Age-dependent changes in ion channel mRNA expression in canine cardiac tissues

Ion channel mRNA distribution and expression in the sinoatrial node and right atrium of dogs and monkeys

There are limited data on the gene expression profiles of ion channels in the sinoatrial node (SAN) of dogs and monkeys. In this study, the messenger RNA (mRNA) expression profiles of various ion channels in the SAN of naïve dogs and monkeys were examined using RNAscope^®in situ hybridization and compared with those in the surrounding right atrium (RA) of each species. Regional-specific Cav1.3 and HCN4 expression was observed in the SAN of dogs and monkeys. Additionally, HCN1 in dogs was only expressed in the SAN. The expression profiles of Cav3.1 and Cav3.2 in the SAN and RA were completely different between dogs and monkeys. Dog hearts only expressed Cav3.2; however, Cav3.1 was detected only in monkeys, and the expression score in the SAN was slightly higher than that in the RA. Although Kir3.1 and NCX1 in dogs were equally expressed in both the SAN and RA, the expression scores of these genes in the SAN of monkeys were slightly higher than those in the RA. The Kir3.4 expression score in the SAN of dogs and monkeys was also slightly higher than that in the RA. The mRNA expression scores of Kv11.1/ERG and KvLQT1 were equally observed in both the SAN and RA of dogs and monkeys. HCN2 was not detected in dogs and monkeys. In summary, we used RNAscope to demonstrate the SAN-specific gene expression patterns of ion channels, which may be useful in explaining the effect of pacemaking and/or hemodynamic effects in nonclinical studies.

In-silico analysis of aging mechanisms of action potential remodeling in human atrial cardiomyocites

Electrophysiology of cardiomyocytes changes with aging. Agerelated ionic remodeling in cardiomyocytes may increase the incidence and prevalence of atrial fibrillation (AF) in the elderly and affect the efficiency of antiarrhythmic drugs. There is the deep lack of experimental data on an action potential and transmembrane currents recorded in the healthy human cardiomyocytes of different age. Experimental data in mammals is also incomplete and often contradicting depending on the experimental conditions. In this in-silico study, we used a population of ionic models of human atrial cardiomyocytes to transfer data on the age- related ionic remodeling in atrial cardiomyocytes from canines and mice to predict possible consequences for human cardiomyocyte activity. Based on experimental data, we analyzes two hypotheses on the aging effect on the ionic currents using two age-related sets of varied model parameters and evaluated corresponding changes in action potential morphology with aging. Using the two populations of aging models, we analyzed the agedependent sensitivity of atrial cardiomyocytes to Dofetilide which is one of the antiarrhythmic drugs widely used in patients with atrial fibrillation.

Splicing regulatory factors, ageing and age-related disease

Alternative splicing is a co-transcriptional process, which allows for the production of multiple transcripts from a single gene and is emerging as an important control point for gene expression. Alternatively expressed isoforms often have antagonistic function and differential temporal or spatial expression patterns, yielding enormous plasticity and adaptability to cells and increasing their ability to respond to environmental challenge. The regulation of alternative splicing is critical for numerous cellular functions in both pathological and physiological conditions, and deregulated alternative splicing is a key feature of common chronic diseases. Isoform choice is controlled by a battery of splicing regulatory proteins, which include the serine arginine rich (SRSF) proteins and the heterogeneous ribonucleoprotein (hnRNP) classes of genes. These important splicing regulators have been implicated in age-related disease, and in the ageing process itself. This review will outline the important contribution of splicing regulator proteins to ageing and age-related disease.

KChIP2 genotype dependence of transient outward current (Ito) properties in cardiomyocytes isolated from male and female mice

The transient outward current (I_to) in cardiomyocytes is largely mediated by Kv4 channels associated with Kv Channel Interacting Protein 2 (KChIP2). A knockout model has documented the critical role of KChIP2 in I_to expression. The present study was conducted to characterize in both sexes the dependence of I_to properties, including current magnitude, inactivation kinetics, recovery from inactivation and voltage dependence of inactivation, on the number of functional KChIP2 alleles. For this purpose we performed whole-cell patch-clamp experiments on isolated left ventricular cardiomyocytes from male and female mice which had different KChIP2 genotypes; i.e., wild-type (KChIP2^+/+), heterozygous knockout (KChIP2^+/-) or complete knockout of KChIP2 (KChIP2^-/-). We found in both sexes a KChIP2 gene dosage effect (i.e., a proportionality between number of alleles and phenotype) on I_to magnitude, however, concerning other I_to properties, KChIP2^+/- resembled KChIP2^+/+. Only in the total absence of KChIP2 (KChIP2^-/-) we observed a slowing of I_to kinetics, a slowing of recovery from inactivation and a negative shift of a portion of the voltage dependence of inactivation. In a minor fraction of KChIP2^-/- myocytes I_to was completely lost. The distinct KChIP2 genotype dependences of I_to magnitude and inactivation kinetics, respectively, seen in cardiomyocytes were reproduced with two-electrode voltage-clamp experiments on Xenopus oocytes expressing Kv4.2 and different amounts of KChIP2. Our results corroborate the critical role of KChIP2 in controlling I_to properties. They demonstrate that the Kv4.2/KChIP2 interaction in cardiomyocytes is highly dynamic, with a clear KChIP2 gene dosage effect on Kv4 channel surface expression but not on inactivation gating.

