About one-third of heart failure (HF) patients have left bundle branch block and resynchronization therapy is beneficial. We have investigated possible mechanisms in a rabbit model of left-sided volume and pressure overload. Male New Zealand rabbits (2,5-3 Kg) were anaesthetised with ketamine and isoflurane. Aortic incompetence was induced by inserting a catheter through the valve leaflets to cause LV volume overload. Then pressure overload was introduced by placing a silver clip (internal diameter 2.44 mm) just above right renal artery, 3 weeks later. The control group underwent sham procedures. The study was conducted in accordance with the Guide for the Care and Use for Laboratory Animals and approved by the local animal research ethics committee. By the end of 8 weeks, the rabbits showed significant LV hypertrophy, diminished LV fractional shortening and increased LV internal dimension. Echocardiography revealed an increase in the left ventricle (LV) internal diameter (in diastole) from 1.49±0.03 to 2.16±0.07 cm (n=6/8; P<0.001) and a decrease in LV fractional shortening from 41.3±1.4 to 25.5±3.2 % (n=6/8; P=0.002). The study was conducted in accordance with the Guide for the Care and Use for Laboratory Animals (US NIH Publication No 85-23, revised 1985). Micro-CT imaging as well as visual inspection showed hypertrophy of the Purkinje fibres. The ECG was recorded from anaesthetized sham-operated and HF rabbits before and after autonomic blockade. In the HF rabbits, there was an increase in the PR interval and duration of the QRS complex: e.g. following autonomic blockade, there was an increase in the PR interval from 68.5±0.9 to 83.9±2.9 ms (n=7/9; P=0.001) and the QRS duration from 38.4±2.0 to 48.2±2.9 ms (n=7/10; P<0.05). This suggests that there is delayed His-Purkinje conduction in the HF model. Free-running Purkinje fibres in the two ventricles were collected and RNA was extracted and reverse transcribed to produce cDNA. Quantitative PCR was carried out to measure the abundance of different ion channel and Ca2+-handling transcripts. In the LV Purkinje fibres from the HF group, we found significant downregulation of many ion channel mRNAs, including mRNAs for funny channels (HCN1 by 66%, HCN4 by 82%), Na+ channels (Nav1.5 by 51%), Ca2+ channels (Cav1.2 by 63%, Cav1.3 by 90%), K+ channels (Kv1.5, ERG, KvLQT1, Kir2.1, Kir3.1, Kir6.2 and SUR2a by 48-93%), Ca+-handling proteins (RYR2, SERCA2 and NCX1 by 55-62%), a Cl- channel (ClC-2 by 46%) and connexins (Cx40 by 50%, Cx43 by 49%). The change in HCN4 appears to have been driven by a significant downregulation in the transcription factor, Tbx3, by 70%. In the RV Purkinje fibres, fewer differences were observed. At the protein level (measured using immunohistochemistry), there was a significant decrease in Cx43 in the LV Purkinje fibres from the HF group. We propose that remodelling of ion channels etc. in the LV His-Purkinje network may occur in response to LV dilatation and consequent stretching of the His-Purkinje tissue, particularly given that stress/stretch is thought to play a role in Purkinje fibre differentiation. Strategies to limit acute ventricular dilatation may be worthwhile in preventing long-term LV dysfunction in HF.