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Introduction Heart failure (HF) is marked by impaired calcium (Ca 2 +) handling, which compromises contractility and heightens arrhythmogenic risk. Ca2 + buffering within cardiac myocytes is essential for stabilizing intracellular Ca2 +. It is underexplored in HF, especially in large-animal models. This study examines how HF alters Ca2 + buffering power and investigates the potential of phosphodiesterase inhibition in restoring Ca2 + dynamics.Methods HF was induced in adult sheep through tachypacing, and animals were divided into three groups: control, HF (n=10), and HF treated with tadalafil (20 mg daily, n=6). Ventricular cardiac myocytes were isolated, and intracellular Ca 2 + was measured using Fluo-5F. Voltage clamp was achieved via a perforated patch clamp. Sarcoplasmic reticulum (SR) Ca2 + content and cellular buffering properties were measured by applying a 5 mM caffeine and 20 mM 2,3-butanedione monoxime solution. Protein levels were quantified using western blotting. All data were analysed with nested statistical models.Results HF significantly reduced Ca 2 + transient amplitude compared to controls (325.2±34.6nmol/L vs. 169.0±16.8nmol/L, p=0.005), with tadalafil treatment showing no notable improvement (149.4±30.5nmol/L, p=0.96 vs. HF). Ca2 + influx via the L-type Ca2 + channel was also reduced in HF (peak current density 1.26±0.17pA/pF vs. 2.10±0.16pA/pF in controls, p=0.007), this remained impaired in the tadalafil group (1.32±0.22pA/pF, p=0.99 vs. HF). SR Ca2 + content showed no change between controls (77.9±9.6nmol/L) and HF (73.9±9.2nmol/L, p=0.99), but decreased significantly in tadalafil-treated HF (49.0±2.9nmol/L, p=0.018 vs. HF).Ca2 + buffering properties were markedly altered in HF, Bmax decreased from 150.1±19.9µM in controls to 105.2±10.7µM (p=0.02). Kd dropped significantly in HF to 0.61±0.1µM (vs. 1.69±0.20µM in controls, p=0.0002), with an overall increase in buffer power (59.2±6.4 in control and 93.5±7.9 in HF p=0.02). Tadalafil treatment resulted in an increase in Kd to 1.25±0.13 µM (p=0.015 vs. HF), Bmax remained similar to HF at 101.8±4.6 µM (p=0.99 vs. HF). Overall buffer power was restored to baseline levels (61.3±6.5 p=0.02 vs HF.) Western blot analysis showed that HF led to a reduction in TnI phosphorylation (p=0.01), which tadalafil partially restored. SERCA levels were equally reduced in both HF and tadalafil groups (p=0.007 and p=0.003 vs. control, respectively).Abstract BS33 Figure 1Buffer properties with tadalafil treatment. Ai: Ca2+ recordings during application of caffeine + BDM solution. Aii: NCX currents during the same application of caffeine. Aiii: Integral of NCX currents in Aii. Aiv: Representative buffer slopes derived from the caffeine method. Bi: Bmax. Bii: Kd, in control (N=12), HF (N=9) and Tadalafil (N=6) groups. Mean±SEM compared using a nested one-way ANOVAAbstract BS33 Figure 2Interplay between buffer power and free and total Ca2+ and tadalafil treatment. Representative trace of free calcium transients (Ai), of total calcium transients (Aii) and the buffer power of cell across the calcium transient (Aiii). Bi: Total calcium amplitude. Bii: Change of total/change of free calcium: ergo cellular buffer power across calcium transient. Biii: Diastolic buffer power. Ci: The relationship between buffer power and free calcium. Cii The relationship between total and free calcium. In control (N=12), HF (N=9) and tadalafil groups (N=6) Mean±SEM compared using nested one-way ANOVAConclusion This study demonstrates that HF results in an increase in Ca 2 + buffering by decreasing Kd, likely due to reduced TnI phosphorylation. Tadalafil restores Ca2 + buffering to baseline by restoring Kd values. This represents a potential therapeutic target for both arrhythmia and contractility in HF. Despite the reduced SR content in tadalafil-treated HF, Ca2 + transient amplitude was preserved, due to the reduction of Ca2 + buffering power seen. These findings propose that targeting Ca2 + buffering with tadalafil may offer a novel strategy to modulate Ca2 + handling in HF.