The cAMP response element binding protein modulates expression of the transient outward current: Implications for cardiac memory

Objective: Long-term cardiac memory (LTCM), expressed as a specific pattern of T-wave change on ECG, is associated with 1) reduced transient outward potassium current ( I to), 2) reduced mRNA for the pore-forming protein of I to, Kv4.3, 3) reduced cAMP response element binding protein (CREB), and 4) diminished binding to its docking site on the DNA, the cAMP response element (CRE). We hypothesized a causal link between the decrease of the transcription factor CREB and down-regulation of I to and one of its channel subunits, KChIP2, in LTCM. Methods: After three weeks of left ventricular pacing to induce LTCM (8 paced, 7 sham control dogs), epicardial KChIP2 mRNA and protein levels were assessed by real-time PCR and Western blotting. Mimicking the CREB down-regulation in LTCM, CREB was knocked down in situ in other dogs using adenoviral anti-sense. Effects on the action potential notch, reflecting I to, were investigated in situ using monophasic action potential (MAP) recordings and at the cellular level by the whole-cell patch clamp technique. CREB binding in the KChIP2 promoter region was ascertained by electrophoretic mobility-shift assays. Results: In LTCM, epicardial KChIP2 mRNA and protein were reduced by 62% and 76%, respectively, compared to shams ( p <0.05). CREB binding by the canine KChIP2 promoter region was demonstrated. CREB knockdown led to disappearance of the phase1 notch in MAP and ablation of I to. Conclusions: These results strengthen the hypothesis that down-regulation of CREB-mediated transcription underlies the attenuation of epicardial I to in LTCM. They also emphasize that ventricular pacing exerts effects at a subcellular level contributing to memory and conceivably to other forms of cardiac remodeling.