Sinus node dysfunction and hyperpolarization-activated (HCN) channel subunit remodeling in a canine heart failure...

The hyperpolarization-activated cation current I(f) contributes significantly to sinoatrial node pacemaker function and possibly to ectopic arrhythmogenesis. Little is known about the expression of corresponding hyperpolarization-activated cyclic nucleotide-gated (HCN) channel subunits in normal hearts and HCN remodeling by diseases, like congestive heart failure (CHF), associated with disturbances of cardiac rhythm.

We assessed expression of HCN1, 2 and 4 in normal mongrel dogs and dogs subjected to 2-week ventricular tachypacing-induced CHF. Competitive RT-PCR, Western blot and immunohistochemistry were used to quantify HCN subunit mRNA and protein expression in the right atrium (RA) and sinoatrial node. CHF approximately doubled sinus node recovery time, indicating suppressed sinus node pacemaker function. HCN expression under control conditions was HCN4 > HCN2 >> HCN1. HCN2 and HCN4 expression was greater at both protein and mRNA levels in sinoatrial node than RA. CHF significantly decreased sinus node HCN expression at both mRNA and protein levels (HCN2 by 78% and 82%; HCN4 by 42% and 77%, respectively). RA HCN2 expression was unaltered by CHF, but HCN4 was significantly upregulated (by 209%).

HCN4 is the dominant subunit in canine sinoatrial node and RA; strong sinus node HCN expression likely contributes to its pacemaker function; downregulation of HCN4 and HCN2 expression contribute to CHF-induced sinus node dysfunction; and upregulation of atrial HCN4 may help to promote atrial arrhythmia formation. These findings provide novel information about the molecular basis of normal and disease-related impairments of cardiac impulse formation.