Inhibition by cytoplasmic nucleotides of a new cation channel in freshly isolated human and rat type II pneumocytes.
Here we report a 26- to 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 Ca(2+) concentrations at the cytoplasmic side in the micromolar range. High concentrations of amiloride (>10 microM) at the extracellular side did not inhibit. The channel was equally permeable for K(+) and Na(+) but was essentially impermeable for Cl(-), Ca(2+), and Mg(2+). It was blocked by adenosine nucleotides (cytoplasmic side) with the following order of potency: AMP approximately ADP (EC(50) </= 10 microM) > ATP >> adenosine >> cyclic AMP. The blocking effect of ATP was reproduced by its nonhydrolyzable analogs AMPPNP or ATP-gamma-S. GTP did not inhibit. Cd(2+) blocked the channel with an EC(50) approximately 55.5 nM. We conclude that type II cells express a Ca(2+)-dependent, nucleotide-inhibited, nonselective, and Ca(2+)-impermeable cation channel (NSC(Ca/AMP)) with tonically suppressed activity. RT-PCR confirmed expression of TRPM4b, a channel with functional characteristics almost identical with NSC(Ca/AMP). Potential physiological roles are discussed.
Department of Physiology, Medical University of Innsbruck, Fritz-Pregl-Str. 3, A-6020 Innsbruck, Austria.
Norbert
N
Mair
Manfred
M
Frick
Cristina
C
Bertocchi
Thomas
T
Haller
Albert
A
Amberger
Helmut
H
Weiss
Raimund
R
Margreiter
Werner
W
Streif
Paul
P
Dietl
D000244:Adenosine Diphosphate / Q000494:pharmacology*; D000249:Adenosine Monophosphate / Q000494:pharmacology*; D000255:Adenosine Triphosphate / Q000494:pharmacology*; D000818:Animals; D002104:Cadmium / Q000494:pharmacology; D002118:Calcium / Q000502:physiology; D027682:Cation Transport Proteins / Q000187:drug effects / Q000502:physiology*; D002413:Cations, Divalent / Q000494:pharmacology; D002448:Cell Adhesion; D003600:Cytosol / Q000502:physiology; D017931:DNA Primers; D006801:Humans; D008297:Male; D008345:Manganese / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D011650:Pulmonary Alveoli / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013324:Strontium / Q000494:pharmacology
D027682:Cation Transport Proteins; D002413:Cations, Divalent; D017931:DNA Primers; D002104:Cadmium; D000249:Adenosine Monophosphate; D008345:Manganese; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D002118:Calcium; D013324:Strontium