Abstract 18171: HCN Channel Distribution in the Human Sinoatrial Node and Latent Atrial Pacemakers (Best of Basic Science Abstract)

Abstract

Background: The hyperpolarization-activated current, I f , plays an important role in cardiac pacemaker cells. However, the distribution and expression of HCN isoforms in the human SAN and latent atrial pacemaker clusters is unknown. Methods: Human atria and entire SAN complexes were isolated from failing (n=5) and non-failing (n=9) human hearts cardioplegically-arrested in the operating room. Three dimensional intramural SAN structure was identified as the fibrotic compact region around the SAN artery with Connexin43-negative pacemaker cardiomyocytes visualized in Masson’s trichrome and immunostained cryosections. Pure SAN tissue was precisely isolated from the frozen SAN cryo blocks using a 16G biopsy needle (Figure). Atrial tissues from different locations were fresh frozen in liquid nitrogen. Immunoblot and immunostaining were used to study the expression pattern of HCN isoforms. Results: Three HCN isoform proteins were detected in the atria and SAN (Figure). HCN1 was predominantly distributed in all the human SAN with a 125.1±40.2 (n=12) expression ratio of SAN to right atrial free wall (RAFW). HCN2 and HCN4 expression levels were higher in SAN than atria with ratios of 6.1±0.9 and 4.6±0.6 (n=12), respectively. HCN2 expression but not HCN1/4 in the latent atrial pacemakers from the right atrioventricular ring (RR) area was significantly higher than in RAFW, with the band density ratio of RR/RAFW= 2.4±0.4 (n=12). Conclusions: This is the first study to conduct precise molecular mapping of the human SAN by isolating pure pacemaker SAN tissue. HCN1 was almost exclusively expressed in SAN and may play a critical role in determining the leading pacemaker in human SAN. Even HCN2 and HCN4 expression is higher in the SAN than RAFW, these isoforms are less SAN-specific than HCN1. SAN and latent atrial pacemakers have different HCN expression patterns, suggesting the utility of HCN isoform specific blockers to selectively modify sinus rhythm or atrial ectopic rhythms.