13Pulmonary hypertension leads to dysfunction and widespread ion channel remodelling within the atrioventricular node in the monocrotaline rat model

Abstract

Introduction: Arrhythmias are common in pulmonary arterial hypertension (PAH) causing severe symptoms and sudden death. Atrioventricular (AV) node dysfunction is seen in PAH but the cause has not been determined. We assessed AV node function, and key ion channels, connexins and intracellular calcium handling proteins in the AV node in the moncrotaline (MCT) rat model of PAH. Methods: Male Wistar rats (200 g) were injected with either MCT (60 mg/kg) or saline (CON). Echo was performed to confirm PAH in the MCT rats. The rats were sacrificed if they showed clinical deterioration or on day 28. For whole heart experiments the heart was excised and mounted on a Langendorff perfusion column at 37°C. For right atrial (RA)/atrioventricular (AV) node experiments, a tissue preparation was superfused at 37°C. Atrio-His (AH) interval, AV effective refractory period (AVERP) and AV functional refractory period (AVFRP) were measured. A separate cohort of hearts was used to assess mRNA expression of key ion channels, connexins and intracellular calcium handling proteins in the AV node using reverse transcription quantitative polymerase chain reaction (RTqPCR). Sections of the AV node were labelled for HCN4 and connexin 43 using immunofluorescence allowing identification of the atrium, ventricle and four regions within the AV node: transitional tissue (TT), inferior nodal extension (INE), compact node (CN) and penetrating bundle (PB). Each region was microdissected and mRNA levels of 96 proteins were measured. Results: The whole heart experiments showed an increase in AVERP (87±1 ms in CON v 94±2 ms in MCT, p<0.05) and AVFRP (115±2 ms in CON v 122±2 ms in MCT, p<0.05) in the MCT rats compared with the CON rats. The isolated RA/AV node preparation demonstrated a 50% incidence of complete heart block in the MCT rats compared with 0% in the CON rats (p<0.05). The RTqPCR experiments demonstrated widespread remodelling of ion channel expression in the AV node in the MCT injected rats. In the TT, there was downregulation of phospholamban and sarcolipin; in the INE there was downregulation of HCN4, Kv2.1, minK and KChiP2, Kir2.4, phospholamban, RyR2, sarcolipin, α1 Na+/K+ ATPase and α2 Na+/K+ ATPase; in the CN there was downregulation of HCN4 and KChiP2. No significant changes were detected in the PB. Changes in HCN4 are shown. Conclusions: PAH leads to AV node dysfunction in the MCT model which is likely due to remodelling of ion channels and intracellular calcium handling proteins in the AV node. Downregulation of HCN4 is of particular interest given recent studies showing that HCN4 knockout causes heart block and suggests HCN4 may play a greater role in AV node function than has previously been realised. ![Graphic][1] Figure HCN4 expression in the AV node. The control rats show the expected distribution of HCN4 across the different regions of the AV node. Significant downregulation of HCN4 is seen in the atrium, INE and CN of the MCT rats [1]: /embed/inline-graphic-1.gif