Heart failure is the leading cause of morbidity and mortality in the world. It is an incurable disease and most treatment strategies aim to treat the symptoms or slow the progression of the condition. Cardiac contractility is governed by calcium homeostasis within cardiac myocytes and is modulated by the sympathetic nervous system. Both mechanisms are detrimentally altered in heart failure. An important group of enzymes, phosphodiesterases, are fundamental to the sympathetic (beta-adrenergic) modulation of calcium cycling in cardiac myocytes. The selective inhibition of phosphodiesterase 5 (PDE5) has recently been considered as a potential therapy for heart failure; having beneficial effects in human and animal models of the disease. The present study employs a large animal model of tachypacing induced heart failure to test the effect of PDE5 inhibition on myocyte and whole heart contractility and beta-adrenergic function, to assess the molecular mechanisms by which PDE5 inhibition is beneficial to the failing myocardium.In initial experiments the PDE5 inhibitor sildenafil was applied acutely to voltage clamped ventricular myocytes from uninstrumented sheep. PDE5 inhibition reduced baseline L-type calcium current and systolic calcium transient amplitude, suggesting it is negatively inotropic. Furthermore, the positive inotropic effects of beta-adrenergic stimulation were somewhat reversed by acute PDE5 inhibition. Interestingly, such negative inotropic effects of acute PDE5 inhibition were not observed in failing ventricular myocytes, which have dysfunctional calcium homeostasis and beta-adrenergic reserve. When delivered chronically over 3 weeks to tachypaced animals, PDE5 inhibition restored and augmented the systolic calcium transient and beta-adrenergic responsiveness at both the whole heart and myocyte level. These effects were associated with changes to the expression and phosphorylation status of the proteins that control calcium homeostasis in left ventricular tissue. In vivo, PDE5 inhibition prolonged longevity and reduced the onset of clinical signs of heart failure in sheep, as well as arresting cardiac dilatation and wall thinning. Chronic PDE5 inhibition however had no effect on cardiac contractility or heart failure induced changes in cardiac electrophysiology.This study presents a novel mechanism by which PDE5 inhibition may be beneficial in a large animal model of heart failure by restoring calcium homeostasis and beta-adrenergic responsiveness. This study may have important implications for the management of heart failure in clinical practice.