Focused electric field (FEF) ablation in a left ventricular and infrared thermal imaging model: a proof-of-concept study.

BACKGROUND: Currently, RF ablation is limited in its ability to deliver deep lesions, as most of the energy delivered to the tissue is dissipated in the first few millimeters from the catheter tip. Focused electric field (FEF) is a novel technology with the potential to ablate deeper than currently available RF catheters. This work is the first proof of concept of FEF technology. OBJECTIVE: To introduce FEF technology and demonstrate its feasibility as an ablation tool. METHODS: We constructed a FEF catheter with a truncated dome-shaped tip, creating a toroidal ablating surface. We performed ablation ex vivo in porcine hearts and examined ablation characteristics using both tissue sectioning and real-time thermal imaging. RESULTS: RF lesions were 9.1 ± 1.0 mm wide by 6.1 ± 1.1 mm deep with ablation using a conventional irrigated tip catheter (Thermocool SF). In contrast, lesions created using FEF ablation were 12.8 ± 1.6 mm wide and 14.0 ± 1.6 mm deep. Steam pops were less frequent in the FEF group. Thermal imaging demonstrated that in contrast to an irrigated tip RF catheter, the FEF catheter generated a uniform temperature profile down to a maximum depth exceeding 15 mm. CONCLUSION: This study is the first proof of concept of FEF technology. Using a novel toroidal catheter tip design, the electric field remains confined to a narrow tissue region, thus avoiding the rapid fall off in energy delivery from the tissue surface inherent to current RF catheter designs. FEF ablation may allow delivery of deeper ablations lesions with potentially lower risk of tissue hyperthermia than conventional catheters. Future studies are needed.

Analysis of the cyclic nucleotide binding domain of the HERG potassium channel and interactions with KCNE2.

Mutations in the cyclic nucleotide binding domain (CNBD) of the human ether-a-go-go-related gene (HERG) K+ channel are associated with LQT2, a form of hereditary Long QT syndrome (LQTS). Elevation of cAMP can modulate HERG K+ channels both by direct binding and indirect regulation through protein kinase A. To assess the physiological significance of cAMP binding to HERG, we introduced mutations to disrupt the cyclic nucleotide binding domain. Eight mutants including two naturally occurring LQT2 mutants V822M and R823W were constructed. Relative cAMP binding capacity was reduced or absent in CNBD mutants. Mutant homotetramers carry little or no K+ current despite normal protein abundance and surface expression. Co-expression of mutant and wild-type HERG resulted in currents with altered voltage dependence but without dominant current suppression. The data from co-expression of V822M and wild-type HERG best fit a model where one normal subunit within a tetramer allows nearly normal current expression. The presence of KCNE2, an accessory protein that associates with HERG, however, conferred a partially dominant current suppression by CNBD mutants. Thus KCNE2 plays a pivotal role in determining the phenotypic severity of some forms of LQT2, which suggests that the CNBD of HERG may be involved in its interaction with KCNE2.

Structural determinants of KvLQT1 control by the KCNE family of proteins.

KvLQT1 is a Shaker-like voltage-gated potassium channel that when complexed with minK (KCNE1) produces the slowly activating delayed rectifier I(ks). The emerging family of KCNE1-related peptides includes KCNE1 and KCNE3, both of which complex with KvLQT1 to produce functionally distinct currents. Namely I(ks), the slowly activating delayed rectifier current, is produced by KvLQT1/KCNE1, whereas KvLQT1/KCNE3 yields a more rapidly activating current with a distinct constitutively active component. We exploited these functional differences and the general structural similarities of KCNE1 and KCNE3 to study which physical regions are critical for control of KvLQT1 by making chimerical constructs of KCNE1 and KCNE3. By using this approach, we have found that a three-amino acid stretch within the transmembrane domain is necessary and sufficient to confer specificity of control of activation kinetics by KCNE1 and KCNE3. Moreover, chimera analysis showed that different regions within the transmembrane domain control deactivation rates. Our results help to provide a basis for understanding the mechanism by which KCNE proteins control K(+) channel activity.

Cyclic AMP regulates the HERG K(+) channel by dual pathways.

Lethal cardiac arrhythmias are a hallmark of the hereditary Long QT syndrome (LQTS), a disease produced by mutations of cardiac ion channels [1]. Often these arrhythmias are stress-induced, suggesting a relationship between beta-adrenergic activation of adenylate cyclase and cAMP-dependent alteration of one or more of the ion channels involved in LQTS. Second messengers modulate ion channel activity either by direct interaction or through intermediary kinases and phosphatases. Here we show that the second messenger cAMP regulates the K(+) channel mutated in the LQT2 form of LQTS, HERG [2], both directly and indirectly. Activation of cAMP-dependent protein kinase (PKA) causes phosphorylation of HERG accompanied by a rapid reduction in current amplitude, acceleration of voltage-dependent deactivation, and depolarizing shift in voltage-dependent activation. In a parallel pathway, cAMP directly binds to the HERG protein with the opposing effect of a hyperpolarizing shift in voltage-dependent activation. The summation of cAMP-mediated effects is a net diminution of the effective current, but when HERG is complexed with with the K(+) channel accessory proteins MiRP1 or minK, the stimulatory effects of cAMP are favored. These findings provide a direct link between stress and arrhythmia by a unique mechanism where a single second messenger exerts complex regulation of an ion channel via two distinct pathways.