Abstract 2894: Essential Role of ERK 1/2 in Sildenafil-Induced Early and Delayed Cardioprotection in Mice
Background: Sildenafil (SIL), a potent inhibitor of phosphodiesterase-5 induces powerful protection against myocardial ischemia-reperfusion (I-R) injury through activation of protein kinase G (PKG). However, the downstream targets of PKG in SIL-induced cardioprotection remain unclear. We hypothesized that PKG-dependent activation of survival kinase, ERK may play a critical role in SIL-induced cardioprotection in mice.
Methods & Results: Ventricular myocytes were isolated from adult male ICR mice and exposed to 40 min of simulated ischemia (SI) with/without 1 hr pre-incubation of SIL (1 μM). Myocyte necrosis and apoptosis were determined after 1 hr or 18 hrs of reoxygenation (RO) using trypan blue or TUNEL assay, respectively. Pretreatment with SIL protected cardiomyocytes after SI-RO (necrosis 18.5±0.5% and apoptosis 6.6±0.7%; n=4, p<0.001) as compared with controls (necrosis 42.1±1.8% and apoptosis 23.3±0.9%). Co-incubation of PD98059 (20 μM), a selective ERK1/2 inhibitor blocked both anti-necrotic and anti-apoptotic protection in cardiomyocytes. Furthermore, intra-coronary infusion of SIL (1 μM) in Langendorff isolated mouse hearts 10 min prior to zero-flow global I (20 min) and R (30 min) significantly reduced myocardial infarct size (from 29.4±2.4% to 16.0±3.0%; p<0.05, n=6). Co-treatment of PD98059 abrogated SIL-induced protection (33.0±5.9; n=4). To evaluate the role of ERK1/2 in delayed cardioprotection, mice were treated with saline or SIL (0.7 mg/kg i.p.) 24 hours before global I-R in Langendorff mode. PD98059 (1 mg/kg) was administered (i.p.) 30 min before the treatment of SIL. Infarct size was reduced from 27.6±3.3% in saline-treated controls to 6.9±1.2% in SIL-treated mice (P<0.05, n=6). The delayed protective effect of SIL was also abolished by PD98059 (22.5±2.3%). Western Blots revealed that SIL significantly increased phosphorylation of ERK1/2 which was blocked by PKG inhibitor, KT5823 in the heart and adult myocytes. Selective knockdown of PKG in cardiomyocytes with short hairpin RNA of PKG also blocked the phosphorylation of ERK1/2.
Conclusion: SIL-induced cardioprotection involves the activation and phosphorylation of ERK which appear to be intimately linked with a PKG-dependent survival pathway.
This research has received full or partial funding support from the American Heart Association, AHA Mid-Atlantic Affiliate (Maryland, North Carolina, South Carolina, Virginia & Washington, DC).