Ionic mechanisms limiting cardiac repolarization reserve in humans compared to dogs

Abstract

Key points• Cardiac repolarization, through which heart-cells return to their resting state after having fired, is a delicate process, susceptible to disruption by common drugs and clinical conditions. • Animal models, particularly the dog, are often used to study repolarization properties and responses to drugs, with the assumption that such findings are relevant to humans. However, little is known about the applicability of findings in animals to man.• Here, we studied the contribution of various ion-currents to cardiac repolarization in canine and human ventricle.• Humans showed much greater repolarization-impairing effects of drugs blocking the rapid delayed-rectifier current I Kr than dogs, because of lower repolarization-reserve contributions from two other important repolarizing currents (the inward-rectifier I K1 and slow delayed-rectifier I Ks ).• Our findings clarify differences in cardiac repolarization-processes among species, highlighting the importance of caution when extrapolating results from animal models to man.Abstract The species-specific determinants of repolarization are poorly understood. This study compared the contribution of various currents to cardiac repolarization in canine and human ventricle. Conventional microelectrode, whole-cell patch-clamp, molecular biological and mathematical modelling techniques were used. Selective I Kr block (50-100 nmol l −1 dofetilide) lengthened AP duration at 90% of repolarization (APD 90 ) >3-fold more in human than dog, suggesting smaller repolarization reserve in humans. Selective I K1 block (10 μmol l −1 BaCl 2 ) and I Ks block (1 μmol l −1 HMR-1556) increased APD 90 more in canine than human right ventricular papillary muscle. Ion current measurements in isolated cardiomyocytes showed that I K1 and I Ks densities were 3-and 4.5-fold larger in dogs than humans, respectively. I Kr density and kinetics were similar in human versus dog. I Ca and I to were respectively ∼30% larger and ∼29% smaller in human, and Na + -Ca 2+ exchange current was comparable. Cardiac mRNA levels for the main I K1 ion channel subunit Kir2.1 and the I Ks accessory subunit minK were significantly lower, but mRNA expression of ERG and KvLQT1 (I Kr and I Ks α-subunits) were not significantly different, in human versus dog. Immunostaining suggested lower Kir2.1 and minK, and higher KvLQT1 protein expression in human versus canine cardiomyocytes. I K1 and I Ks inhibition increased the APD-prolonging effect of I Kr block more in dog (by 56% and 49%, respectively) than human (34 and 16%), indicating that both currents contribute to increased repolarization reserve in the dog. A mathematical model incorporating observed human-canine ion current differences confirmed the role of I K1 and I Ks in repolarization reserve differences. Thus, humans show greater repolarization-delaying effects of I Kr block than dogs, because of lower repolarization reserve contributions from I K1 and I Ks , emphasizing species-specific determinants of repolarization and the limitations of animal models fo...

hERG 1b is critical for human cardiac repolarization

“…S2) (30-34). These attributes as well as biochemical and morphological characteristics suggest that the iPSC-CMs are at an immature developmental stage (35)(36)(37)(38)(39) compared with a fully developed ventricular cardiomyocyte (diastolic potential < −80 mV; dV/dt MAX = 150-350 V/s) (40,41). Thus, iPSC-CMs could prove extremely valuable as a model for the developing heart, especially for cases of intrauterine fetal death and LQTS in the young (11,18,42).…”

I Kr Impact on Repolarization and Its Variability Assessed by Dynamic Clamp

“…Overall, changes in activation/deactivation gating affected I Kr during Phase 3 only ( Figure 7A); indeed, during Phases 1 and 2, corresponding to the AP plateau, I Kr was largely inactivated, thus making changes in activation/deactivation inconsequential. Although this is true in guinea-pig myocytes, 11 I Kr inactivation may be slightly weaker in the human AP, 16 and this might partially unveil the effect of activation/deactivation gating. Downregulation of channel expression or channel blockade (both represented by g max reduction) are the most common I Kr abnormality underlying QT prolongation 17 ; therefore, the relatively small effect of g max changes on APD ( Figure 3D) was unexpected.…”

“…18,19 This interpretation implies that activation/ deactivation gating might be more relevant to APD modulation if I Kr inactivation during Phase 1 were less complete, as may be the case in canine and human myocytes. 16 …”

Human ex-vivo action potential model for pro-arrhythmia risk assessment

“…Various academic groups have used, to a very limited extent, cardiac tissues from healthy donors (Boukens et al, 2015;Jost et al, 2005Jost et al, , 2013 and patients (Brandenburger et al, 2015;Näbauer, Beuckelmann, Überfuhr, & Steinbeck, 1996;Näbauer & Kääb, 1998;Nawrath & Eckel, 1979;Sivagangabalan et al, 2014;Wettwer et al, 1993;Wettwer, Amos, Posival, & Ravens, 1994) to investigate the role of ion channels, specifically potassium channels, in human ventricular repolarization. Additionally, dofetilide, Sotalol and quinidine have been found to prolong ventricular repolarization in healthy human hearts (QT duration;Johannsen et al, 2014;Lande et al, 1998;Vicente et al, 2015) and patients (QT duration (Echt et al, 1982;McComb, McGovern, McGowan, Ruskin, & Garan, 1987); monophasic AP recordings (Melicherick et al, 1999;Nademanee et al, 1990;Yuan, Wohlfart, Rasmussen, Olsson, & Blomstrom-Lundqvist, 1994)).…”

“…While access to animal tissues is typically plentiful, the availability of human cardiac samples is limited. When available, human tissues have typically been limited to fixed or frozen samples or very small amounts of fresh and viable tissues which were obtained from surgical discards (Boukens et al, 2015;Jost et al, 2005Jost et al, , 2013Brandenburger et al, 2015;Näbauer, Beuckelmann, Überfuhr, & Steinbeck, 1996;Näbauer & Kääb, 1998;Wettwer et al, 1993;Wettwer, Amos, Posival, & Ravens, 1994). However, in the past, tissue recovery procedures, as well as tissue quality and quantity have been extremely variable, resulting in an unreliable source for robust and reproducible drug safety data.…”