Sildenafil citrate (
Viagra) is the most widely used
drug for treating
erectile dysfunction in men. We recently demonstrated that it induces potent protective effects against
ischemia-reperfusion (I-R) injury in rabbit hearts through the opening of mitochondrial
ATP-dependent K+ channels. In the present study, we investigated the role of the NO-dependent signaling pathway in delayed cardioprotection by
sildenafil. Adult male ICR mice were treated with saline or
sildenafil (0.7 mg/kg
IP) 24 hours before global I-R in the Langendorff mode.
Infarct size was reduced from 27.6+/-3.3% in saline-treated control mice to 6.9+/-1.2% in
sildenafil-treated mice (mean+/-SEM, P<0.05) without compromising cardiac function. Reverse transcription-polymerase chain reaction revealed a transient increase in endothelial and inducible
NO synthase (eNOS and iNOS, respectively)
mRNA in
sildenafil-treated mice, peaking at 45 minutes (eNOS) and 2 hours (iNOS) after
sildenafil injection. The magnitude of
mRNA increase was more pronounced for iNOS than for eNOS. In addition, a significant increase in both iNOS and eNOS
protein was detected 24 hours after
sildenafil treatment. A selective inhibitor of iNOS, 1400W (10 mg/kg IP given 30 minutes before I-R), abolished
sildenafil-induced protection (23.7+/-2.8%, P<0.05 versus
sildenafil). These data suggest that the induction of
NO synthase isoforms is an essential component of the signaling mechanism for
sildenafil-induced delayed preconditioning. However, iNOS appears to be the primary
isoform that mediates the robust cardioprotection.