Even in the face of improving therapeutic approaches, heart disease remains a major cause of premature death and morbidity. The two main causes of death in diseases such as heart failure are arrhythmias and deteriorating contractile performance. This presentation will introduce both recently published (e.g. Briston et al 2011) and unpublished work from our laboratory demonstrating a pivotal role for changes in intracellular calcium handling and β-adrenergic signalling as contributors to the occurrence of contractile dysfunction and arrhythmias using an ovine model of heart failure. All experiments were conducted on enzymatically dissociated single myocytes isolated from the hearts of either control (non-failing) sheep or sheep in which heart failure had been induced by right ventricular tachypacing (pacemaker insertion under 1-3 % isoflurane inhalational anaesthesia with 0.5 mg/kg meloxicam analgesia s/c). The tachypacing model produces a dilated cardiomyopathy with substantial atrial and ventricular dilatation and contractile dysfunction. Once symptoms of heart failure were present (lethargy, dyspnoea) animals were humanely killed (pentobarbitone 200 mg/kg i/v) and cells isolated. Changes in intracellular calcium concentration were measured with fluorescent indicators using either confocal microscopy or standard epifluorescence techniques. Cells were also voltage clamped at 37 °C using either the whole cell or perforated patch techniques as appropriate for the experiment being conducted. Under baseline conditions, heart failure resulted in a reduction in the amplitude of the systolic calcium transient which was entirely attributable to a reduction in the L-type calcium current. Moreover, during β-adrenergic stimulation with the mixed agonist isoprenaline the systolic calcium transient remained reduced in the heart failure cells compared to non-failing cells. However, under these circumstances the reduction in calcium transient amplitude was due to the combined effects of a smaller L-type calcium current and a smaller sarcoplasmic reticulum (SR) calcium content. To determine the mechanism as to why SR calcium content failed to increase appropriately in heart failure we undertook protein expression analysis using Western blotting which revealed a reduced phosphorylation of phospholamban at both the serine16 and threonine17 sites. There was also an increase in protein phosphatase and G-protein receptor kinase (GRK-2) protein expression and a reduction in protein kinase A (PKA) activity. Given the requirement for a threshold SR calcium content to be reached for the occurrence of arrhythmogenic diastolic calcium waves (Venetucci et al, 2007) we then sought to determine if changes in the threshold SR calcium content were involved in the increased propensity for arrhythmias in heart failure. Diastolic calcium waves were produced by raising the extracellular calcium concentration to 10 mM. In both control conditions and following β-adrenergic stimulation with isoprenaline the threshold SR calcium content was reduced in heart failure cells compared to non-failing cells. Importantly, in non-failing cells the threshold SR calcium content was greater than the SR calcium content before arrhythmogenic diastolic calcium waves occurred. However, in heart failure cells during β-adrenergic stimulation the threshold SR calcium content was no different to the cells SR calcium content before calcium waves occurred. We conclude therefore that changes to intracellular calcium homeostasis and β-adrenergic signalling both contribute to the pathophysiology of heart failure. Specifically, in this particular model of heart failure, an important role exists for the smaller L-type calcium current, enhanced protein phosphatase and GRK-2 expression and reduced PKA activity in contributing to the reduced calcium transient amplitude. Furthermore, we suggest that in heart failure the similarity between the cells ‘operating’ SR calcium content and threshold SR calcium content for arrhythmogenic diastolic calcium release during β-adrenergic stimulation provides a possible mechanism for the increased prevalence of arrhythmias in patients with heart failure particularly following an increase in catecholamine production.