Heart failure (HF) is a major cause of premature mortality and increased morbidity. One of the hallmarks of HF is a decrease in responsiveness to beta-adrenergic (β-AR) stimulation. As HF patients have elevated levels of circulating catecholamines, it is possible that changes to β-AR signalling are caused by desensitisation of the β-adrenoreceptors. The second messengers cAMP and cGMP enable correct β-AR signalling by facilitating phosphorylation of a number of excitation contraction-coupling (ECC) targets. It is believed that remodelling of the cAMP pathway in HF may lead to β-AR desensitisation. In a similar vein to beta-blocker therapy, inhibitory cGMP pathway activation may improve contractility in HF patients through modulation of the remodelled cAMP pathway. Thus, we sought to investigate the effects on ECC parameters in the presence of a selective inhibitor of the main cGMP-hydrolysing phosphodiesterase isoform, PDE5. Heart failure was induced in sheep using tachypacing. Animals were anaesthetised for pacemaker implantation (isoflurane, 1-3% in oxygen) and perioperative analgesia provided (meloxicam, 0.5 mg/kg). After 7 days recovery, right ventricular pacing (210-220 bpm) was applied until clinical symptoms of HF were evident. Animals were sacrificed by I.V. injection of pentobarbitone (200mg/kg) and collagenase digestion was used to isolate mid-myocardial left ventricular myocytes for whole cell voltage clamp studies. In the presence of the selective PDE5 inhibitor sildenafil (1 µM), systolic transient amplitude and L-type calcium current (ICa-L) were significantly reduced in both control (n=6, P<0.005) and HF (n=5, P<0.05) myocytes. When co-stimulated with the non-selective β-AR agonist isoprenaline (100 nM), PDE5 inhibition attenuated increases in ICa-L (n=5, P<0.05) with no effect on calcium transient amplitude in control myocytes. No effects of PDE5 inhibition were observed on the responses to β-AR stimulation in HF myocytes. In HF sheep chronically treated (~4 weeks) with the selective PDE5 inhibitor tadalafil (20 mg/kg) preliminary analysis of the ICa-L current properties, in comparison to control myocytes, showed a significant rightward shift of current inactivation and decreased window current. Significance assessed using one way repeated measures ANOVA and student’s t-test. The present data shows that PDE5 inhibition, possibly through increased PKG activity, can reduce baseline ICa-L and systolic calcium transient amplitude in ventricular myocytes from both normal and HF sheep. However, under stimulation of an already impaired β-AR pathway in HF myocytes, the suppressive effects of PDE5 inhibition are not observed. This may suggest a role for remodelling of the signalling pathways governing interaction of cAMP and cGMP. Experiments were carried out in accordance with the UK Home Office Animals (Scientific Procedures) Act 1986.