Modulation of collision-induced changes in canine heart repolarization by cycle length

HCN3 contributes to the ventricular action potential waveform in the murine heart

RATIONALE

The hyperpolarization-activated current I(h) that is generated by hyperpolarization-activated cyclic nucleotide-gated channels (HCNs) plays a key role in the control of pacemaker activity in sinoatrial node cells of the heart. By contrast, it is unclear whether I(h) is also relevant for normal function of cardiac ventricles.

OBJECTIVE

To study the role of the HCN3-mediated component of ventricular I(h) in normal ventricular function.

METHODS AND RESULTS

To test the hypothesis that HCN3 regulates the ventricular action potential waveform, we have generated and analyzed a HCN3-deficient mouse line. At basal heart rate, mice deficient for HCN3 displayed a profound increase in the T-wave amplitude in telemetric electrocardiographic measurements. Action potential recordings on isolated ventricular myocytes indicate that this effect was caused by an acceleration of the late repolarization phase in epicardial myocytes. Furthermore, the resting membrane potential was shifted to more hyperpolarized potentials in HCN3-deficient mice. Cardiomyocytes of HCN3-deficient mice displayed approximately 30% reduction of total I(h). At physiological ionic conditions, the HCN3-mediated current had a reversal potential of approximately -35 mV and displayed ultraslow deactivation kinetics.

CONCLUSIONS

We propose that HCN3 together with other members of the HCN channel family confer a depolarizing background current that regulates ventricular resting potential and counteracts the action of hyperpolarizing potassium currents in late repolarization. In conclusion, our data indicate that HCN3 plays an important role in shaping the cardiac action potential waveform.

Effects of activation sequence on monophasic action potential configuration in the dog

The effects of altered activation sequence on the monophasic action potential (MAP) in in situ beating hearts are not known, although its effects on refractory periods are well documented. In nine anesthetized, open-chest dogs, complete atrioventricular block was produced, and the heart was driven by either right ventricular or left ventricular stimulation. The MAPs of the right and left ventricles were recorded by contact electrodes at cycle lengths of 1,000, 800, 600, and 400 ms. The MAP configuration was evaluated with regard to the difference between phase 1 and phase 2 MAP amplitudes and MAP duration at 50 and 90% repolarization. An MAP recorded from the ventricle that was being electrically stimulated was designated an ipsilateral ventricular stimulation, whereas the MAP recorded from the nonstimulated ventricle was termed a contralateral ventricular stimulation. The difference in amplitude and the 50% and 90% MAP durations for ipsilateral ventricular stimulation were consistently larger than for contralateral ventricular stimulation at all cycle lengths tested. Transient outward current did not appear to play a major role in producing such differences in MAP because intravenous treatment with 4-aminopyridine, a blocker of transient outward current, did not affect the configuration of the MAP. These findings provide an insight on the influence of ventricular activation sequence on the shape of the transmembrane action potential.