Increased open probability of single cardiac L-type calcium channels in patients with chronic atrial fibrillation. role of phosphatase 2A.
BACKGROUND
The L-type calcium channel (LCC) plays a crucial role in the electrical remodeling of atrial fibrillation (AF). AF is associated with reduction of L-type calcium current density, due to a transcriptional downregulation of the pore forming alpha(1c)-subunit of LCC. However, it is unclear, whether this current reduction is related to a decrease in channel number or to alterations in channel function. Hence, we performed a single LCC analysis to assess channel gating and function in human AF.


RESULTS
We used the cell-attached patch-clamp technique in isolated atrial human cardiomyocytes of 25 patients with sinus rhythm (SR) and 15 patients with chronic AF. Protein expression of the pore-forming alpha(1c)-subunit of LCC was reduced by 40% in AF. Single channel peak average current was 1.7-fold higher in AF than in SR, due to a 3.1-fold higher open probability of LCC. Since phosphatase 2A (PP2A) is known to preferentially reduce LCC open probability via channel dephosphorylation, we assessed whether PP2A expression or activity is reduced in AF. Okadaic acid, an inhibitor of phosphatases, increased channel open probability in SR, but not in AF. However, Western blot analysis of atrial homogenates of the same patient population revealed unchanged expression of PP2A.


CONCLUSIONS
Human AF is characterized by increased single LCC activity, due to an increase of channel open probability. The blunted effect of PP2A on LCC as shown by single channel analysis may be related to a reduction of cytosolic PP2A activity or impaired local interaction between PP2A and LCC in AF.
Klein
Gunnar
G
Department of Cardiovascular Medicine, Hannover Medical School, Carl-Neuberg-Strasse 1, 30625 Hannover, Germany. gunnarklein@yahoo.de
Schröder
Frank
F
Vogler
David
D
Schaefer
Arnd
A
Haverich
Axel
A
Schieffer
Bernhard
B
Korte
Thomas
T
Drexler
Helmut
H
D000318:Adrenergic beta-Agonists / Q000494:pharmacology; D000319:Adrenergic beta-Antagonists / Q000494:pharmacology; D001281:Atrial Fibrillation / Q000378:metabolism* / Q000503:physiopathology; D015153:Blotting, Western / Q000379:methods; D020746:Calcium Channels, L-Type / Q000378:metabolism*; D016022:Case-Control Studies; D002908:Chronic Disease; D004562:Electrocardiography; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D006801:Humans; D015640:Ion Channel Gating / Q000187:drug effects; D008297:Male; D008875:Middle Aged; D009206:Myocardium / Q000201:enzymology; D019319:Okadaic Acid / Q000494:pharmacology; D010749:Phosphoprotein Phosphatases / Q000032:analysis / Q000378:metabolism*; D010744:Phosphoric Monoester Hydrolases / Q000037:antagonists & inhibitors; D054648:Protein Phosphatase 2
D000318:Adrenergic beta-Agonists; D000319:Adrenergic beta-Antagonists; D020746:Calcium Channels, L-Type; D004791:Enzyme Inhibitors; D019319:Okadaic Acid; D010749:Phosphoprotein Phosphatases; D054648:Protein Phosphatase 2; D010744:Phosphoric Monoester Hydrolases