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

Abstract

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 Ϫ46.9 mV in mE324A versus Ϫ66.1 mV in mHCN2 (PϽ0.05). mE324A manifested a positive shift of voltage dependence of gating kinetics of activation and deactivation compared with mHCN2 (PϽ0.05) 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; PϽ0.05). When spontaneous rate fell below 45 bpm, EPM intervened at that rate, triggering 83% of beats in control, contrasting (PϽ0.05) with 26% (mHCN2) and 36% (mE324A). On day 14, epinephrine (1 g/kg per minute IV) induced a 50% heart rate increase in all mE324A, one third of mHCN2, and one fifth of control (PϽ0.05 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.

Properties of WT and mutant hERG K+ channels expressed in neonatal mouse cardiomyocytes

“…Compared with adult mouse cardiomyocytes, neonatal mouse cardiomyocytes lack transverse tubules, which may have an advantage in voltage-clamp experiments (27). Neonatal mouse cardiomyocytes have been used as a model for ion channel expression, including for the Na ϩ channel Nav1.5 (39,44), Tand L-type Ca 2ϩ channels (17,21), the K ϩ channel Kv4.2 (18), and the pacemaker HCN channel (9). The study of WT and mutant ion channels in native cardiac myocyte models is important since ion channel mutations have been reported to function differently in native myocyte and HEK-293 cell expression systems (26).…”

Biological Pacemakers – A Review

“…Increased escape rate was demonstrated by injection of adenovirus carrying HCN2 in canine left atrium [44] and in the canine left bundle branch [34]. In a canine model of International Journal of Cardiovascular Practice atrioventricular block, implanted with a pacemaker set at a rate of 45 beats per minute, requirement of pacing was reduced after injection of the wild-type HCN2 [45]. Another group showed reduction of pacing requirement in a porcine model of sick sinus syndrome with a pacemaker set at 60 beats per minute.…”

Biologic Pacemaker - Role of Gene and Cell Therapy in Cardiac Arrhythmias

“…Tandem pacing is the proposed way to proceed clinically (patients with chronic atrial fibrillation and complete heart block); i.e. implant both a biological pacemaker and an electronic demand pacemaker in the same individual , this has been tested in dogs in complete heart block an adenoviral HCN2 construct (into the left bundle-branch system) were delivered and an electronic demand unit, the electrode of which was placed in the right ventricular endocardial apex (Bucchi et al 2006). The biological pacemaker fired 70% of the time and was catecholamine responsive.…”