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Am J Physiol Lung Cell Mol Physiol (August 20, 2004). doi:10.1152/ajplung.00177.2004
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Submitted on May 17, 2004
Accepted on August 12, 2004

Inhibition by cytoplasmic nucleotides of a new cation channel in freshly isolated human and rat type II pneumocytes

Norbert Mair1, Manfred Frick1, Cristina Bertocchi1, Thomas Haller1, Albert Amberger2, Helmut Weiss3, Raimund Margreiter4, Werner Streif5, and Paul Dietl1*

1 Department of Physiology, Medical University of Innsbruck, Innsbruck, Austria
2 Tyrolean Cancer Research Institute, Innsbruck, Austria
3 Department of General and Transplant Surgery, Medical University of Innsbruck, Innsbruck, Austria
4 Department of General and Transplant Surgery, Medical University of Innsbruck, Innsbruck, Austria; Tyrolean Cancer Research Institute, Innsbruck, Austria
5 Department of Pediatrics, Medical University of Innsbruck, Innsbruck, Austria

* To whom correspondence should be addressed. E-mail: paul.dietl{at}uibk.ac.at.

Here we report a 26-29 pS cation channel abundantly expressed in freshly isolated and primary-cultured type II cells from rat or healthy human lungs. The channel was never spontaneously active in cell-attached patches but could be activated by cell permeabilization with {beta}-escin. Excised patch clamp experiments revealed activation by Ca2+ concentrations at the cytoplasmic side ([Ca2+]cyt) in the µM range. High concentrations of amiloride [>10 µM] at the extracellular side did not inhibit. The channel was equally permeable for K+ and Na+, but essentially impermeable for Cl-, Ca2+ and Mg2+. It was blocked by adenosine nucleotides (cytoplasmic side) with the following order of potency: AMP ~ ADP (EC50 ≤ 10 µM) > ATP >> adenosine >> cyclic AMP. The blocking effect of ATP was reproduced by its nonhydrolyzable analogs AMPPNP or ATP-{gamma}-S. GTP did not inhibit. Cd2+ blocked the channel with an EC50 ~ 55.5 nM. We conclude that type II cells express a Ca2+-dependent, nucleotideinhibited, non-selective and Ca2+-impermeable cation channel (NSCCa/AMP) with tonically suppressed activity. RT-PCR confirmed expression of TRPM4b, a channel with functional characteristics almost identical with NSCCa/AMP. Potential physiological roles are discussed.




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