Published Online
on August 21, 2006

A more recent version of this article appeared on September 5, 2006

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Submitted on February 6, 2006
Revised on June 5, 2006
Accepted on June 9, 2006

Wild-Type and Mutant HCN Channels in a Tandem Biological-Electronic Cardiac Pacemaker

Annalisa Bucchi PhD, Alexei N. Plotnikov MD, Iryna Shlapakova MD, Peter Danilo Jr PhD, Yelena Kryukova PhD, Jihong Qu PhD, Zhongju Lu MD, PhD, Huilin Liu MD, Zongming Pan PhD, Irina Potapova PhD, Bruce Ken Knight PhD, Steven Girouard PhD, Ira S. Cohen MD, PhD, Peter R. Brink PhD, Richard B. Robinson PhD, and Michael R. Rosen MD^*

From the Departments of Pharmacology (A.B., A.N.P., I.S., P.D., Y.K., I.S.C., R.B.R., M.R.R.) and Pediatrics (M.R.R.), Center for Molecular Therapeutics, Columbia University, New York, NY; Guidant Corporation, St Paul, Minn (J.Q., B.K., S.G.); and Departments of Physiology and Biophysics, Institute of Molecular Cardiology, State University of New York at Stony Brook (Z.L., H.L., Z.P., I.P., I.S.C., P.R.B., M.R.R.).

^* To whom correspondence should be addressed. E-mail: mrr1{at}columbia.edu.

Background--Biological pacemakers (BPM) implanted in canine left bundle branch function competitively with electronic pacemakers (EPM). We hypothesized that BPM engineered with the use of mE324A mutant murine HCN2 (mHCN2) genes would improve function over mHCN2 and that BPM/EPM tandems confer advantage over either approach alone.

Methods and Results--In cultured neonatal rat myocytes, activation midpoint was -46.9 mV in mE324A versus -66.1 mV in mHCN2 (P<0.05). mE324A manifested a positive shift of voltage dependence of gating kinetics of activation and deactivation compared with mHCN2 (P<0.05) in myocytes as well as Xenopus oocytes. In intact dogs in complete atrioventricular block, saline (control), mHCN2, or mE324A virus was injected into left bundle branch, and EPM were implanted (VVI 45 bpm). Twenty-four-hour ECGs were monitored for 14 days. With EPM discontinued, there was no difference in duration of overdrive suppression among groups. However, basal heart rates in controls were less than those in mHCN2, which did not differ from those in E324A (45 versus 57 versus 53 bpm; P<0.05). When spontaneous rate fell below 45 bpm, EPM intervened at that rate, triggering 83% of beats in control, contrasting (P<0.05) with 26% (mHCN2) and 36% (mE324A). On day 14, epinephrine (1 µg/kg per minute IV) induced a 50% heart rate increase in all mE324A, one third of mHCN2, and one fifth of control (P<0.05 mE324A versus control or mHCN2).

Conclusions--mE324A induces faster, more positive pacemaker current activation than mHCN2 and stable, catecholamine-sensitive rhythms in situ that compete with EPM comparably but more catecholamine responsively than mHCN2. BPM/EPM tandems function reliably, reduce the number of EPM beats, and confer sympathetic responsiveness to the tandem.

Key words: electrocardiography • electrophysiology • gene therapy • heart block • pacemakers

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