Abstract
Sildenafil induces powerful cardioprotection against ischemia/reperfusion (I/R) injury. Since adenosine is known to be a major trigger of ischemic preconditioning, we hypothesized that A(1) adenosine receptor (A(1)AR) activation plays a role in sildenafil-induced cardioprotective signaling. Adult male C57BL wild-type (WT) mice or their corresponding A(1)AR knockout (A(1)AR-KO) mice were treated intraperitoneally (i.p.) with either sildenafil (0.71 mg/kg, equivalent to 50 mg dose for a 70 kg patient) or volume-matched saline. The selective A(1)AR antagonist 8-cyclopentyl-1,3-dipropyxanthine (DPCPX; 0.1 mg/kg, i.p.) was administered 30 min before sildenafil. The hearts were isolated 24 h later and subjected to 30 min of global ischemia and 1 h of reperfusion in Langendorff mode. Post-ischemic myocardial infarct size (mean+/-SEM; % of risk area) was reduced in C57BL-WT mice treated with sildenafil (5.6+/-0.9) versus saline control group (27.3+/-2.1; p<0.05; n=6/each). However, sildenafil failed to protect the A(1)AR-KO hearts (31.6+/-1.9 vs. 32.3+/-1.5 with saline, p>0.05). Additionally, DPCPX treatment abolished the infarct limiting effect of sildenafil (27.3+/-3.2, p<0.05). DPCPX alone had no effect on infarct size as compared with the control group. No significant changes in post-ischemic recovery of left ventricular pressure and heart rate were observed in the sildenafil-treated group. We further examined the effect of sildenafil in protection against simulated ischemia and reoxygenation injury in adult cardiomyocytes derived from WT and A(1)AR-KO mice. WT myocytes treated with sildenafil (1 microM) demonstrated significantly lower trypan blue-positive necrotic cells. However, cardiomyocytes derived from A(1)AR-KO mice or DPCPX-treated WT cells failed to show protection against necrosis with sildenafil. These results suggest that A(1)AR activation following treatment with sildenafil plays an integral role in the signaling cascade responsible for delayed protection against global I/R injury.