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1 Department of Anesthesiology, University of Alabama at Birmingham, Birmingham, AL, USA
2 Department of Anesthesiology, University of Alabama at Birmingham, Birmingham, AL, USA; Department of Physiology and Biophysics, University of Alabama at Birmingham, Birmingham, AL, USA
* To whom correspondence should be addressed. E-mail: sadis{at}uab.edu.
We recorded apical membrane potentials (Va) of H441 cells (a human lung cell line exhibiting both ENaC and CFTR-type channels) grown as confluent monolayers, using the microelectrode technique in current clamp mode before, during, and after perfusion of the apical membranes with 10 µM forskolin. When perfused with normal Ringer's, Va was -43 ± 10 mV (mean ±SD; n = 31). Perfusion with forskolin resulted in sustained depolarization by 25.0 ±3.5 mV (mean ±SD; n = 23) and increased the number, open time and the open probability of a 4.2 pS ENaC-type sodium (Na+) channel. In contrast to a previous report (Am.J.Physiol. 275: C1610-C1620, 1998) no transient hyper-polarization was observed. The forskolin-induced depolarization of Va was almost totally prevented by pre-treatment of monolayers with 10 µM amiloride or by substitution of Na+ ions in the bath solution with N-Methyl-D-Glucamine (NMDG+). These findings indicate that cAMP stimulation of Na+ influx across H441 confluent monolayers results from activation of an amiloride sensitive apical Na+ conductance and not from Va hyper-polarization due to Cl- influx through CFTR-type channels.
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