Abstracts

Rationale: Brain neuronal and cardiac activity is regulated by many different ion channels, including the hyperpolarization-activated cyclic nucleotide-gated cation channels (HCN). Current generated from HCN channels (Ih) are thought to play a role in spontaneous pacemaker activity in the heart and brain, and alteration in expression and function of these channels has been reported in brain structures in human and animal models of epilepsy. Whether these changes are also seen in the heart has not been explored. This study investigated expression of mRNA for HCN channel isoforms in the heart chambers of Genetic Absence Epilepsy Rats from Strasbourg (GAERS), a polygenic rat model of idiopathic generalised epilepsy, and for any associated changes in cardiac electrophysiology, as compared to their non-epileptic control (NEC) strain. Methods: RNA was extracted RNA was extracted from the 4 cardiac chambers of adult GAERS and NEC rats using RNeasy mini kit (QIAGEN) and treated with DNase I (QIAGEN) to remove any contaminating genomic DNA. 500ng of RNA was reversed transcribed to cDNA using the Omniscript Reverse Transcription kit (QIAGEN). Quantitative PCR (qPCR) was performed on 25ng cDNA using custom designed gene expression assays for HCN1 and HCN2 (Applied Biosystems). HCN mRNA levels were compared to mRNA levels of the housekeeping gene β-actin using a custom designed gene expression assay for this gene (Applied Biosystems). Analysis was performed using the ΔΔCT method. Electrocardiograms (ECG) were recorded from adult GAERS and NEC rats under neurolept anaesthesia. Cardiac variables measured from the ECG trace included: ST interval, QRS complex, QT interval and heart rate. Since QT interval varies with heart rate and rodent heart rate is higher than human, Bazzet’s correction formula was used to correct its value based on the heart rate (QTc). Results: HCN2 mRNA expression was significantly decreased in all 4 cardiac chambers in GAERS (n=9-16) compared to NEC (n=10-12) rats with a 37% and 32% reduction observed in left (p<0.01) and right (p<0.01) atria respectively and a 19% and 18% reduction observed in left (p<0.01) and right (p<0.05) ventricles respectively. Cardiac function was also altered in GAERS (n=10) with a shorter QRS complex (p<0.05) (reflective of ventricular depolarization) and a longer ST interval (p<0.05) (reflective of ventricular repolarization) compared to NEC rats (n=5). Overall there was no significant difference in heart rate between GAERS and NEC rats. Conclusions: GAERS have decreased expression of HCN2 channels in the heart compared to NEC rats. This would be expected to result in significant effects on the kinetics of Ih in the heart, and is consistent with the electrophysiological changes demonstrated. These results demonstrate that changes in ion channel expression and function in the epileptic brain may also be present in the heart, and this may have clinical implications by increasing the risk for cardiac arrhythmias and even sudden death in epilepsy.