Essential role of mitochondrial Ca2+-activated and ATP-sensitive K+ channels in sildenafil-induced late cardioprotection

Sildenafil (Viagra), a phosphodiesterase type-5 inhibitor used in treatment of male erectile dysfunction and pulmonary hypertension can induce cardioprotection through opening of mitochondrial ATP-sensitive K channels (mitoK(ATP)). Recent studies suggest that activation of mitochondrial Ca2+-activated K+ channels (mitoK(Ca)) also has anti-ischemic effects. However, the relative role of mitoK(Ca) and mitoK(ATP) in sildenafil-induced cardioprotection remains unknown. In the present study, adult male ICR mice were pretreated with sildenafil (0.71 mg/kg, i.p.) 24 h prior to 20 min of global ischemia followed by 30 min of reperfusion in Langendorff mode. Paxilline (blocker of K-Ca) or 5-hydroxydecanoic acid (5-HD; blocker of mitoK(ATP)) was administered either 30 min before sildenafil or 10 min prior to ischemia. Treatment with sildenafil reduced infarct size, which was abolished by either paxilline or 5-HD. Furthermore, in vivo gene knockdown of beta 1 subunit of K-Ca (K-Ca-beta 1) using small interfering RNA (siRNA) administered 48 h before sildenafil injection blocked the infarct limiting effect of sildenafil. The protective effect of sildenafil was preserved in mice treated with non-target siRNA. Western blots demonstrated selective protein expression of K-Ca-beta 1 in cardiac mitochondria and the gene knockdown effect of siRNA on K-Ca-beta 1. The level of K-Ca-beta 1 protein was not upregulated following treatment with sildenafil. We conclude that both mitoK(Ca) and mitoK(ATP) play a critical role in triggering and mediating sildenafil-induced delayed cardioprotection. The results suggest that activation of mitoK(Ca) and mitoK(ATP) are crucial for maintaining mitochondrial homeostasis and reducing cell death in sildenafil-induced preconditioning against ischemia-reperfusion injury. (C) 2007 Elsevier Inc. All rights reserved.