Developmental changes in expression and biophysics of ion channels in the canine ventricle

BACKGROUND

Developmental changes in the electrical characteristics of the ventricular myocardium are not well defined. This study examines the contribution of inwardly rectifying K(+) current (IK1), transient outward K(+) current (Ito), delayed rectifier K(+) currents (IKr and IKs) and sodium channel current (INa) to repolarization in the canine neonate myocardium.

METHODS

Single myocytes isolated from the left ventricle of 2-3week old canine neonate hearts were studied using patch-clamp techniques.

RESULTS

Neonate cells were ~6-fold smaller than those of adults (28.8±8.8 vs. 176±6.7pF). IK1 was larger in neonate myocytes and displayed a substantial inward component and an outward component with negative slope conductance, peaking at -60mV (4.13 pA/pF). IKr tail currents (at -40mV), were small (<20pA). IKs could not be detected, even after exposure to isoproterenol (100nM). Ito was also absent in the neonate, consistent with the absence of a phase 1 in the action potential. Peak INa, late INa and ICa were smaller in the neonate compared with adults. KCND3, KCNIP2 and KCNQ1 mRNA expression was half, while KCNH2 was equal and KCNJ2 was greater in the neonate when compared with adults.

CONCLUSIONS

Two major repolarizing K(+) currents (IKs and Ito) present in adult ventricular cells are absent in the 2week old neonate. Peak and late INa are significantly smaller in the neonate. Our results suggest that the absence of these two currents in the neonate heart may increase the susceptibility to arrhythmias under certain long QT conditions.

Electrophysiological Characterization and Antiarrhythmic Efficacy of the Mixed Potassium Channel-Blocking Antiarrhythmic Agent AZ13395438 In Vitro and In Vivo

Objective:

To examine the electrophysiological, hemodynamic, and antiarrhythmic effects of the novel antiarrhythmic agent AZ13395438.

Methods:

The ion channel-blocking potency of AZ13395438 was assessed in Chinese hamster ovary cells stably expressing various human cardiac ion channels and in human atrial myocytes. The in vivo electrophysiological, hemodynamic, and antiarrhythmic effects of intravenously administered AZ13395438 were examined in anesthetized rabbits, in anesthetized naive dogs, and in dogs subjected to rapid atrial pacing (RAP) for 8 weeks. Pharmacokinetic/pharmacodynamic (PKPD) modeling was applied to predict the potency of AZ13395438 in increasing atrial and ventricular refractoriness.

Results:

AZ13395438 potently and predominantly blocked the atrial repolarizing potassium currents IKur, IAch, and Ito in vitro. In vivo, AZ13395438 caused a concentration-dependent and selective increase in atrial refractoriness with no or small effects on ventricular refractoriness and repolarization and on hemodynamics in both rabbits and dogs. The PKPD modeling predicted unbound plasma concentrations of AZ13395438 of 0.20 ± 0.039, 0.38 ± 0.084, and 0.34 ± 0.057 µmol/L to increase the right atrial effective refractory period by 20 milliseconds in the rabbit and in the naive and the RAP dogs, respectively. In the RAP dog with atrial fibrillation (AF), AZ13395438 significantly increased AF cycle length and successfully converted AF to sinus rhythm in 12 of the 12 occasions at an unbound plasma concentration of 0.48 ± 0.076 µmol/L. During saline infusion, conversion was seen only in 4 of the 10 occasions ( P = .003 vs AZ13395438). Furthermore, AZ13395438 reduced AF inducibility by burst pacing from 100% to 25% ( P < .001).

Conclusion:

AZ13395438 can be characterized as a mixed potassium ion channel-blocking agent that selectively prolongs atrial versus ventricular refractoriness and shows promising antiarrhythmic efficacy in a clinically relevant animal model of AF.