Right ventricle (RV) failure is the leading cause of death in patients with pulmonary arterial hypertension (PAH). Diastolic dysfunction is associated with poor prognosis, though the cellular and extracellular causes require further study. Creatine kinase normally buffers ADP but its expression is reduced in heart failure, leading to diastolic dysfunction in isolated cardiac myocytes (Fowler et al., 2012). We hypothesised loss of creatine kinase reduces RV compliance in rats with monocrotaline (MCT)-induced PAH by affecting crossbridge cycling.Male Wistar rats (200g) were injected with 60 mg.kg-1 MCT (MCT; N=25) or an equivalent volume of saline (CON; N=28). In vivo RV haemodynamic measurements were taken in anaesthetised rats. MCT rats were killed by stunning and cervical dislocation upon showing symptoms of HF, CON rats were killed on equivalent days. In isolated Langendorff perfused arrested hearts passive stiffness was measured by inflating an indwelling fluid filled balloon in the RV; balloon pressure was measured simultaneously and passive pressure-volume (PV) curves constructed. Baseline PV curves were recorded in 0mM Ca2+ Krebs solution (0Ca) then repeated after 30 min with or without the creatine kinase inhibitor DNFB (20µM). Hearts were then frozen and 10µm sections stained for collagen using picrosirius red. Data are presented as mean±SEM. Paired or unpaired t-tests or 2 way ANOVA with Bonferroni corrections were used as appropriate. P<0.05 was considered significant.A steeper in vivo end-diastolic PV relationship indicated diastolic dysfunction in MCT rats (CON 0.04±0.01 mmHg/µL, MCT 0.25±0.07 mmHg/µL, p=0.021). In isolated hearts the slope of the passive PV curve between 0-30 mmHg was not different between CON and MCT, nor RV volume at a given pressure (Figure 1A). The slope of the PV curve was not affected by DNFB. Percentage collagen staining was unchanged in MCT rats (CON 3.1±0.9%, MCT 2.3±0.4%, p>0.05). Collagen content did not correlate with stiffness (r2=0.059). After repeated distensions and 30 min Langendorff perfusion in 0Ca both CON and MCT PV curves shifted to larger volumes for a given pressure. The change in volume from baseline to 30 min was compared between DNFB treated hearts and time-matched hearts in 0Ca; DNFB prevented distension in CON hearts (p<0.025) (Figure 1B).In vivo diastolic dysfunction in our MCT rats is not due to fibrosis and inhibition of creatine kinase reduced compliance possibly through impaired crossbridge cycling. Thus loss of creatine kinase in heart failure may contribute to diastolic dysfunction. However, the use of an intraventricular balloon in the RV may not be sensitive enough to detect changes in passive PV slope between CON and MCT hearts.