HCN212-channel biological pacemakers manifesting ventricular tachyarrhythmias are responsive to treatment with I(f) blockade.
BACKGROUND
A potential concern about biological pacemakers is their possible malfunction, which might create ventricular tachycardias (VTs).


OBJECTIVE
The purpose of this study was to test our hypothesis that should VTs complicate implantation of HCN-channel-based biological pacemakers, they would be suppressed by inhibitors of the pacemaker current, I(f).


METHODS
We created a chimeric channel (HCN212) containing the N- and C-termini of mouse HCN2 and the transmembrane region of mouse HCN1 and implanted it in HEK293 cells. Forty-eight hours later, in whole-cell patch clamp recordings, mean steady state block induced by 3 microM ivabradine (IVB) showed HCN1 = HCN212 > HCN2 currents. The HCN212 adenoviral construct was then implanted into the canine left bundle branch in 11 dogs. Complete AV block was created via radiofrequency ablation, and a ventricular demand electronic pacemaker was implanted (VVI 45 bpm). Electrocardiogram, 24-hour Holter monitoring, and pacemaker log record check were performed for 11 days.


RESULTS
All dogs developed rapid VT (>120 bpm, maximum rate = 285 +/- 37 bpm) at 0.9 +/- 0.3 days after implantation that persisted through 5 +/- 1 days. IVB, 1 mg/kg over 5 minutes, was administered during rapid VT, and three dogs received a second dose 24 hours later. While VT terminated with IBV in all instances within 3.4 +/- 0.6 minutes, no effect of IVB on sinus rate was noted.


CONCLUSIONS
We conclude that (1) I(f)-associated tachyarrhythmias-if they occur with HCN-based biological pacemakers-can be controlled with I(f)-inhibiting drugs such as IVB; (2) in vitro, IVB appears to have a greater steady state inhibiting effect on HCN1 and HCN212 isoforms than on HCN4; and (3) VT originating from the HCN212 injection site is suppressed more readily than sinus rhythm. This suggests a selectivity of IVB at the concentration attained for ectopic over HCN4-based pacemaker function. This might confer a therapeutic benefit.
Plotnikov
Alexei N
AN
Center for Molecular Therapeutics, Columbia University, New York, New York 10032, USA.
Bucchi
Annalisa
A
Shlapakova
Iryna
I
Danilo
Peter
P
Brink
Peter R
PR
Robinson
Richard B
RB
Cohen
Ira S
IS
Rosen
Michael R
MR
D000818:Animals; D001552:Benzazepines / Q000494:pharmacology*; D015220:Calcium Channels; D002304:Cardiac Pacing, Artificial; D002317:Cardiovascular Agents / Q000494:pharmacology*; D017115:Catheter Ablation; D017147:Defibrillators, Implantable; D004285:Dogs; D004594:Electrophysiology; D000077550:Ivabradine; D008297:Male; D032342:Muscle Cells; D051381:Rats; D012307:Risk Factors; D017180:Tachycardia, Ventricular / Q000188:drug therapy / Q000503:physiopathology* / Q000628:therapy*
D001552:Benzazepines; D015220:Calcium Channels; D002317:Cardiovascular Agents; D000077550:Ivabradine