrations, which likely resulted from saturation of the single channel conductance. The channel activity (nPo) was significantly decreased when cytosolic Na+ concentration was increased from 0 to 50 mM in inside-out patches. Whole-cell Na+ conductance (with Li+ as a charge carrier) was inhibited by the addition of ionomycin (1 µM) and Ca2+ (1 mM) to the bath. Dialysis of the cells with a
pipette solution containing 1 µM Ca2+ caused a biphasic inhibition, with time constants of 1.7 ± 0.3 min (n = 3)
and 128.4 ± 33.4 min (n = 3). An increase in cytosolic Ca2+ concentration from <1 nM to 1 µM was accompanied
by a decrease in channel activity. Increasing cytosolic Ca2+ to 10 µM exhibited a pronounced inhibitory effect. Single channel conductance, however, was unchanged by increasing free Ca2+ concentrations from <1 nM to 10 µM.
Collectively, these results provide the first characterization of rENaC heterologously expressed in a mammalian
epithelial cell line, and provide evidence for channel regulation by cytosolic Na+ and Ca2+.
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