Remodeling of Na,K-ATPase, and membrane fluidity after atrial fibrillation in sheep.

Atrial fibrillation (AF) is accompanied by various changes in ion channels that cause atrial electrophysiological remodeling. The enzyme Na,K-ATPase is also a major cellular mechanism for the regulation of ion homeostasis. During AF, Na,K-ATPase may be regulated by synthesis of its alpha- and beta-subunits as well as changes in membrane fluidity. To test this hypothesis, we studied the effect of pacing-induced AF in sheep on atrial Na,K-ATPase alpha- and beta-subunits and on membrane fluidity as well. Methods: A group of six sheep (AF group) was subjected to overdrive electrical stimulation of the right atrium in order to induce AF. A group of six sham operated sheep served as control. All paced sheep developed multiple episodes of sustained-AF with a mean total duration of 110 min over a 2-hours period. Protein expression of Na,K-ATPase alpha- and beta-subunits in atrial microsomal membranes was assayed by Western blotting analysis. When significant changes in membrane expression were observed, transcriptional regulation was analysed by Northern blotting. Membrane fluidity was assessed on atrial microsomal fractions by anisotropy measurements using the fluorescent probe diphenylhexatriene. Results: Atrial fibrillation enhanced the expression of the Na,K-ATPase beta(1)-subunit at both membrane and mRNA levels. Anisotropy values were higher in AF group than in control group, indicating a decreased fluidity of the membranes isolated from paced sheep atria. Conclusion: These data are the first evidence for an enhanced Na,K-ATPase beta(1)-subunit expression in membrane during AF. Membrane rigification represents a new factor of tachycardia-induced atrial remodeling.