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179 Phosphodiesterase-5 inhibition with sildenafil suppresses calcium waves by reducing sarcoplasmic reticulum content
  1. David Hutchings,
  2. Katharine Dibb,
  3. Charles Pearman,
  4. George Madders,
  5. Lori Woods,
  6. David Eisner,
  7. Andrew Trafford
  1. University of Manchester


Rationale: Occurrence of diastolic Ca2+ waves in cardiac myocytes leads to arrhythmias by inducing delayed after-depolarisations. Waves are initiated when sarcoplasmic reticulum (SR) content reaches a critical threshold level. The phosphodiesterase-5 inhibitor sildenafil (Sil) is antiarrhythmic in mammalian myocardial ischaemia models, while Sil reduces Ca2+ transient amplitude and sarcoplasmic reticulum (SR) Ca2+ content in rat myocytes.

Objective: To determine effects of Sil on propensity to Ca2?+ waves in the large mammal.

Methods: Sheep ventricular myocytes were voltage clamped and intracellular Ca2+ measured using Fura-2. Cells were paced at 0.5 Hz with depolarisations from −40 mV to+10 mV. When at steady state, waves were induced with 10–15 mM Ca2+. Upon regular waving, Sil (1µM) was applied. To determine threshold SR content, caffeine (10 mM) was added immediately following a wave, and both wave and caffeine-induced I NCX integrated. Differences between groups were determined using students paired t tests.

Results: Increasing external Ca2+ to 10–15 mM increased SR content and induced diastolic waves. Sildenafil abolished waves in 9/15 cells. In cells where Sil terminated waves, SR content was reduced below threshold. In addition, Sil treatment was associated with a reduced rate constant of SERCA (kSERCA−66.0±9.9% of control, p<0.005), an initial (first 4 s) increase in sarcolemmal efflux via the I NCX tail current (+142± 36.4%, p<0.01), and reduced sarcolemmal influx via I Ca-L (–30.5±5.6%, p<0.005). In cells continuing to wave in Sil, SR threshold for waves was unchanged (126.9 µmolL-1 ctrl vs 147.2 µmolL-1 Sil, p=0.6). In unstimulated cells spontaneously waving in 10–15 mM Ca²+, sildenafil reduced wave frequency (6.3 waves per 20 s vs 2.7, p<0.005). The protective effect of sildenafil on both wave models was abolished when cells were pre-incubated with the PKG inhibitor, KT5823. Sildenafil suppression of waves was also observed in cells from animals in end-stage heart failure, while Sil suppressed ventricular ectopy and episodes of torsades de pointes in vivo in a sheep model of LQT2.

Conclusions: Sildenafil suppresses waves induced by elevated external Ca2+ via a PKG-dependent mechanism, and mediated by a reduction in SR content, which itself is caused by reduced SERCA function ± reduced I Ca-L. These findings highlight novel antiarrhythmic properties of PDE5 inhibition and translate to suppression of triggered arrhythmias in vivo.

Funding: British Heart Foundation.

  • Sildenafil
  • Calcium
  • Wave

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